Mitochondrial respiratory chain disorders in the Old Order Amish population

The Old Order Amish populations in the US are one of the Plain People groups and are descendants of the Swiss Anabaptist immigrants who came to North America in the early eighteenth century. They live in numerous small endogamous demes that have resulted in reduced genetic diversity along with a high prevalence of specific genetic disorders, many of them autosomal recessive. Mitochondrial respiratory chain deficiencies arising from mitochondrial or nuclear DNA mutations have not previously been reported in the Plain populations. Here we present four different Amish families with mitochondrial respiratory chain disorders. Mutations in two mitochondrial encoded genes leading to mitochondrial respiratory chain disorder were identified in two patients. In the first case, MELAS syndrome caused by a mitochondrial DNA (mtDNA) mutation (m.3243A>G) was identified in an extended Amish pedigree following a presentation of metabolic strokes in the proband. Characterization of the extended family of the proband by a high resolution melting assay identified the same mutation in many previously undiagnosed family members with a wide range of clinical symptoms. A MELAS/Leigh syndrome phenotype caused by a mtDNA mutation [m.13513G>A; p.Asp393Asn] in the ND5 gene encoding the ND5 subunit of respiratory chain complex I was identified in a patient in a second family. Mutations in two nuclear encoded genes leading to mitochondrial respiratory chain disorder were also identified in two patients. One patient presented with Leigh syndrome and had a homozygous deletion in the NDUFAF2 gene, while the second patient had a homozygous mutation in the POLG gene, [c.1399G>A; p.Ala467Thr]. Our findings identify mitochondrial respiratory chain deficiency as a cause of disease in the Old Order Amish that must be considered in the context of otherwise unexplained systemic disease, especially if neuromuscular symptoms are present.