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Hemophagocytic Lymphohistiocytosis, Familial

Synonyms: Familial Erythrophagocytic Lymphohistiocytosis, FHLH

, MD, MBA, , MD, , MS, , MD, and , MT, MBA.

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Initial Posting: ; Last Update: January 17, 2013.

Estimated reading time: 37 minutes


Clinical characteristics.

Familial hemophagocytic lymphohistiocytosis (FHL) is characterized by proliferation and infiltration of hyperactivated macrophages and T-lymphocytes manifesting as acute illness with prolonged fever, cytopenias, and hepatosplenomegaly. Onset is typically within the first months or years of life and, on occasion, in utero, although later childhood or adult onset is more common than previously suspected. Neurologic abnormalities may be present initially or may develop later; they may include increased intracranial pressure, irritability, neck stiffness, hypotonia, hypertonia, convulsions, cranial nerve palsies, ataxia, hemiplegia, quadriplegia, blindness, and coma. Rash and lymphadenopathy are less common. Other findings include liver dysfunction and bone marrow hemophagocytosis. The median survival of children with typical FHL, without treatment, is less than two months; progression of hemophagocytic lymphohistiocytosis and infection account for the majority of deaths in untreated individuals.


The diagnosis of FHL is made based on the presence of clinical criteria and is confirmed by molecular genetic testing. Five disease subtypes (FHL1, FHL2, FHL3, FHL4, and FHL5) are described. Four genes in which pathogenic variants are causative have been identified: PRF1 (FHL2), UNC13D (FHL3), STX11 (FHL4), and STXBP2 (FHL5).


Treatment of manifestations: Antibiotics or antiviral agents are used to treat or prevent infections that may have triggered the exaggerated inflammatory response. Individuals suspected of having FHL or diagnosed with FHL are treated with chemotherapy and immunotherapy to achieve clinical stability prior to allogeneic hematopoietic cell transplantation (HCT).

Prevention of primary manifestations: Allogeneic HCT is the only curative therapy and is undertaken as early in life as feasible in children with confirmed FHL.

Surveillance: Following HCT, annual neuropsychological evaluation and regular follow up by a transplant specialist to monitor for late complications related to growth and hormone function is recommended.

Agents/circumstances to avoid: Live vaccines; exposure to infections.

Evaluation of relatives at risk: Molecular genetic testing of at-risk sibs for the familial pathogenic variants enables consideration of presymptomatic bone marrow transplantation and timely treatment.

Genetic counseling.

FHL is inherited in an autosomal recessive manner. Each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once an at-risk sib is known to be unaffected, the risk of his/her being a carrier is 2/3. Carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk are possible if the two pathogenic variants in the family are known.


Clinical Diagnosis

The diagnosis of familial hemophagocytic lymphohistiocytosis (FHL), a cellular immunologic disorder resulting from genetic defects in cytotoxic cell function that lead to hyperactivation, proliferation, and infiltration of macrophages and T-lymphocytes, can be established if 1 and/or 2 is present:

1. Biallelic pathogenic variants in any one of PRF1, UNC13D (also known as MUNC13-4), STX11, or STXBP2

2. At least five of the eight following criteria based on the guidelines of the Histiocyte Society [Henter et al 2007]:

  • Prolonged fever (>7 days)
  • Cytopenias affecting two or three of the three lineages in the peripheral blood:
    • Hemoglobin <90 g/L (for infants age <4 weeks: Hgb <100 g/L)
    • Platelets <100x109/L
    • Neutrophils <1.0x109/L
  • Splenomegaly
  • Hypertriglyceridemia and/or hypofibrinogenemia
    • Fasting triglycerides ≥2.0 mmol/L or >3 SD of the normal value for age
    • Fibrinogen ≤1.5 g/L
  • Hemophagocytosis. Non-malignant, mixed lymphohistiocytic accumulation in the reticuloendothelial system; the spleen, liver, lymph nodes, bone marrow, and CNS are most frequently involved.
    Note: (1) Hemophagocytosis may not be apparent early in the course of the disease. (2) Hemophagocytosis is seen less often in the liver, where lymphocytic infiltration of the portal areas is typical [Kapelari et al 2005].
  • Low or absent natural killer (NK) cell activity. This is common prior to and during active disease, as well as after remission following chemotherapy in a significant proportion of individuals with FHL. Normal or raised NK cell activity has also been observed in some affected individuals, including those with UNC13D pathogenic variants [Yamamoto et al 2004, Ishii et al 2005].
    Note: (1) NK cell activity normative values are per local laboratory reference. (2) The number of circulating NK cells (CD56+/16+) is generally within normal limits. (3) In secondary (acquired or reactive) hemophagocytic lymphohistiocytosis (HLH), NK cell activity has been reported to normalize during remission [Horne et al 2005b, Ishii et al 2005].
  • Hyperferritinemia. Serum ferritin concentration ≥500 µg/L (normal 10-290 µg/L)
  • High plasma concentrations of soluble CD25 (soluble IL2Rα); ≥2400 U/mL
    Note: (1) Normal range depends on test methodology used. (2) Results must be compared to age-matched controls.

The presence of inflammatory cells in the spinal fluid in association with increased CSF protein and/or characteristic findings on brain MRI (including gross demyelination or failure of normal myelination during the first year of life, multifocal inflammation involving gray and white matter and intracranial bleeding, generalized atrophy, or brain edema are also highly suspicious for HLH [Filipovich 2011] but are not part of the formal diagnostic criteria.

In addition, a positive family history of affected sibs and/or parental consanguinity in a symptomatic individual supports the diagnosis of FHL.


Protein analysis

  • Perforin (the protein product of PRF1) analysis by flow cytometry:
    • In general, absent or markedly decreased perforin protein expression is highly likely to be associated with homozygous or compound heterozygous pathogenic variants in PRF1.
    • Of note, normal perforin protein expression may be associated with PRF1 pathogenic variants because some missense variants alter perforin protein function without significantly changing protein expression.
    • If perforin protein expression is normal or increased, UNC13D or STXBP2 molecular genetic testing should be considered first.
  • Natural killer (NK) cell activity analysis:
    • Low or absent NK cell activity is an important biomarker in persons with FHL.
    • Low or absent NK cell activity is usually observed in persons with confirmed biallelic pathogenic variants in PRF1, UNC13D, STXBP2 or STX11 [Sieni et al 2012a].
    • In secondary (acquired or reactive) hemophagocytic lymphohistiocytosis (HLH), NK cell activity may fluctuate over time [Horne et al 2005b, Ishii et al 2005].
  • NK cell degranulation (CD107a) analysis:
    • NK cell degranulation can be quantified by measurement of upregulation of surface CD107a [Bryceson et al 2007].
    • Defective CD107a surface expression in NK cells is a frequent finding in persons with UNC13D pathogenic variants (FHL3) and STX11 pathogenic variants (FHL4), in contrast to healthy controls or persons with PRF1 pathogenic variants (FHL2) [Marcenaro et al 2006]. Individuals with pathogenic variants in STXBP2 (FHL5) also have defective CD107a expression [zur Stadt et al 2009].
  • Granzyme B initiates caspase-dependent and caspase-independent apoptotic killing of target cells.
    • In most types of HLH, including those caused by mutation of PRF1 and UNC13D as well as secondary HLH, granzyme B ranges from increased to markedly increased.
    • Increased amounts of granzyme B in persons with HLH do not necessarily indicate normal killing, but most likely indicate that the granules housing the granzymes are not able to mobilize normally.
  • Soluble IL2R is an indicator of prolonged activation of T cells.
    • As the IL-2 receptor (CD25) forms on the surface of T cells during activation, increasing density of the receptor causes shedding into the plasma.
    • This soluble form of the receptor is useful as a diagnostic criterion of any of the forms of HLH and for monitoring reoccurrence of the disease.
    • Elevated soluble IL-2 receptor levels do not differentiate between primary (genetic) and secondary HLH.
    • It is expected for children to have different expression of CD25 and thus the soluble form of the IL2R in plasma at different ages. Therefore, when interpreting results in children, age-based reference ranges should be used.
  • Serum ferritin concentration is a marker for generalized inflammation.
    • It is, however, markedly elevated in the majority of persons with HLH and is a very sensitive indicator of HLH when serum concentrations are markedly increased.
    • Increased serum ferritin concentrations make no distinction between genetic and secondary HLH.

Molecular Genetic Testing

Genes/loci. Five loci (FHL1, FHL2, FHL3, FHL4, and FHL5) are associated with familial hemophagocytic lymphohistiocytosis; the five disease subtypes are based on these loci (Table 1). Four genes have been identified and characterized: PRF1 (FHL2), UNC13D (FHL3), STX11 (FHL4), and STXBP2 (FHL5).

Table 1.

Locus Names, Genes, and Pathogenic Variants Associated with Familial Hemophagocytic Lymphohistiocytosis

Locus NameGene% of FHLVariants Identified
FHL1N/A4 consanguineous Pakistani families 1N/A
FHL2PRF120%-30% 2, 3 worldwide
>50% in African American families 4
Multiple distinctive pathogenic variants throughout entire coding region
~20%-30% 3, 5 worldwideMultiple distinctive pathogenic variants throughout entire coding region & splicing sites
Deep intronic pathogenic variant & large inversion also reported 6
FHL4STX11~20% of Turkish/Kurdish families 5, 7
Biallelic pathogenic variants identified in other ethnic groups (at a low frequency) 8
3 recurrent pathogenic variants identified in Turkish/Kurdish families
Other pathogenic variants identified in other populations
FHL5STXBP216% in Central Europeans, Turks, & Saudis 9
~20% in North Americans w/FHL 10
Multiple distinctive pathogenic variants throughout entire coding region & splicing sites

Linkage analysis using homozygosity mapping in four consanguineous families of Pakistani descent with hemophagocytic lymphohistiocytosis (HLH) identified a locus (FHL1) on chromosome 9q21.3 22 [Ohadi et al 1999]. No gene in which mutation is causative has been identified at this locus.


Marsh et al [2010b]; Weitzman, personal communication (2010)


Evidence for locus heterogeneity. Approximately 30% of individuals diagnosed with FHL do not have identified pathogenic variants in any of the four genes listed (see Table 1). Ménasché et al [2005] provide evidence that additional genetic loci may be responsible for FHL.

Table 2.

Molecular Genetic Testing Used in Familial Hemophagocytic Lymphohistiocytosis

Locus Name / Gene 1% of FHL Attributed to Pathogenic Variants in This GeneTest MethodVariants Detected 2
FHL2 /
20%-30% 3
>50% 4
Sequence analysis 5 / scanning for pathogenic variants 6Sequence variants 7
FHL3 /
20%-30% 3Sequence analysis 5 / scanning for pathogenic variants 6Sequence variants 7, 8
UnknownTargeted analysis for pathogenic variants 9253-kb inversion 9
FHL4 /
~20% in Turks/Kurds 10
1% in North Americans 11
5% in Central Europeans 10
Sequence analysis 5 / scanning for pathogenic variants 6Sequence variants
UnknownDeletion/duplication analysis 12Exon- & whole-gene deletion 13
FHL5 /
~20% in North Americans 14Sequence analysis 5Sequence variants 7

See Molecular Genetics for information on allelic variants.


Pathogenic variants in both PRF1 and UNC13D have been identified with almost equal frequency (~20%-30%) in individuals of western European and Japanese descent [Göransdotter Ericson et al 2001, Molleran Lee et al 2004, Ishii et al 2005, Lee et al 2006].


For individuals of African American descent [Molleran Lee et al 2004]


Sequence analysis detects variants that are benign, likely benign, of uncertain significance, likely pathogenic, or pathogenic. Pathogenic variants may include small intragenic deletions/insertions and missense, nonsense, and splice site variants; typically, partial-, whole-, or multigene deletions/duplications are not detected. For issues to consider in interpretation of sequence analysis results, click here.


Sequence analysis and scanning of the entire gene for pathogenic variants can have similar variant detection frequencies; however, variant detection rates for scanning may vary considerably between laboratories depending on the specific protocol used.


Single pathogenic variants have been identified in PRF1 [Johnson, unpublished data (2012)], UNC13D [zur Stadt et al 2005; Qian, personal communication (2012)], and STXBP2 [Johnson, unpublished (2012)] in a minority of individuals with HLH. These data suggest that a second pathogenic variant in (a) the other allele not detectable by direct sequencing, or in (b) a different HLH-related gene may be responsible for a minority of cases. Based on family studies, there is no evidence of a dominant model of disease in FHL2, FHL3, FHL4, or FHL5.


A deep intronic pathogenic variant has been reported; this must be included in design of sequence analysis approach (see Molecular Genetics) [Meeths et al 2011].


Targeted PCR specific for the breakpoints of the 253-kb inversion (see Molecular Genetics) [Meeths et al 2011]


Testing that identifies deletions/duplications not readily detectable by sequence analysis of the coding and flanking intronic regions of genomic DNA; methods used may include: quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), and chromosomal microarray (CMA) that includes this gene/chromosome segment.


A whole-gene deletion in STX11 has been reported [zur Stadt et al 2005].


Interpretation of test results

  • Some individuals with FHL have only one identified pathogenic variant; it is possible that the second allele is mutated in the promoter region, deeper in the introns in other non-coding regions of the gene, or is a large complex rearrangement and is thus undetectable by methods in current use. Alternately, a second pathogenic variant could be present in a different HLH-related gene for which clinical testing is not currently possible.
  • Approximately 30% of North American individuals with FHL do not have pathogenic variants in PRF1, UNC13D, STX11 or STXBP2 [Johnson, unpublished data (2012)]; therefore, normal test results do not exclude the diagnosis of FHL.

Testing Strategy

To confirm/establish the diagnosis in a proband. The recommended genetic workup of a person with HLH is complex and depends on the individual's race/ethnicity, immunologic test results, and clinical presentation, among other factors (see Genotype/Phenotype Correlations). A testing algorithm is available to assist clinicians in the prioritization of testing [Jordan et al 2011].

No unique clinical findings distinguish between FHL2, FHL3, FHL4, and FHL5. However, immunologic testing may be useful in directing the genetic workup of a person with FLH (see Genotype/Phenotype Correlations and testing algorithm).

  • PRF1 sequence analysis / scanning for pathogenic variants is generally performed first, as more than 80% of African Americans and 20% of North Americans of northern European background with FHL have at least one PRF1 pathogenic variant [Johnson, unpublished data (2012)].
    Sequence analysis typically does not detect gross deletions (exon, multiexon, or whole-gene deletions), insertions, or some complex gene rearrangements. However, no gross deletions, insertions, or complex gene rearrangements have been identified in PRF1 to date.
  • In persons of northern European background, UNC13D molecular genetic testing is generally performed next. UNC13D pathogenic variants are rare in African Americans. Sequencing of the entire coding region and exon/intron boundaries of UND13D including targeted analyses of the deep intronic splicing variant and the large inversion should be performed.
    Approximately 1%-2% of North Americans of northern European background have the deep intronic splicing variant or the large inversion [Qian, personal communication (2012)].
  • STXBP2 pathogenic variants account for about 20% of pathogenic variants in North American persons with FHL and have been identified in all ethnic/racial groups tested [Johnson, unpublished data (2012)]. Therefore, sequence analysis for STXBP2 is appropriate in any person with suspected FHL for whom molecular genetic testing of PRF1 and UNC13D is normal. Of note, no gross deletions, insertions, or complex rearrangements have been identified in STXBP2 to date.
  • Pathogenic variants in STX11 account for about 1% of the pathogenic variants in North American individuals and about 5% of central European individuals with HLH. Sequence analysis or scanning of STX11 for pathogenic variants should follow PRF1, UNC13D, and STXBP2 variant analyses that do not identify at least one pathogenic variant. If sequence analysis is normal, deletion/duplication analysis of STX11 can be considered, as gross deletions have been reported in STX11. Pathogenic variants in STX11 have been identified in most racial/ethnic groups [Marsh et al 2010b].

In symptomatic individuals not found to have pathogenic variants in PRF1, UNC13D, STX11, or STXBP2, sequencing of RAB27A for Griscelli syndrome type 2 (see Differential Diagnosis) should be considered.

Note: Approximately 2%-3% of North American individuals with FHL have pathogenic variants in RAB27A. These individuals may not show evidence of partial albinism or other pigmentary abnormalities typically reported with Griscelli syndrome type 2 [Johnson, unpublished data (2012)].

In males with symptoms of HLH not found to have pathogenic variants in PRF1, UNC13D, STX11, STXBP2, or RAB27A, molecular genetic testing of XIAP (also known as BIRC4) and SH2D1A should be considered (see Differential Diagnosis).

Carrier testing for at-risk relatives requires prior identification of two pathogenic variants in the family.

Note: Carriers are heterozygotes for this autosomal recessive disorder and are not at risk of developing the disorder.

Predictive testing for at-risk asymptomatic sibs of a proband requires prior identification of two pathogenic variants in the family.

Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of two pathogenic variants in the family.

Clinical Characteristics

Clinical Description

Symptoms of familial hemophagocytic lymphohistiocytosis (FHL) are usually evident within the first months or years of life [Aricò et al 1996] and may even develop in utero [Malloy et al 2004]. However, symptomatic presentation throughout childhood and even into adulthood has been observed in some cases [Aricò et al 1996, Allen et al 2001, Clementi et al 2002, Zhang et al 2011, Sieni et al 2012b].

FHL usually presents in childhood as an acute illness with prolonged fever (>7 days), cytopenias, and hepatosplenomegaly. Rash and lymphadenopathy are less frequently observed. Liver dysfunction (elevated serum aminotransferase and bilirubin), neurologic dysfunction (irritability or lethargy, hypotonia or hypertonia, seizures, ataxia, and cranial nerve involvement presenting as nerve palsy and blindness), and bone marrow hemophagocytosis complete the typical presentation of the disorder.

Oftentimes, FHL is associated with documented infection, especially herpes viral infections [Imashuku et al 2005] and leishmaniasis [Filipovich 2011].

In adults, FHL may present with acute onset of full-blown HLH, or as a more insidious illness with recurrent bouts of nonspecific symptoms of HLH which may resolve spontaneously or with steroid treatment alone. HLH in adults is often erroneously attributed to an infectious etiology without a full genetic workup being undertaken.

Common signs and symptoms

  • Fever is frequently indolent and protracted but may decline spontaneously. In a few children, it may develop late in the course of the disease.
  • Splenomegaly and hepatomegaly are usually pronounced and progressive.
  • Skin rash is uncharacteristic, transient, and often associated with high fever.
  • Lymph node enlargement develops in fewer than 50% of individuals but may occasionally be significant.
  • Neurologic abnormalities may be present at initial clinical presentation or in many instances may develop later in the course of the disease. The presentation may include irritability, bulging fontanel in infants, neck stiffness, hypotonia, hypertonia, and convulsions. Cranial nerve VI and/or VII palsy, ataxia, hemiplegia, quadriplegia, blindness, and coma may occur, as well as nonspecific signs of increased intracranial pressure.
    On examination of CSF, elevated protein, increased number of mononuclear cells, and occasionally hemophagocytosis are characteristic findings.
    MRI may show gross demyelination or failure of normal myelination during the first year of life. MRI may also show multifocal inflammation involving gray and white matter. Bleeding, generalized atrophy, and brain edema may be observed. Hyperdense areas on CT may be falsely interpreted as calcifications.
  • Life expectancy. Median survival in typical FHL, without treatment, is less than two months. Progressive hemophagocytic lymphohistiocytosis (HLH) and invasive infection account for the majority of deaths in untreated children [Sung et al 2002]. Although children can survive without hematopoietic cell transplantation (HCT) [Henter et al 2002], the five-year survival of children treated with chemotherapy alone was only 10.1% in one large series. Allogeneic hematopoietic stem cell transplantation is potentially curative (see Management).
  • The course of the disease and life expectancy are not well studied in adults with FHL.

Genotype-Phenotype Correlations

Both Feldmann and Molleran Lee documented poor correlation between phenotype and genotype in FHL2 with different classes of PRF1 pathogenic variants [Feldmann et al 2002, Molleran Lee et al 2004].

In general, age of onset tends to be younger in those with nonsense versus missense PRF1 variants [Clementi et al 2002, Feldmann et al 2002, zur Stadt et al 2006, Trizzino et al 2008]. In particular, homozygosity for the PRF1 p.Leu17ArgfsTer34 pathogenic variant, commonly seen in affected individuals of African descent, is associated with an earlier onset (median age 3 months; range 1.5-10 months) compared to other PRF1 pathogenic missense variants [Lee et al 2006]. However, affected individuals within the same family may develop evidence of disease at different ages [Allen et al 2001].

In a study of 35 Japanese individuals with FHL, Ishii et al [2005] concluded that persons with pathogenic variants of PRF1 had earlier disease onset than those with pathogenic variants of UNC13D or FHL for whom no genetic basis was established.

Deficient NK cell activity persisted after chemotherapy in all persons with FHL2, whereas some individuals with FHL3 or the non-FHL2/FHL3 subtype showed partial recovery of NK cell activity during remission.

Alloantigen-specific CTL-mediated cytotoxicity was deficient in individuals with FHL2 with PRF1 pathogenic nonsense variants and very low in individuals with FHL3, but was only moderately reduced in individuals with FHL2 with PRF1 pathogenic missense variants [Ishii et al 2005].

Ueda et al [2006] found that the incidence of deficient NK cell activity was higher in those with PRF1 pathogenic variants than in those with UNC13D or uncharacterized pathogenic variants. In addition, pathogenic nonsense variants in PRF1 were associated with significantly higher levels of ferritin and soluble Il2Ra than observed in other subtypes of FHL.

Horne et al [2008] concluded that persons with pathogenic variants of PRF1 had earlier onset of symptoms than those with pathogenic variants of STX11, and that the latter had a somewhat milder disease course than those with PRF1 variants, UNC13D variants, or uncharacterized pathogenic variants. Rudd et al [2006] reported long remissions without specific treatments in some individuals with STX11 pathogenic variants. They also reported a higher than expected incidence of psychomotor retardation and myelodysplastic syndrome or acute myelogenous leukemia in their small group with STX11 pathogenic variants. Marsh et al [2010b] noted that pathogenic missense variants in STX11 were associated with later onset and preserved NK cell function compared to nonsense variants in STX11, which resulted in abrogation of NK cell function.

In general, the age of onset of disease tends to be later in individuals with pathogenic variants of STXBP2 [Meeths et al 2010b] than it is in individuals with pathogenic variants of PRF1 or UNC13D; however, age of onset is variable, even within the same family. Later age at onset in persons with the common c.1247-1G>C variant in STXBP2 (both in the homozygous and compound heterozygous states) in comparison to individuals with various missense variants has been reported [zur Stadt et al 2009, Pagel et al 2012]. In contrast, the authors have observed onset of HLH in infancy in more than half of individuals with the c.1247-1G>C variant, as well as considerable discordance in phenotype and age at onset between sibs with the same genotype [Johnson, unpublished data (2012)]. Gastrointestinal disorders, bleeding diathesis, and hypogammaglobulinemia, with onset often prior to symptoms of HLH, has been reported in a subset of individuals with pathogenic variants of STBXP2 [Meeths et al 2010b].

Adult onset of disease in individuals with FHL-causing pathogenic variant(s) has been well described [Allen et al 2001, Clementi et al 2002, Ueda et al 2006]. Missense and splice site variants in PRF1, UNC13D and STXBP2 were recently reported in 14% of adults with HLH [Zhang et al 2011]. Similarly, Sieni et al [2012b] identified 11 adults with FHL secondary to pathogenic variants in PRF1, UNC13D, STXBP2, and SH2D1A. In both studies, the p.Ala91Var variant in PRF1 was the most commonly identified variant. While p.Ala91Var is not by itself pathogenic, it does have an effect on cytotoxic function [Voskoboinik et al 2005] and may function as a genetic modifier of disease in some affected individuals.


Classification. The widely used modern classification of histiocytic disorders was proposed by the Histiocyte Society in 1987 and refined in 1998.

The Writing Group of the Histiocyte Society recommended a division of the histiocytic disorders into three classes:

  • Dendritic cell-related disorders, of which Langerhan's cell histiocytosis (LCH) is by far the most common
  • Macrophage-related disorders (including hemophagocytic lymphohistiocytosis [HLH])
  • Malignant disorders


  • FHL was once called familial erythrophagocytic lymphohistiocytosis (FEL).
  • FHL occurring in conjunction with identified viral infections was termed VAHS (virus-associated hemophagocytic syndrome); however, it is now recognized that FHL may be triggered by viral infection.
  • The general term hemophagocytic lymphohistiocytosis (HLH) encompasses all forms of the disorder.


The estimated prevalence of FHL is 1:50,000 births [Henter et al 1998] with equal gender distribution.

A common PRF1 pathogenic variant, p.Leu17ArgfsTer34, has been observed at a high frequency in individuals with FHL in the African American population [Lee et al 2006].

Differential Diagnosis

Secondary (acquired or reactive) hemophagocytic lymphohistiocytosis (HLH) is difficult to distinguish from familial (primary) HLH by clinical or histologic findings alone. Given the rapid advances in genetic diagnosis of FHL, molecular genetic testing is recommended even in HLH suspected to be acquired.

The diagnosis of secondary HLH is usually made in association with infection by viruses, bacteria, fungi, or parasites or in association with lymphoma, autoimmune disease, or metabolic disease [Imashuku et al 2000, Imashuku et al 2005]. Acquired HLH may have decreased, normal, or increased NK cell activity.

Secondary HLH appears to be self-limited because some affected individuals are able to fully recover having received supportive medical treatment (i.e., IV immunoglobulin) alone. However, long-term remission without the use of cytotoxic and immune-suppressive therapies is highly unlikely in adults with HLH and in individuals with CNS involvement [Imashuku et al 1999].

Secondary HLH is usually associated with:

  • Infection, particularly involving the herpes virus group, usually in older children and adolescents [Fisman 2000]. An example is EBV (Epstein-Barr virus)-associated HLH, which is more common in Asians than in whites or Africans [Ma et al 2001]. Also in one case, compound heterozygosity for two pathogenic missense variants in PRF1 was associated with chronic active EBV infection with hemophagocytic lymphohistiocytosis [Katano et al 2004].
  • "Macrophage activation syndrome" (MAS), the most serious and life-threatening complication of systemic-onset juvenile idiopathic arthritis (sJIA). MAS was first described by Hadchouel et al [1985] as a hemorrhagic syndrome associated with mental status changes, hepatosplenomegaly, increased serum concentrations of liver enzymes, and a sharp fall in blood counts. The term MAS was coined by Stéphan et al [1993] in reference to the bone marrow histologic findings of numerous, well-differentiated macrophages (or histiocytes) actively phagocytosing hematopoietic elements. Such cells may infiltrate many organs in MAS. MAS responds to therapies similar to those typically effective in HLH. The amount of perforin expressed in NK cells from individuals with severe sJRA is less than in normal controls [Normand et al 2000], suggesting a mechanism of immune susceptibility to hemophagocytic lymphohistiocytosis (HLH) similar to FHL2. More recent studies also show that natural killer cell dysfunction is a distinguishing feature of sJIA and MAS [Villanueva et al 2005].
  • Autoimmune diseases, such as rheumatologic disorders, treated with immune suppressive agents [Janka et al 1998]. Secondary HLH is most commonly associated with sJIA, but has also been observed in systemic lupus erythematosus [Muiesan et al 1998].
  • Inborn errors of metabolism including biotinidase deficiency [Kardas et al 2012], lysinuric protein intolerance [Duval et al 1999], galactosemia [Marcoux et al 2005], multiple sulfatase deficiency, Gaucher disease, Pearson syndrome, galactosialidosis, methylmalonic acidemia, and propionic acidemia [Gokce et al 2012] have all been reported in association with secondary hemophagocytic lymphohistiocytosis in some individuals. It is not known how these metabolic disorders lead to HLH.

Inherited immune disorders can be associated with highly lethal hemophagocytic syndromes, sometimes triggered by exposure to EBV or other viruses. These include the following:

X-linked lymphoproliferative disease (XLP)

  • XLP1, caused by SAP deficiency, results from hemizygous mutation of SH2D1A. The three main phenotypes of this type of XLP are an inappropriate immune response to Epstein-Barr virus (EBV) infection resulting in unusually severe and often fatal infectious mononucleosis, dysgammaglobulinemia, and/or lymphoproliferative disorders typically of B cell origin. Clinical manifestations of XLP1 vary, even among affected family members. The most common presentation is a near-fatal or fatal EBV infection associated with an unregulated and exaggerated immune response with widespread proliferation of cytotoxic T cells, EBV-infected B cells, and macrophages. Mortality is greater than 90%. In about one third of males with XLP1, hypogammaglobulinemia of one or more immunoglobulin subclasses is diagnosed prior to EBV infection or in rare survivors of EBV infection. The prognosis for males with this phenotype is more favorable if they are managed with regular IV IgG. Lymphomas or other lymphoproliferative disease occur in about one third of males with XLP1, some of whom have hypogammaglobulinemia or have survived an initial EBV infection. The lymphomas seen in XLP1 are typically high-grade B cell lymphomas, non-Hodgkin type, often extranodal, and particularly involving the intestine. Allogeneic HSCT is the only curative therapy for XLP1 and should be undertaken in confirmed cases of XLP1 as early in life as feasible. Average life expectancy without curative bone marrow transplantation has been estimated as less than ten years. Inheritance is X-linked recessive.
  • XLP2, caused by XIAP deficiency and resulting from mutation of XIAP (aka BIRC4), is located (as the name implies) on the X chromosome [Rigaud et al 2006]. The most common manifestation of XIAP deficiency is the development of HLH, which is often recurrent. Because of this, it has recently been suggested that XIAP deficiency may be best classified as a cause of X-linked HLH [Marsh et al 2010a].
    Hypogammaglobulinemia and colitis have also been described in individuals with XIAP deficiency. In contrast to XLP1, no cases of lymphoma have been observed in individuals with XIAP deficiency [Pachlopnik Schmid et al 2011]. Patient management includes treatment of HLH when it occurs, immunoglobulin replacement if needed, and consideration of allogeneic bone marrow transplant.

Chediak-Higashi syndrome is a complex syndrome characterized by partial albinism, abnormal platelet function, and severe immunodeficiency. This disorder is caused by mutation of CHS1, which encodes a protein involved in intracellular vesicle formation, resulting in a failure to fuse lysosomes properly with phagosomes. Chediak-Higashi syndrome can be differentiated from FHL based on the presence of huge secretory lysosomes in the neutrophils and lymphocytes and giant melanosomes on skin biopsy. Inheritance is autosomal recessive.

Griscelli syndrome type 2 (GS2) is a disorder of cytotoxic T lymphocytes caused by mutation of a small GTPase, RAB27A, which controls the movement of vesicles within cells [Ménasché et al 2000]. GS2 is usually associated with neurologic abnormalities in addition to partial albinism with fair skin and silvery-grey hair and has an extremely high incidence of HLH. A recent Swedish study identified RAB27A pathogenic variants in 5% of individuals with primary HLH and without a known diagnosis of GS2 [Meeths et al 2010a]. GS2 and FHL3 may share a common pathway: Neeft et al [2005] showed that the protein encoded by UNC13D is a direct partner of the protein RAB27A, and that the RAB27A/UNC13D complex is essential in the regulation of secretory granule fusion with the plasma membrane in hematopoietic cells. Inheritance is autosomal recessive.


Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with hemophagocytic lymphohistiocytosis (HLH), as well as to determine the appropriate therapy, the following evaluations are recommended:

  • Physical examination to evaluate for rashes, lymphadenopathy, hepatosplenomegaly, and neurologic dysfunction
  • Evaluation of blood and bone marrow compartments (CBC and BM biopsy)
  • Determination of the extent of liver involvement by measuring serum concentration of transaminases, bilirubin, triglycerides, sodium, and lactate dehydrogenase
  • Identification of potential infectious co-factors, especially viral infection or reactivation, which would require specific treatment
  • Establishing the presence or extent of CNS involvement by evaluating the CSF and performing neuroimaging and neuropsychological assessment
  • Testing of NK cell activity, intracellular perforin and granzyme B expression, and CD107a mobilization if these tests are available
  • Evaluation of inflammatory factors such as serum concentrations of ferritin, sIL2Rα, and other cytokines
  • Evaluation and monitoring of PT, PTT, and fibrinogen
  • Clinical genetics consultation
  • Molecular genetic testing, if not performed previously, which may help to determine if an affected individual is a candidate for bone marrow transplantation:
    • Genetic testing for PRF1, UNC13D, STX11, and STXBP2 pathogenic variants
    • Consideration of testing for RAB27A pathogenic variants in any individual with HLH, even with no obvious evidence of depigmentation
    • Consideration of testing for XIAP (formerly BIRC4) and SH2D1A pathogenic variants in any male with HLH
    • Collection of materials for future characterization of underlying genetic defects, most significantly in individuals with terminal disease

Treatment of Manifestations

For a detailed explanation of treatment for HLH, see Jordan et al [2011] (full text).

HLH-1994. To improve survival, in 1994 the Histiocyte Society initiated a prospective international collaborative therapeutic study. It combined chemotherapy and immunotherapy (etoposide, corticosteroids, cyclosporine A, and intrathecal methotrexate for individuals with CNS diseases), followed by HSCT in persistent, recurring, and/or familial disease. Although HLH-94 was primarily designed for the treatment of FHL, it was also open to all individuals with non-familial HLH.

HLH-2004. The HLH-2004 protocol was opened on January 1, 2004, and was designed for individuals with FHL and non-familial HLH. This protocol was based on the HLH-94 protocol with minor therapeutic modifications such as initiation of cyclosporine from onset of induction therapy.

The two protocols developed by the Histiocyte Society are available from the Histiocyte Society website.

Given the poor prognosis of individuals with HLH without prompt and effective treatment, it is recommended that treatment be initiated when clinical suspicion is high, even if all recommended studies are incomplete. In general, treatment involves the following:

  • Chemotherapy and immunotherapy to achieve a clinically stable resolution prior to hematopoietic cell transplantation (HCT)
  • Antibiotics or antiviral agents to treat or prevent infections that may have triggered the exaggerated inflammatory response
  • Allogeneic HCT, the only curative therapy, which should be undertaken in children with confirmed FHL as early in life as feasible:
    • Presymptomatically if status is confirmed by family history of clinical HLH, or
    • After achievement of clinical remission in simplex cases (i.e., a single occurrence in a family)
  • Use of HCT has improved survival significantly [Henter et al 2002]. Initially, three-year-survival in children who received HLH-94 therapy was approximately 64% [Horne et al 2005a]. More recently, reduced intensity regimens prior to HCT have diminished the early transplant mortality and increased three-year-survival rates to 92% [Marsh et al 2010c]. Long-term follow up reveals that after HCT most children treated early in the disease course return to normal or near-normal quality of life. Brain atrophy secondary to steroid therapy often identified on neuroimaging studies during the acute phase of HLH gradually resolves after HCT [Shuper et al 1998]. However, children who experienced serious CNS involvement may have irreversible neurologic problems and learning disabilities despite adequate disease control following HCT.

Prevention of Primary Manifestations

Allogeneic HCT is the only curative therapy and should be undertaken in children with confirmed familial HLH as early in life as possible.

Prevention of Secondary Complications

Prompt treatment of HLH with infection prophylaxis is indicated in immunocompromised individuals.


The following are recommended annually after HCT:

  • Neuropsychological evaluation
  • Regular follow-up by transplant specialists to monitor for late complications relating to growth and hormone function

Agents/Circumstances to Avoid

The following should be avoided:

  • Live vaccinations
  • Exposure to infections

Evaluation of Relatives at Risk

Molecular genetic testing of at-risk sibs for the family-specific pathogenic variants is appropriate to identify those who are affected before symptoms occur for the purpose of early medical management and consideration of presymptomatic bone marrow transplantation.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Pregnancy Management

To date, pregnancy in a woman with primary FHL has not been described. Secondary hemophagocytic lymphohistiocytosis is a rare complication of pregnancy and is beyond the scope of this review. Primary hemophagocytic lymphohistiocytosis in the fetus is a (reportedly) rare but recognized entity which is typically characterized by nonimmune fetal hydrops [Malloy et al 2004, Nitta et al 2007, Woods et al 2009], preterm delivery [Woods et al 2009], and/or fetal demise. In utero chemotherapy followed by post-natal HCT has been described [Shah et al 2009].

Therapies Under Investigation

Search in the US and in Europe for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.


The treatment of FHL is also the most effective treatment for secondary HLH and the hemophagocytic syndrome associated with other inherited immunodeficiencies.

Genetic Counseling

Genetic counseling is the process of providing individuals and families with information on the nature, inheritance, and implications of genetic disorders to help them make informed medical and personal decisions. The following section deals with genetic risk assessment and the use of family history and genetic testing to clarify genetic status for family members. This section is not meant to address all personal, cultural, or ethical issues that individuals may face or to substitute for consultation with a genetics professional. —ED.

Mode of Inheritance

Familial hemophagocytic lymphohistiocytosis (FHL) is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

  • The parents of an affected child are obligate heterozygotes and therefore carry one mutated allele.
  • Heterozygotes (carriers) are asymptomatic.

Sibs of a proband

  • Each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
  • Once an at-risk sib is known to be unaffected, the risk of his/her being a carrier is 2/3.
  • Heterozygotes (carriers) are asymptomatic.

Offspring of a proband. The offspring of an individual with FHL are obligate heterozygotes (carriers).

Other family members of a proband. Each sib of the proband's parents is at a 50% risk of being a carrier.

Carrier Detection

Carrier testing is possible if the pathogenic variants in the family are known.

Related Genetic Counseling Issues

See Management, Evaluation of Relatives at Risk for information on evaluating at-risk relatives for the purpose of early diagnosis and treatment.

Testing of at-risk asymptomatic sibs for FHL is possible using the techniques described in Molecular Genetic Testing. Such testing is not useful in accurately predicting age of onset, severity, type of symptoms, or rate of progression in asymptomatic individuals. However, expectant management is warranted for individuals with the same genotype as the symptomatic proband and HCT prior to onset of symptoms may improve outcome.

Family planning

  • The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy.
  • It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected, are carriers, or are at risk of being carriers.

DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, allelic variants, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals.

Prenatal Testing and Preimplantation Genetic Diagnosis

Once the pathogenic variants have been identified in an affected family member, prenatal diagnosis for a pregnancy at increased risk and preimplantation genetic diagnosis are possible.


GeneReviews staff has selected the following disease-specific and/or umbrella support organizations and/or registries for the benefit of individuals with this disorder and their families. GeneReviews is not responsible for the information provided by other organizations. For information on selection criteria, click here.

  • Histiocytosis Association
    Phone: 856-589-6606
    Fax: 856-589-6614
  • European Society for Immunodeficiencies (ESID) Registry
    Dr. Gerhard Kindle
    University Medical Center Freiburg Centre of Chronic Immunodeficiency
    Engesserstr. 4
    79106 Freiburg
    Phone: 49-761-270-34450

Molecular Genetics

Information in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.

Table B.

OMIM Entries for Hemophagocytic Lymphohistiocytosis, Familial (View All in OMIM)

170280PERFORIN 1; PRF1
605014SYNTAXIN 11; STX11


Gene structure. PRF1 has three exons; exons 2 and 3 encode a 555-amino acid polypeptide. Genomic DNA of PRF1 spans about 5 kb. For a detailed summary of gene and protein information, see Table A, Gene.

Benign variants. To date, several benign variants have been observed (see Table 3). These have been evaluated by biochemical and functional testing in vitro and are highly unlikely to have pathogenic effects on perforin expression.

Pathogenic variants. To date, more than 115 pathogenic variants have been identified. They have been found throughout the entire coding region. Pathogenic variants in PRF1 include nonsense and missense variants, small deletions, and small insertions [Voskoboinik et al 2005]. To date, no gross deletions, splice site variants, or complex rearrangements have been reported.

The variant c.272C>T has been reported as a pathogenic variant [Clementi et al 2001]. This sequence variant is identified in about 3% of the North American general population [Molleran Lee et al 2004] and in a much higher proportion of patients with FHL [Busiello et al 2006, Zhang et al 2007]. Earlier studies have shown that the sequence change c.272C>T results in reduced levels of perforin expression [Voskoboinik et al 2005] and that p.Ala91Val perforin has reduced cytotoxicity in CTL and NK cells [Trambas et al 2005]. In addition, Voskoboinik showed that the c.272C>T (p.Ala91Val) substitution in perforin not only causes reduced steady state levels of expression in effector cells, but also results in a reduced intrinsic capacity for lysis and even had some dominant-negative effect on the wild type perforin [Voskoboinik et al 2007]. Even though c.272C>T by itself is not disease causing, these cumulative data suggest that the sequence variant may be an important genetic susceptibility factor and may play a role in the disease process.

Table 3.

Selected PRF1 Variants

Variant ClassificationDNA Nucleotide ChangePredicted Protein Change 1
(Alias 2)
Reference Sequences
Pathogenicc.272C>T 3p.Ala91Val 3

Note on variant classification: Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.

Note on nomenclature: GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen​ See Quick Reference for an explanation of nomenclature.


p.(=) designates that protein has not been analyzed, but no change is expected


Variant designation that does not conform to current nomenclature


Not disease-causing, but proposed as a genetic susceptibility factor or disease modifier (see PRF1, Pathogenic variants)

Normal gene product. PRF1 encodes the cytolytic effector perforin, which is expressed by cytolytic T lymphocytes and NK cells [Stepp et al 1999]. Perforin expression is higher in resting than in activated cells. Perforin induces apoptotic cell death in response to granzymes (serine esterases that represent most of the granule content of T cytotoxic cells) independent from the FAS-mediated apoptotic machinery.

Abnormal gene product. PRF1 pathogenic variants affect cellular cytotoxicity, resulting in impaired antiviral defense and dysregulation of apoptotic mechanisms involved in the regulation of immune responses to inappropriate proliferation of cells, such as T cells and macrophages. Although the mechanisms of regulating perforin expression are incompletely understood, it is known that methylation of a promoter-specific region does affect the level of perforin expression.

UNC13D (MUNC13-4)

Gene structure. UNC13D consists of 32 exons ranging from 36 to 235 bp in size. For a detailed summary of gene and protein information, see Table A, Gene.

Benign variants. A dozen benign variants, which are highly unlikely to have pathogenic effects on UNC13D protein expression, have been observed.

Pathogenic variants. To date, more than 100 pathogenic variants have been identified throughout the entire coding region and the exon/intron boundaries of UNC13D. Pathogenic variants that have been identified include missense, nonsense, and splice site variants and small deletions and/or insertions. One deep intronic variant (c.118-308C>T) and one large gene inversion have been detected [Meeths et al 2011]. A large 253-kb inversion straddles the UNC13D 3’ end and adjacent sequences; the breakpoints have been mapped and it is detectable by targeted analysis for pathogenic variants (Table 2). The deep intronic variant lies in a region typically not sequenced; detection will require specific primers.

Normal gene product. The protein encoded by UNC13D has 1090 amino acids and is a member of the Munc13 protein family, which is an essential effector of the cytolytic secretory pathway. UNC13D differs from the other Munc13 proteins, as it is not expressed in the brain but rather is highly expressed in hematopoietic tissues including T- and B-lymphocytes and monocytes and in non-hematopoietic tissues, such as the lung and the placenta.

UNC13D is involved in vesicle-plasma membrane fusion during exocytosis of perforin- and granzyme-containing granules by cytotoxic T cells and NK cells [Feldmann et al 2002, Ménager et al 2007], which follows granule docking and precedes plasma granule membrane fusion [Ménasché et al 2005].

UNC13D is a direct partner of RAB27A. The two proteins are highly expressed in cytotoxic T-cells (CTLs) and mast cells, where they colocalize on secretory lysosomes. Together, they assist priming the vesicles for exocytosis [Ménager & de Saint Basile 2007]. The region comprising the Munc13 homology domains is essential for the localization of UNC13D to secretory lysosomes [Neeft et al 2005].

Abnormal gene product. Pathogenic variants in UNC13D prevent interaction with RAB27A and abolish cytolytic secretion [Neeft et al 2005]. Individuals with UNC13D pathogenic variants have impaired lymphocyte cytotoxic function with normal or increased perforin expression. Interestingly, the c.118-308C>T pathogenic variant impairs transcription in lymphocytes, but not some other cell types [Meeths et al 2011].

Table 4.

Selected UNC13D Pathogenic Variants

DNA Nucleotide ChangePredicted Protein ChangeReference Sequences
(253-kb inversion)NA

Note on variant classification: Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.

Note on nomenclature: GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen​ See Quick Reference for an explanation of nomenclature.

NA = not applicable


Gene structure. STX11 consists of two exons. Exon 2 encodes 287 amino acid residues. Genomic DNA of STX11 spans about 37 kb. For a detailed summary of gene and protein information, see Table A, Gene.

Benign variants. To date, three benign variants have been identified [Zhang, unpublished (2010)]; they are highly unlikely to have pathogenic effects on STX11 protein expression. See Table 5.

Pathogenic variants. A total of nine distinct pathogenic variants have been reported, including in three families in highly consanguineous Turkish/Kurdish FHL4 kindreds. STX11 pathogenic variants have been found in individuals with FHL from other ethnic backgrounds, albeit at a low frequency [Marsh et al 2010b]. See Table 5. Pathogenic nonsense and missense STX11 variants have been reported in most affected individuals to date. A whole-gene deletion in STX11 has been reported [zur Stadt et al 2005].

Table 5.

Selected STX11 Variants

DNA Nucleotide ChangePredicted Protein ChangeReference
Pathogenicc.[369_370delAG; 374_379delCGC]
(5-bp deletion)
(19.2-kb deletion)

Note on variant classification: Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.

Note on nomenclature: GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen​ See Quick Reference for an explanation of nomenclature.

Normal gene product. The gene product of STX11 is known for its function in vesicle transport. It is a member of soluble N-ethylmaleimide sensitive factor attachment protein receptors present on target membranes (t-SNAREs) [zur Stadt et al 2005]. STX11 is expressed in peripheral mononuclear cells, as well as in unstimulated NK cells and CD8+ T cells [Bryceson et al 2007].

Abnormal gene product. The absence of the STX11 protein has been observed in monocytes and lymphocytes in persons with FHL4. The abrogation of degranulation and cytotoxicity by resting PBLs observed in this disorder are believed to be caused by the defects in STX11 [Arneson et al 2007].


Gene structure. STXBP2 is located on chromosome 19p13.3-p13.2 and its genomic DNA spans about 11 kb. STXBP2 comprises 19 exons. For a detailed summary of gene and protein information, see Table A, Gene.

Benign variants. To date, more than two dozen benign variants have been identified [Zhang, unpublished (2012)]; they are highly unlikely to have pathogenic effects on STXBP2 protein expression. For example, the benign variant p.Ala433Val did not affect binding between syntaxin-11 and UNC13B [zur Stadt et al 2009].

Pathogenic variants. To date, 27 pathogenic variants have been identified throughout the entire coding region and the exon/intron boundaries of STXBP2. Identified variants include pathogenic missense, nonsense, and splice site variants as well as small deletions and/or insertions. Gross deletions, insertions, and other complex variants have not been observed to date [Zhang, unpublished (2012)].

Table 6.

Selected STXBP2 Variants

Variant ClassificationDNA Nucleotide ChangePredicted Protein ChangeReference Sequences

Note on variant classification: Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.

Note on nomenclature: GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen​ See Quick Reference for an explanation of nomenclature.

NA = not applicable

Normal gene product. STXBP2 encodes syntaxin-11, a polypeptide of 593 amino acids and a member of the STXBP/Munc-18/Sec1 family. Syntaxin-11, formerly known as UNC18B, is expressed predominantly as a 2.4-kb message in placenta, lung, liver, kidney, and pancreas, as well as in peripheral blood lymphocytes [Ziegler et al 1996]. It interacts with soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) syntaxins, and regulates intracellular vesicle trafficking [Côte et al 2009]. Like other Sec/Munc18 family members, STXBP2 has three domains and forms an arch-shaped structure. The central cavity, formed by domains 1 and 3a, provide the binding surface for syntaxin-11. In addition, STXBP2 and STX11 share binding sites and are colocalized in CD8+ T and NK cells. Stimulation with IL2 can further enhance colocalization in CD8+ and NK cells [zur Stadt et al 2009].

Abnormal gene product. zur Stadt et al [2009] showed that missense variants in STXBP2 can lead to a complete loss of the ability to bind to syntaxin-11. Cultured NK cells from patients with pathogenic variants in STXBP2 exhibit impaired cytotoxic granule exocytosis with decreased or absent CD107 expression. Ex vivo cytotoxicity in NK and cytotoxic T-cell were both decreased [Meeths et al 2010b]. However, degranulation and cytotoxicity could be at least partially corrected by in vitro stimulation with IL2 [zur Stadt et al 2009]. Defects in STXBP2 can reduce or completely abolish syntaxin-11 expression. In contrast, STXBP2 expression does not require syntaxin-11 [Côte et al 2009].


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Chapter Notes

Revision History

  • 17 January 2013 (me) Comprehensive update posted live
  • 7 September 2010 (cd) Revision: sequence analysis of STXBP2 for familial hemophagocytic lymphohistiocytosis 5 available clinically
  • 11 March 2010 (me) Comprehensive update posted live
  • 9 March 2007 (cd) Revision: clinical testing and prenatal diagnosis available for FHL4
  • 22 March 2006 (me) Review posted live
  • 21 April 2005 (jj, af, rw) Original Submission
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