Haplogroup analysis supports a pathogenic role for the 7510T>C mutation of mitochondrial tRNA(Ser(UCN)) in sensorineural hearing loss

We ascertained a large North American family, LMG309, with matrilineal transmission of non-syndromic, progressive sensorineural hearing loss (SNHL). There was no history of aminoglycoside exposure, and penetrance was complete. We sequenced the entire mitochondrial genome and identified the previously reported 7510T>C transition in the tRNA(Ser(UCN)) gene. The 7510T>C was homoplasmic in all affected members. The LMG309 mitochondrial sequence belongs to an unnamed subgroup of mitochondrial haplogroup H. We demonstrate that the previously reported Spanish family S258 carries 7510T>C on a different mitochondrial sub-haplogroup, H1. We did not detect 7510T>C among 79 Caucasian haplogroup H control samples, including 11 from sub-haplogroup H1 and one from the same sub-haplogroup as LMG309. Our results provide strong genetic evidence that 7510T>C is a pathogenic mutation that causes non-syndromic SNHL.

The deoxyguanosine kinase gene is mutated in individuals with depleted hepatocerebral mitochondrial DNA

Mitochondrial DNA (mtDNA)-depletion syndromes (MDS; OMIM 251880) are phenotypically heterogeneous, autosomal-recessive disorders characterized by tissue-specific reduction in mtDNA copy number. Affected individuals with the hepatocerebral form of MDS have early progressive liver failure and neurological abnormalities, hypoglycemia and increased lactate in body fluids. Affected tissues show both decreased activity of the mtDNA-encoded respiratory chain complexes (I, III, IV, V) and mtDNA depletion. We used homozygosity mapping in three kindreds of Druze origin to map the gene causing hepatocerebral MDS to a region of 6.1 cM on chromosome 2p13, between markers D2S291 and D2S2116. This interval encompasses the gene (DGUOK) encoding the mitochondrial deoxyguanosine kinase (dGK). We identified a single-nucleotide deletion (204delA) within the coding region of DGUOK that segregates with the disease in the three kindreds studied. Western-blot analysis did not detect dGK protein in the liver of affected individuals. The main supply of deoxyribonucleotides (dNTPs) for mtDNA synthesis comes from the salvage pathway initiated by dGK and thymidine kinase-2 (TK2). The association of mtDNA depletion with mutated DGUOK suggests that the salvage-pathway enzymes are involved in the maintenance of balanced mitochondrial dNTP pools.

The spectrum of mutations, including four novel ones, in the thiamine-responsive megaloblastic anemia gene SLC19A2 of eight families

Thiamine responsive megaloblastic anemia (TRMA) is an autosomal recessive disorder with a triad of symptoms: megaloblastic anemia, deafness, and non-type 1 diabetes mellitus. Occasionally, cardiac abnormalities and abnormalities of the optic nerve and retina occur as well. Patients with TRMA often respond to treatment with pharmacological doses of thiamine. Recently, mutations were found in patients with TRMA in a thiamine transporter gene (SLC19A2). We here describe the mutations found in eight additional families. We found four novel mutations and three that were previously described. Of the novel ones, one is a nonsense mutation in exon 1 (E65X), two are missense mutations in exon 2 (S142F, D93H), and another is a mutation in the splicing donor site at the 5' end of intron 4 (C1223+1G>A). We also summarize the state of knowledge on all mutations found to date in TRMA patients. SLC19A2 is the first thiamine transporter gene to be described in humans. Reviewing the location and effect of the disease causing mutations can shed light on the way the protein functions and suggest ways to continue its investigation.

Atrichia with papular lesions resulting from a nonsense mutation within the human hairless gene

Atrichia with papular lesions is a rare autosomal recessive form of alopecia characterized by hair loss soon after birth and the development during childhood of a diffuse papular rash. We have previously shown that this disorder results from a deleterious mutation in the human hairless gene, a gene also involved in the pathogenesis of a related but clinically distinct form of congenital alopecia, termed alopecia universalis congenita. In this report, we describe a novel nonsense mutation in exon 4 of the human hairless gene in a consanguineous kindred affected with atrichia with papular lesions. This report provides additional evidence for phenotypic heterogeneity among inherited atrichias and for an association between the papular rash of atrichia with papular lesions and nonsense mutations in the human hairless gene.

Mutations in SLC19A2 cause thiamine-responsive megaloblastic anaemia associated with diabetes mellitus and deafness

Thiamine-responsive megaloblastic anaemia (TRMA), also known as Rogers syndrome, is an early onset, autosomal recessive disorder defined by the occurrence of megaloblastic anaemia, diabetes mellitus and sensorineural deafness, responding in varying degrees to thiamine treatment (MIM 249270). We have previously narrowed the TRMA locus from a 16-cM to a 4-cM interval on chromosomal region 1q23.3 (refs 3,4) and this region has been further refined to a 1.4-cM interval. Previous studies have suggested that deficiency in a high-affinity thiamine transporter may cause this disorder. Here we identify the TRMA gene by positional cloning. We assembled a P1-derived artificial chromosome (PAC) contig spanning the TRMA candidate region. This clarified the order of genetic markers across the TRMA locus, provided 9 new polymorphic markers and narrowed the locus to an approximately 400-kb region. Mutations in a new gene, SLC19A2, encoding a putative transmembrane protein homologous to the reduced folate carrier proteins, were found in all affected individuals in six TRMA families, suggesting that a defective thiamine transporter protein (THTR-1) may underlie the TRMA syndrome.

Atrichia with papular lesions maps to 8p in the region containing the human hairless gene

Atrichia with papular lesions (APL) (OMIM 209500) is a hereditary form of alopecia. Hair loss occurs soon after birth and is followed years later by the development of a diffuse papular eruption. Its mode of transmission is still uncertain. A related but clinically distinct form of alopecia, known as alopecia universalis (OMIM 203655), has recently been found to be associated with a mutation in the human hairless gene. The present report describes the largest consanguineous kindred of APL reported to date and provides strong evidence for autosomal recessive inheritance of this rare disorder. On the basis of a linkage analysis of this kindred using six microsatellite markers spanning the human hairless gene region, we found that the APL locus maps to chromosome region 8p12 in a 5 cM interval between marker D8S560 and marker D8S1739. A maximum lodscore of 3.7 was obtained with marker D8S1786, at a recombination fraction of 0. Our results suggest phenotypic variability at the hairless locus although they do not rule out the existence of a gene cluster associated with hair disorders in the same region.