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Amish Lethal Microcephaly

Synonyms: Amish Microcephaly, MCPHA

, PhD and , MD.

Author Information
, PhD
National Human Genome Research Institute
National Institutes of Health
Bethesda, MD
, MD
Genetic Disease Research Branch
National Human Genome Research Institute
National Institutes of Health
Bethesda, MD

Initial Posting: ; Last Update: June 16, 2011.

Summary

Disease characteristics. Amish lethal microcephaly is characterized by microcephaly and early death. The occipitofrontal circumference is typically six to 12 standard deviations below the mean; anterior and posterior fontanels are closed at birth and facial features are distorted. The average life span is between five and six months.

Diagnosis/testing. Amish lethal microcephaly is diagnosed by presence of microcephaly and a tenfold increase in the levels of the urinary organic acid 2-ketoglutarate. SLC25A19 (also known as DNC or MUP1) is the only gene known to be associated with Amish lethal microcephaly. All affected individuals within the Old Order Amish population are homozygous for the same single-base pair substitution.

Management. Treatment of manifestations: Treatment is supportive only. Phenobarbital has been used to treat a few children with seizures. Physical therapy may alleviate contractures or other secondary neurologic manifestations. Infectious illnesses are managed to minimize acidosis.

Genetic counseling. Amish lethal microcephaly is inherited in an autosomal recessive manner. At conception, 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 family members and prenatal testing for pregnancies at increased risk are possible once the disease-causing mutations have been identified in the family.

Diagnosis

Clinical Diagnosis

The major clinical finding in individuals with Amish lethal microcephaly (MCPHA) is severe microcephaly present at birth [Kelley et al 2002]. Occipitofrontal circumference (OFC) is typically six to 12 standard deviations below the mean.

Recently a child with severe microcephaly, partial absence of the corpus callosum, and closed spinal dysraphism was reported [Siu et al 2010], expanding the phenotype.

Testing

Urinary organic acid 2-ketoglutarate. All affected children of Lancaster Amish heritage reported have highly elevated, at least tenfold-increased levels of the urinary organic acid 2-ketoglutarate, usually in the absence of increased levels of other citric acid cycle intermediates or lactate [Kelley et al 2002].

The patient reported by Siu et al [2010] did not have elevated urinary 2-ketoglutarate but did have elevated plasma concentrations of lactate.

Note: No other metabolic abnormalities are consistently found. However, during intercurrent illnesses such as viral syndromes, some children have metabolic acidosis with increased concentration of lactate in blood and urine.

Molecular Genetic Testing

Gene. SLC25A19 (also known as DNC or MUP1) is the only gene in which mutations are known to cause MCPHA.

Clinical testing

  • Sequence analysis. All affected individuals within the Old Order Amish population are homozygous for the same single-base pair substitution, a c.530G>C transversion in SLC25A19 [Rosenberg et al 2002]. The same mutation was identified in an Amish-Mennonite family not closely related to the Lancaster Amish [Siu et al 2010].
  • Deletion/duplication analysis. The usefulness of deletion/duplication analysis has not been demonstrated, as no deletions or duplications involving SLC25A19 as causative of Amish lethal microcephaly have been reported.

Table 1. Summary of Molecular Genetic Testing Used in Amish Lethal Microcephaly

Gene SymbolTest MethodMutations DetectedMutation Detection Frequency 1
SLC25A19Sequence analysis c.530G>C and other sequence variants 2100% 3
Deletion / duplication analysis 4Deletion / duplication of one or more exons or the whole gene 5Unknown; none reported to date

1. The ability of the test method used to detect a mutation that is present in the indicated gene

2. Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations; typically, exonic or whole-gene deletions/duplications are not detected.

3. Note that the 100% mutation detection rate is for testing among the Amish; the yield would be expected to be lower in non-Amish individuals.

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

5. No deletions or duplications involving SLC25A13 as causative of Amish lethal microcephaly have been reported. (Note: By definition, deletion/duplication analysis identifies rearrangements that are not identifiable by sequence analysis of genomic DNA.)

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Information on specific allelic variants may be available in the Molecular Genetics section (see Table A and/or Molecular Genetics, Pathologic allelic variants).

Testing Strategy

To confirm/establish the diagnosis in a proband. Perform molecular genetic testing to identify disease-causing mutations using sequence analysis.

Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family.

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

Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutations in the family.

Clinical Description

Natural History

Amish lethal microcephaly (MCPHA) is a distinct disorder with little variability in its presentation, at least among the Old Order Amish from Lancaster County, Pennsylvania [Kelley et al 2002]. Affected infants have severe microcephaly at birth. The cranial vault is extremely underdeveloped as a result of the small brain size. Anterior and posterior fontanels are closed, and ridging from premature sutural fusion may be evident. The facial features are distorted as a result of the profound microcephaly. The only non-CNS physical anomaly is moderate micrognathia. Mild hepatomegaly has been observed in several affected individuals, usually during acute illnesses associated with metabolic acidosis.

Many affected infants have difficulty maintaining body temperature. After the first two or three months of life, increasing irritability of unknown causes commonly develops [Kelley et al 2002]. Although no changes in physical or neurologic examination accompany the irritability, the Lancaster Amish children are more likely to die within 24-48 hours of developing their next viral illness. The average life span of an affected infant is between five and six months among the Lancaster Amish, but the affected Amish-Mennonite child reported by Siu et al [2010] was alive (albeit with severe developmental delay) at age seven years.

Neuropathology. A partial autopsy of an affected four-month-old infant gave insight into the neuropathology of the disorder [Strauss et al 2002]. The severity of the malformation is more pronounced in the anterior portion of the brain. Frontal lobes are smooth and rudimentary. Increasing convolution and lamination progress occipitotemporally. Regions that are most hypoplastic are most disorganized histologically.

No pathology on the case reported by Siu et al [2010] was available.

Genotype-Phenotype Correlations

With only two known mutations, such correlations are difficult to make.

Prevalence

MCPHA has been found primarily in the Old Order Amish who have ancestors in Lancaster County, Pennsylvania. At least 61 affected infants have been born to 33 nuclear families in the past 40 years. In this population, incidence is approximately one in 500 births.

The report of Siu et al [2010] shows that the phenotype is not limited to the Old Order Amish population, although prevalence is difficult to estimate.

Differential Diagnosis

Microcephaly has a wide variety of causative factors. It can be syndromic or isolated, environmental or genetic, congenital or acquired [Battaglia & Carey 2003]. A metabolic screen (including urine organic acids, plasma amino acids, lactate, pyruvate, and electrolytes) is indicated for all children with congenital microcephaly. Further specific evaluations are performed as indicated based on the results of this screen.

Microcephaly. The differential diagnosis for isolated congenital microcephaly includes single-gene disorders inherited in an autosomal recessive manner.

Primary autosomal recessive microcephalies are a group of nonsyndromic disorders characterized by a small-sized brain which are not associated with gross anomalies of brain architecture or malformations in other organ systems. Genes for five of the seven MCPH (for MiCrocePHaly) loci (MCPH1-7) are known. Affected individuals are either homozygous for the same mutation or are compound heterozygous for two mutations in one MCPH-related gene.

The remaining mapped locus for which a gene is not known:

  • MCPH2 (19q13.1-q13.2)

Other causative genes for isolated congenital microcephaly are unknown and unmapped.

The degree of microcephaly is much greater in Amish lethal microcephaly than in any of these other genetically defined microcephaly syndromes. Additionally, 2-ketoglutaric aciduria has not been reported as a finding with these disorders.

Microcephaly may also be inherited in an autosomal dominant manner and possibly in an X-linked manner, though the causative genes have not been identified [Battaglia & Carey 2003].

Alpha-ketoglutarate. Elevated levels of urinary alpha-ketoglutarate may also be seen in individuals with mutations in the alpha-ketoglutarate dehydrogenase complex, but this phenotype does not typically present with microcephaly [Dunckelmann et al 2000].

Increased levels of urinary 2-ketoglutarate are common in a wide variety of disorders of mitochondrial dysfunction, including those caused by mutations of both mtDNA and nuclear DNA genes.

Among other genetic malformation syndromes, a similar level of urinary 2-ketoglutarate is also characteristic of the autosomal recessive form of DOOR syndrome (deafness, onychodystrophy, osteodystrophy, and mental retardation; OMIM 220500), for which the causative genetic defect has not yet been determined [van Bever et al 2007].

Note to clinicians: For a patient-specific ‘simultaneous consult’ related to this disorder, go to Image SimulConsult.jpg, an interactive diagnostic decision support software tool that provides differential diagnoses based on patient findings (registration or institutional access required).

Management

Evaluation Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with Amish lethal microcephaly (MCPHA), appropriate imaging studies should be performed. Refer to general references on microcephaly for guidance; no recommendations are specific to MCPHA.

Treatment of Manifestations

This disorder is fatal within the first year of life in children of the Old Order Amish of Lancaster County, Pennsylvania. No intervention — including mitochondrial vitamin treatment — has shown promise for treatment or amelioration in this population. However, most experience with this disorder is based on the Old Order Amish; therefore, most clinical experience is based on a single allele. It is now known that this same allele can manifest in a more mild form [Siu et al 2010]. In this case, it was shown that a ketogenic diet improved the acidosis.

Seizures have occurred in some affected infants; the few children who were treated responded well to phenobarbital.

Physical therapy may be considered if the affected child develops contractures or other secondary neurologic manifestations.

Early intervention to stimulate and comfort the infant may be considered.

Support and respite for the family may be needed during the stressful terminal irritability phase of the disease, which can last for several weeks.

Prevention of Primary Manifestations

Routine childhood illnesses should be managed to minimize the acidosis associated with acute illnesses. Many affected infants in the Lancaster Amish population have died during metabolic exacerbations associated with an intercurrent infectious illness.

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 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.

Other

Vitamin supplementation does not appear to be effective in most patients.

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

Amish lethal microcephaly (MCPHA) is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

  • The biologic parents of an affected individual are obligate heterozygotes; therefore, each carries one mutant allele.
  • Heterozygotes (carriers) are asymptomatic and have normal urinary excretion of 2-ketoglutarate.

Sibs of a proband

  • At conception, each full 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. MCPHA is lethal before reproductive age.

Other family members of a proband. The risk to each sib of the proband's parents of being a carrier is at least 50% and may be 67% (2/3) if the proband's parent had an affected sib.

Carrier Detection

Carrier testing for at-risk family members is possible if the disease-causing mutations in the family have been identified.

Related Genetic Counseling Issues

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, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals.

Prenatal Testing

Molecular genetic testing. If the disease-causing mutations have been identified in the family, prenatal diagnosis for pregnancies at increased risk is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis (usually performed at ~15-18 weeks’ gestation) or chorionic villus sampling (usually performed at ~10-12 weeks’ gestation).

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Fetal ultrasound examination. Three fetal ultrasound examinations performed after 20 weeks' gestation in two pregnancies of babies ultimately found to have MCPHA revealed marked deceleration of head growth [Kelley et al 2002]. The sensitivity and specificity of fetal ultrasound for the prenatal diagnosis of Amish lethal microcephaly has not been evaluated.

Biochemical genetic testing. Prenatal diagnosis by measurement of 2-ketoglutarate in amniotic fluid has also has not been evaluated.

Preimplantation genetic diagnosis (PGD) may be an option for some families in which the disease-causing mutations have been identified.

Resources

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.

  • Medline Plus
  • National Institute of Neurological Disorders and Stroke (NINDS)
    PO Box 5801
    Bethesda MD 20824
    Phone: 800-352-9424 (toll-free); 301-496-5751; 301-468-5981 (TTY)

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 A. Amish Lethal Microcephaly: Genes and Databases

Data are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.

Table B. OMIM Entries for Amish Lethal Microcephaly (View All in OMIM)

606521SOLUTE CARRIER FAMILY 25 (MITOCHONDRIAL THIAMINE PYROPHOSPHATE CARRIER), MEMBER 19; SLC25A19
607196MICROCEPHALY, AMISH TYPE; MCPHA

Normal allelic variants. SLC25A19 contains nine exons that span 16.5 kb [Iacobazzi et al 2001]. Translation begins in exon 4 and there is evidence of alternate splicing of the three untranslated 5’ exons.

Pathologic allelic variants. One known genetic alteration results in Amish lethal microcephaly (MCPHA) [Rosenberg et al 2002]. A single-nucleotide substitution that predicts p.Gly177Ala (see Table 2; OMIM 606521) was found in homozygous form in affected individuals. This substitution was not found in 252 chromosomes from non-Amish individuals tested by a PCR-amplified restriction fragment length polymorphism assay. (For more information, see Table A.)

Table 2. Selected SLC25A19 Pathologic Allelic Variants

DNA Nucleotide Change Protein Amino Acid Change Reference Sequences
c.530G>Cp.Gly177AlaNM_021734​.4
NP_068380​.3
c.373G>Ap.Gly125SerSame

Note on variant classification: Variants listed in the table have been provided by the author(s). 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 (www​.hgvs.org). See Quick Reference for an explanation of nomenclature.

Normal gene product. The protein encoded by SLC25A19 contains three predicted mitochondrial carrier motifs. These motifs are characteristic of proteins, including the 2-ketoglutarate transporter, found in the inner mitochondrial membrane [Dolce et al 2001]. Functional analysis using bacterially expressed protein reconstituted in proteoliposomes revealed that DNC can catalyze the transport of deoxynucleotide diphosphates, deoxynucleotide triphosphates, and dideoxynucleotide triphosphates in exchange for dNDPs, ADP, or ATP. However, recent studies showed that this is not the primary physiologic function of this protein [Lindhurst et al 2006]. Transport assays revealed that the SLC25A19 protein also catalyzes the exchange of thiamine pyrophosphate (ThPP) and thiamine monophosphate (ThMP). ThPP is a co-factor for the pyruvate dehydrogenase (PDH) and the alpha-ketoglutarate dehydrogenase (KGDH) complexes that are both found in the mitochondria, and are key enzymes in oxidative metabolism.

Abnormal gene product. A bacterially expressed human SLC25A19 protein containing the p.Gly177Ala substitution reconstituted in proteoliposomes was unable to catalyze the exchange of α-S35dATP for ADP, ThPP or ThMP at 37°C and had reduced activity at 25°C [Rosenberg et al 2002].

Knockout mouse embryos homozygous for a null allele of Slc25a19 have a neural tube closure defect, yolk sac erythropoiectic failure, and elevated alpha-ketoglutarate in the amniotic fluid, and are lethal by embryonic day 12 [Lindhurst et al 2006]. Fibroblasts generated from E10.5 mouse embryos had normal levels of mitochondrial ribo- and deoxyribonucleoside triphosphates, but ThPP or ThMP were not detectable in their mitochondria. Ribo- and deoxyriboncleoside triphosphate levels were also normal in mitochondria of lymphoblasts from individuals with MCPHA; ThPP and ThMP levels were markedly reduced indicating that the p.Gly177Ala substitution is a hypomorphic allele. Activity of the PDH and KGDH complexes was greatly reduced in both the mouse and human cells, which explains the alpha-ketoglutaric aciduria in individuals with MCPHA and emphasizes the importance of oxidative metabolism in early embryogenesis. These findings provide a basis for understanding how a mutation in this transporter can cause metabolic acidosis with elevated alpha-ketoglutarate.

The findings in the child with severe microcephaly, partial absence of the corpus callosum, and closed spinal dysraphism reported by Siu et al [2010] parallel the mouse model of MCPHA [Lindhurst et al 2006].

References

Literature Cited

  1. Battaglia A, Carey JC. Microcephaly. In: Rudolph CD, Rudolph AM, eds. Rudolph's Pediatrics. 21 ed. New York, NY: McGraw-Hill; 2003:784-6.
  2. Dolce V, Fiermonte G, Runswick MJ, Palmieri F, Walker JE. The human mitochondrial deoxynucleotide carrier and its role in the toxicity of nucleoside antivirals. Proc Natl Acad Sci U S A. 2001;98:2284–8. [PMC free article: PMC30130] [PubMed: 11226231]
  3. Dunckelmann RJ, Ebinger F, Schulze A, Wanders RJ, Rating D, Mayatepek E. 2-ketoglutarate dehydrogenase deficiency with intermittent 2-ketoglutaric aciduria. Neuropediatrics. 2000;31:35–8. [PubMed: 10774994]
  4. Iacobazzi V, Ventura M, Fiermonte G, Prezioso G, Rocchi M, Palmieri F. Genomic organization and mapping of the gene (SLC25A19) encoding the human mitochondrial deoxynucleotide carrier (DNC). Cytogenet Cell Genet. 2001;93:40–2. [PubMed: 11474176]
  5. Kelley RI, Robinson D, Puffenberger EG, Strauss KA, Morton DH. Amish lethal microcephaly: a new metabolic disorder with severe congenital microcephaly and 2-ketoglutaric aciduria. Am J Med Genet. 2002;112:318–26. [PubMed: 12376931]
  6. Lindhurst MJ, Fiermonte G, Song S, Struys E, De Leonardis F, Schwartzberg PL, Chen A, Castegna A, Verhoeven N, Mathews CK, Palmieri F, Biesecker LG. Knockout of Slc25a19 causes mitochondrial thiamine pyrophosphate depletion, embryonic lethality, CNS malformations, and anemia. Proc Natl Acad Sci U S A. 2006;103:15927–32. [PMC free article: PMC1595310] [PubMed: 17035501]
  7. Rosenberg MJ, Agarwala R, Bouffard G, Davis J, Fiermonte G, Hilliard MS, Koch T, Kalikin LM, Makalowska I, Morton DH, Petty EM, Weber JL, Palmieri F, Kelley RI, Schäffer AA, Biesecker LG. Mutant deoxynucleotide carrier is associated with congenital microcephaly. Nat Genet. 2002;32:175–9. [PubMed: 12185364]
  8. Siu VM, Ratko S, Prasad AN, Prasad C, Rupar CA. Amish microcephaly: Long-term survival and biochemical characterization. Am J Med Genet A. 2010;152A:1747–51. [PubMed: 20583149]
  9. Spiegel R, Shaag A, Edvardson S, Mandel H, Stepensky P, Shalev SA, Horovitz Y, Pines O, Elpeleg O. SLC25A19 mutation as a cause of neuropathy and bilateral striatal necrosis. Ann Neurol. 2009;66:419–24. [PubMed: 19798730]
  10. Strauss KA, Pfannl R, Morton DH. The neuropathology of Amish lethal microcephaly. Am J Hum Genet. 2002;71S:260.
  11. van Bever Y, Balemans W, Duval EL, Jespers A, Eyskens F, van Hul W, Courtens W. Exclusion of OGDH and BMP4 as candidate genes in two siblings with autosomal recessive DOOR syndrome. Am J Med Genet A. 2007;143:763–7. [PubMed: 17343268]

Chapter Notes

Author Notes

Dr. Lindhurst is a geneticist and senior biologist trained at the University of Chicago. Dr. Biesecker is a clinical geneticist and human geneticist trained at the University of Michigan. Both are based at the National Human Genome Research Institute at the NIH.

Revision History

  • 16 June 2011 (me) Comprehensive update posted live
  • 5 May 2009 (me) Comprehensive update posted
  • 20 December 2005 (me) Comprehensive update posted to live Web site
  • 4 September 2003 (me) Review posted to live Web site
  • 24 June 2003 (mjl) Original submission

Note: Pursuant to 17 USC Section 105 of the United States Copyright Act, the GeneReview ‘Amish Lethal Microcephaly’ is in the public domain in the United States of America.

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