Clinical characteristics.

Mucolipidosis III gamma (ML IIIγ) is a slowly progressive inborn error of metabolism mainly affecting skeletal, joint, and connective tissues. Clinical onset is in early childhood; the progressive course results in severe functional impairment and significant morbidity from chronic pain. Cardiorespiratory complications (restrictive lung disease from thoracic involvement and thickening and insufficiency of the mitral and aortic valves) are rarely clinically significant. Motor milestones can be affected due to joint disease. A few (probably <5%) affected individuals display mild cognitive impairment.

Diagnosis/testing.

The diagnosis of ML IIIγ is established in a proband with suggestive clinical and radiographic findings and biallelic pathogenic variants in GNPTG identified by molecular genetic testing.

Management.

Treatment of manifestations: No measures are known to be effective in treating the progressive limitation of motion in large and small joints. Physical and occupational therapy adapted to individual's needs; low-impact aqua therapy is usually well tolerated. Braces (especially of the hands) at night may improve daily functions; carpal tunnel and, rarely, tarsal tunnel syndrome may require surgical release procedures for temporary relief; treatment of spine deformities per specialists. Anesthesia requires careful multidisciplinary planning due to multisystem involvement (atlantoaxial instability, cardiac, airway, and respiratory). In older adolescents and adults, joint replacement has been successful in relieving hip pain and knee pain. Later in the disease course, when bone pain of variable intensity may become frequent, management focuses on pain relief. Bisphosphonate treatment in individuals with significant skeletal disease and markedly decreased bone mineral densitometry can be considered. Treatment of restricted lung disease per pulmonologist. When significant cardiac valvular dysfunction disrupts ventricular function, valve replacement needs to be considered. Developmental and educational support as needed. Addressing the social and emotional needs of affected individuals and their families is recommended.

Surveillance: Annual assessment (unless more frequent pain, cardiac, and/or respiratory monitoring is warranted) of musculoskeletal manifestations, gross motor and fine motor function, pain level, growth, dental, vision, respiratory and cardiac function, development, educational needs, psychological issues, and utilization of community resources. Frequency of DXA scans depends on age and results of prior studies. Pulmonary function studies every five years.

Agents/circumstances to avoid: Vigorous stretching exercises because they are ineffective, painful, and may damage the surrounding joint capsule and adjacent tendons.

Genetic counseling.

ML IIIγ is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for a GNPTG pathogenic variant, each sib of an affected individual has at conception 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 the GNPTG pathogenic variants have been identified in an affected family member, molecular genetic carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible.

Suggestive Findings

ML IIIγ should be suspected in individuals with the following clinical and radiographic findings [Raas-Rothschild et al 2004, Tüysüz et al 2018, Nampoothiri et al 2019] and family history.

Clinical findings

• Slowly progressive joint stiffness of the fingers, shoulders, and hips with progressive osteoarthritis
• Genu valgum
• Growth rate deceleration resulting in short stature
• Early-onset carpal tunnel syndrome
• Spinal deformities including scoliosis and hyperlordosis
• Gradual mild coarsening of facial features
• Corneal clouding
• Cardiac valve disease
• Absence of organomegaly

Radiographic findings. In early childhood, skeletal radiographs reveal mild-to-moderate dysostosis multiplex:

• Pelvis and hips. Hypoplastic iliac bones with flared iliac wings, shallow and irregular acetabula, and moderate-to-severe dysplasia of the proximal femoral epiphyses – giving rise to coxa valga – are the most striking radiologic abnormalities.
• Hands and feet. Diaphyses of metacarpals and phalanges are mildly shortened with "bullet-shaped" distal end of phalanges; carpal bones may be smaller than normal and with osteoporotic changes.
• Ribs. Widening especially in the lateral and anterior costochondral junctions
• Spine. Generalized platyspondyly; irregularity of the anterior upper and lower vertebral end plates; wedge-shaped and small ovoid vertebral bodies

In late childhood or adolescence, the changes on skeletal radiographs worsen with the development of generalized osteopenia.

Family history is consistent with autosomal recessive inheritance (e.g., affected sibs and/or parental consanguinity). Absence of a known family history does not preclude the diagnosis.

Establishing the Diagnosis

The diagnosis of ML IIIγ is established in a proband with suggestive clinical and radiographic findings and biallelic pathogenic (or likely pathogenic) variants in GNPTG identified by molecular genetic testing (see Table 1).

Note: (1) Per American College of Medical Genetics and Genomics / Association for Molecular Pathology variant interpretation guidelines, the terms "pathogenic variant" and "likely pathogenic variant" are synonymous in a clinical setting, meaning that both are considered diagnostic and can be used for clinical decision making [Richards et al 2015]. Reference to "pathogenic variants" in this GeneReview is understood to include likely pathogenic variants. (2) Identification of biallelic GNPTG variants of uncertain significance (or of one known GNPTG pathogenic variant and one GNPTG variant of uncertain significance) does not establish or rule out the diagnosis.

Molecular genetic testing approaches can include a combination of gene-targeted testing (multigene panel) and comprehensive genomic testing (exome sequencing, genome sequencing). Gene-targeted testing requires that the clinician determine which gene(s) are likely involved (see Option 1), whereas comprehensive genomic testing does not (see Option 2).

Clinical Description

Mucolipidosis III gamma (ML IIIγ) is a slowly progressive inborn error of metabolism mainly affecting skeletal, joint, and connective tissues. Clinical onset is in early childhood (median age of onset is 3.0 years). The progressive joint involvement results in severe functional impairment and significant morbidity. Adults can have mild-to-moderate restrictive lung disease and cardiac valve disease. A few (probably <5%) affected individuals may display mild cognitive impairment [Nampoothiri et al 2019, Dogterom et al 2021], but the majority do not.

Onset. The initial manifestation in most affected individuals is joint stiffness in fingers as early as age 18 months [Tüysüz et al 2018].

Skeletal / soft connective tissue. A moderate-to-severe claw-like flexion deformity of the fingers worsens with time. Limited range of motion of the shoulders is common early in the disease course. Genu valgum deformity occurs in all affected individuals early in the disease.

Hip involvement usually develops during the end of adolescence in ML IIIγ (earlier in mucolipidosis III alpha/beta [ML IIIα/β]). Hip involvement progresses over years, finally resulting in destruction of the proximal femoral epiphyses. Limited hip mobility and lower-limb pain can be significant and may result in waddling gait with age and requires total hip replacement in most individuals.

Carpal tunnel syndrome develops in most affected individuals and may be clinically significant in the second and third decade [Raas-Rothschild et al 2004, Tüysüz et al 2018, Nampoothiri et al 2019].

Spinal deformities develop over time and include scoliosis and hyperlordosis. In one individual atlantoaxial instability required corrective surgery; however, this complication is probably very uncommon in individuals with ML IIIγ [Tüysüz et al 2018, Nampoothiri et al 2019, Dogterom et al 2021, Erdem et al 2025].

Short neck reported in several individuals had no clinical significance [Tüysüz et al 2018].

Chronic pain syndrome is common in individuals with ML IIIγ and significantly impairs the quality of life. It mainly involves the hips, knees, or entire legs, and sometimes the hands and shoulders. Chronic pain syndrome is attributed to skeletal and connective tissue disease but the exact etiology is unknown. In some affected individuals spinal cord compression due to spinal stenosis (decreased diameter of the spinal canal) and vertebral osteoarthritic changes may also contribute to the chronic pain syndrome.

Osteopenia, confirmed by reduced bone mineral densitometry measured by dual-energy x-ray absorptiometry (DXA) scan, is common.

Growth. Weight and length at birth are within normal limits. Gradual slowing of growth rate begins in early childhood. Worsening hip and knee contractures contribute to reduced linear growth. The height of individuals with ML IIIγ is often below the tenth centile on standard growth curves.

Craniofacial. Dysmorphic facial features are absent or minimal in younger children. Although most individuals with ML IIIγ develop coarsening of facial features, this often occurs in the first two decades of life, which is more gradual than in mucolipidosis III alpha/beta (ML IIIα/β). Orofacial abnormalities including limited mouth opening, condylar defects, and impacted teeth may be seen in some individuals [de Bode et al 2022].

Ophthalmologic. While the corneas are clear by routine clinical inspection, opacities that do not cause ophthalmologic impairment may be appreciated by slit lamp examination in some individuals [Tüysüz et al 2018]. There is one report of late-onset retinitis pigmentosa in an individual with ML IIIγ [De Geer et al 2023].

Respiratory. Individuals with ML IIIγ generally do not have pulmonary impairment; however, mild-to-moderate restrictive lung disease may be present in adults due to abnormalities of the spine and ribs that reduce lung capacity [Oussoren et al 2018].

Cardiovascular. Individuals with ML IIIγ are at risk for cardiac involvement. Although gradual thickening and subsequent mitral valve prolapse and insufficiency of the mitral and aortic valves are common from late childhood onward, cardiac function is normal in most affected adults [Oussoren et al 2018, Tüysüz et al 2018, Erdem et al 2025]. Life-threatening valvular disease is uncommon, and valve replacement is rarely required [Dogterom et al 2021].

Neuromotor development and intellect. While motor milestones may be delayed, other aspects of development including language and learning skills are as expected for age. Affected children may require school assistance mostly because of physical limitations. While cognitive function is within the normal range in most affected individuals, a few individuals (probably <5%) may have cognitive deficiency. It is still to be determined if this manifestation is related to ML IIIγ.

Integument. The skin may become mildly thickened with time. Scleroderma-like changes have been reported in individuals with Moroccan ancestry [Zrhidri et al 2017].

Gastrointestinal. Hepatomegaly and splenomegaly are absent.

Prognosis. Data on life expectancy in individuals with ML IIIγ is lacking but calculated median survival in those with ML III (including ML III α/β and ML IIIγ) is 62 years [Dogterom et al 2021], although this may be biased due to inclusion of individuals reported before 1980. Overall, survival is significantly improved in individuals reported after 2000 and it is presumed that life expectancy is only slightly decreased compared to the normal population [Erdem et al 2025].

Genotype-Phenotype Correlations

Although data is limited, predicted loss-of-function pathogenic variants such as the recurrent variants c.347_349delACA, c.445delG, c.499dupC, and c.607dupC are associated with a severe phenotype [Tüysüz et al 2018, Dogterom et al 2021].

Nomenclature

The enzyme UDP-N-acetylglucosamine: lysosomal hydrolase N-acetylglucosamine-1-phosphotransferase is the product of two genes: GNPTAB, encoding the alpha and beta subunits, and GNPTG, encoding the gamma subunit [Bao et al 1996].

• Pathogenic variants in GNPTAB cause GNPTAB-related disorders, which include mucolipidosis II (ML II) and mucolipidosis III alpha/beta (ML IIIα/β), distinct clinical disorders with different age of onset and severity.
• Pathogenic variants in GNPTG cause ML IIIγ [Cathey et al 2008].

The trivial name of this enzyme is UDPGlcNAc 1-P-transferase; thus, the three mucolipidosis phenotypes can be considered "UDPGlcNAc 1-P-transferase deficiency disorders" [Leroy 2007].

ML IIIγ was previously referred to as variant pseudo-Hurler polydystrophy* or mucolipidosis IIIC [Cathey et al 2008].

* Pseudo-Hurler polydystrophy was the term used from 1966 by Maroteaux and Lamy when they first delineated ML III. They used this term because of the resemblance of ML III to Hurler disease, or mucopolysaccharidosis I (MPS I) [Kornfeld & Sly 2001].

Prevalence

The worldwide estimated incidence of ML II, ML IIIα/β, and ML IIIγ varies between 0.22 to 2.70 in 100,000 live births [Dogterom et al 2021]. The exact prevalence of ML IIIγ is unknown; it is considered an ultra-rare disease.

Most individuals with ML IIIγ known to the authors originated from the Mediterranean region [Raas-Rothschild et al 2004, Persichetti et al 2009, Tüysüz et al 2018, Erdem et al 2025].

A GNPTG founder pathogenic variant, c.499dupC, is reported in the Druze and Bedouin population in northern Israel (with a carrier frequency of 1/59) [Raas-Rothschild et al 2000, Avnat et al 2023], the Sephardic Jewish population from Tunisia [Raas-Rothschild et al 2004, Zlotogora 2015], and in other populations including Turkey [Erdem et al 2025].

With increased use of next-generation sequencing, individuals with ML IIIγ have been identified from other geographic regions including China, India, South America (Brazil), North America, and North Africa [Persichetti et al 2009, Gao et al 2011, Nampoothiri et al 2019, Velho et al 2019], suggesting that the disorder is pan ethnic.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with ML IIIγ, the evaluations summarized in Table 3 (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Treatment of Manifestations

Supportive care to improve quality of life, maximize function, and reduce complications is recommended. This ideally involves multidisciplinary care by specialists in relevant fields (see Table 4).

Surveillance

To monitor existing manifestations, the individual's response to supportive care, and the emergence of new manifestations, the evaluations summarized in Table 5 are recommended.

Agents/Circumstances to Avoid

Vigorous stretching exercises are not recommended because they are ineffective, painful, and may damage the surrounding joint capsule and adjacent tendons.

Evaluation of Relatives at Risk

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

Therapies Under Investigation

Search ClinicalTrials.gov in the US and EU Clinical Trials Register 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.

Mode of Inheritance

Mucolipidosis III gamma (ML IIIγ) is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

• The parents of an affected child are presumed to be heterozygous for a GNPTG pathogenic variant.
• Molecular genetic testing is recommended for the parents of the proband to confirm that both parents are heterozygous for a GNPTG pathogenic variant and to allow reliable recurrence risk assessment.
• If a pathogenic variant is detected in only one parent and parental identity testing has confirmed biological maternity and paternity, it is possible that one of the pathogenic variants identified in the proband occurred as a de novo event in the proband or as a postzygotic de novo event in a mosaic parent. In one family, only the father of the proband was heterozygous for a GNPTG pathogenic variant (a GNPTG pathogenic variant was not detected in maternal DNA) and the proband was presumed to have ML IIIγ as the result of one inherited and one de novo GNPTG pathogenic variant [Ludwig et al 2017].
  If the proband appears to have homozygous pathogenic variants (i.e., the same two pathogenic variants), additional possibilities to consider include:
  • A single- or multiexon deletion in the proband that was not detected by sequence analysis and that resulted in the artifactual appearance of homozygosity;
  • Uniparental isodisomy for the parental chromosome with the pathogenic variant that resulted in homozygosity for the pathogenic variant in the proband.
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Sibs of a proband

• If both parents are known to be heterozygous for a GNPTG pathogenic variant, each sib of an affected individual has at conception 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.
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Offspring of a proband. Unless an affected individual's reproductive partner also has ML IIIγ or is a carrier, offspring will be obligate heterozygotes (carriers) for a pathogenic variant in GNPTG.

Other family members. If both parents are heterozygous for a GNPTG pathogenic variant, each sib of the proband's parents are at a 50% risk of being carriers of a GNPTG pathogenic variant.

Carrier Detection

Molecular genetic carrier testing for at-risk relatives requires prior identification of the GNPTG pathogenic variants in the family.

Note: Biochemical testing cannot be used to identify heterozygotes.

Related Genetic Counseling Issues

Family planning

• The optimal time for determination of genetic risk and discussion of the availability of prenatal/preimplantation genetic 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.
• Carrier testing should be considered for the reproductive partners of individuals affected with ML IIIγ and individuals known to be carriers of a GNPTG pathogenic variant, particularly if consanguinity is likely and/or if both partners are of the same ethnic background. A GNPTG founder variant has been identified in the Druze, Bedouin, Sephardic Jewish, and Turkish populations (see Table 6).

Prenatal Testing and Preimplantation Genetic Testing

Once the GNPTG pathogenic variants have been identified in an affected family member, prenatal and preimplantation genetic testing are possible.

Differences in perspective may exist among medical professionals and within families regarding the use of prenatal and preimplantation genetic testing. While most health care professionals would consider use of prenatal and preimplantation genetic testing to be a personal decision, discussion of these issues may be helpful.

Molecular Pathogenesis

N-acetylglucosamine-1-phosphotransferase is a hexameric enzyme complex composed of two alpha, two beta, and two gamma subunits. The membrane-bound alpha and beta subunits are synthesized as a common alpha/beta precursor encoded by GNPTAB, whereas the soluble gamma subunit is encoded by GNPTG.

In the Golgi apparatus the gamma subunit directly binds to the alpha subunit and enhances N-acetylglucosamine-1-phosphotransferase activity for mannose-6-phosphate (M6P) modification of specific lysosomal enzymes. Once modified with M6P, the lysosomal hydrolases can attach to the M6P receptor and be targeted to the mature lysosome.

Most disease-associated variants are predicted to be loss-of-function variants. The majority of reported missense variants are located in the M6P receptor homology (MRH) domain. It is speculated that these variants impair MRH domain function, which plays a major role in binding phosphorylated and non-phosphorylated high-mannose-type N-glycans.

Mechanism of disease causation. Loss of function

Acknowledgments

We thank the "Vaincre les Maladies Lysosomales" association for their continuous support and for the research grants for our research projects on ML IIIγ and ML II.

We thank the families for their cooperation.

Revision History

• 23 April 2026 (sw) Comprehensive update posted live
• 21 November 2019 (bp) Comprehensive update posted live
• 5 July 2012 (me) Comprehensive update posted live
• 28 January 2010 (me) Review posted live
• 28 August 2009 (arr) Original submission

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