Heimler Syndrome is Caused by Hypomorphic Mutations in the Peroxisome-Biogenesis Genes PEX1 and PEX6

Heimler syndrome (HS) is a rare recessive disorder characterized by sensorineural hearing loss (SNHL), amelogenesis imperfecta, nail abnormalities and occasional or late onset retinal pigmentation. We ascertained eight families with HS, and - using a whole exome sequencing approach - identified biallelic mutations in PEX1 or PEX6 in six of them. Loss of function mutations in both genes are known causes of a spectrum of autosomal recessive peroxisome biogenesis disorders (PBDs), including Zellweger syndrome. PBDs are characterized by leukodystrophy, hypotonia, SNHL, retinopathy, and skeletal, craniofacial, and liver abnormalities. We demonstrate that each HS family has at least one hypomorphic allele that results in extremely mild peroxisomal dysfunction. Although individuals with HS share some subtle clinical features found in PBDs, the overlap is minimal and the diagnosis was not suggested by routine blood and skin fibroblast analyses used to detect PBDs. In conclusion, our findings define Heimler syndrome as a mild PBD, expanding the pleiotropy of mutations in PEX1 and PEX6

Exome sequencing identifies a dominant tnnt3 mutation in a large family with distal arthrogryposis

Distal arthrogryposis (DA) is a group of rare, clinically and genetically heterogeneous disorders primarily characterized by congenital contractures of the distal limb joints without a neuromuscular disease. Mutations in at least 8 different genes have been shown to cause DA. Here, we report a 4-generation Indian family with 18 affected members presenting variable features of camptodactyly, brachydactyly, syndactyly, decreased flexion palmar creases, ulnar deviation of the hands, sandal gaps and club feet. We undertook exome sequencing of 3 distantly related affected individuals. Bioinformatics filtering revealed a known pathogenic missense mutation c.188G>A (p.Arg63His) in TNNT3 in all 3 affected individuals that segregated with the phenotype. The affected individuals exhibit significant phenotypic variability. This study demonstrates the value of exome sequencing helping to define the causative variant in genetically heterogeneous disorders

Mutations of Human NARS2, Encoding the Mitochondrial Asparaginyl-tRNA Synthetase, Cause Nonsyndromic Deafness and Leigh Syndrome.

Here we demonstrate association of variants in the mitochondrial asparaginyl-tRNA synthetase NARS2 with human hearing loss and Leigh syndrome. A homozygous missense mutation ([c.637G>T; p.Val213Phe]) is the underlying cause of nonsyndromic hearing loss (DFNB94) and compound heterozygous mutations ([c.969T>A; p.Tyr323*] + [c.1142A>G; p.Asn381Ser]) result in mitochondrial respiratory chain deficiency and Leigh syndrome, which is a neurodegenerative disease characterized by symmetric, bilateral lesions in the basal ganglia, thalamus, and brain stem. The severity of the genetic lesions and their effects on NARS2 protein structure cosegregate with the phenotype. A hypothetical truncated NARS2 protein, secondary to the Leigh syndrome mutation p.Tyr323* is not detectable and p.Asn381Ser further decreases NARS2 protein levels in patient fibroblasts. p.Asn381Ser also disrupts dimerization of NARS2, while the hearing loss p.Val213Phe variant has no effect on NARS2 oligomerization. Additionally we demonstrate decreased steady-state levels of mt-tRNAAsn in fibroblasts from the Leigh syndrome patients. In these cells we show that a decrease in oxygen consumption rates (OCR) and electron transport chain (ETC) activity can be rescued by overexpression of wild type NARS2. However, overexpression of the hearing loss associated p.Val213Phe mutant protein in these fibroblasts cannot complement the OCR and ETC defects. Our findings establish lesions in NARS2 as a new cause for nonsyndromic hearing loss and Leigh syndrome

Mutations of human NARS2, encoding the mitochondrial asparaginyl-tRNA synthetase, cause nonsyndromic deafness and Leigh syndrome.

Here we demonstrate association of variants in the mitochondrial asparaginyl-tRNA synthetase NARS2 with human hearing loss and Leigh syndrome. A homozygous missense mutation ([c.637G>T; p.Val213Phe]) is the underlying cause of nonsyndromic hearing loss (DFNB94) and compound heterozygous mutations ([c.969T>A; p.Tyr323*] + [c.1142A>G; p.Asn381Ser]) result in mitochondrial respiratory chain deficiency and Leigh syndrome, which is a neurodegenerative disease characterized by symmetric, bilateral lesions in the basal ganglia, thalamus, and brain stem. The severity of the genetic lesions and their effects on NARS2 protein structure cosegregate with the phenotype. A hypothetical truncated NARS2 protein, secondary to the Leigh syndrome mutation p.Tyr323* is not detectable and p.Asn381Ser further decreases NARS2 protein levels in patient fibroblasts. p.Asn381Ser also disrupts dimerization of NARS2, while the hearing loss p.Val213Phe variant has no effect on NARS2 oligomerization. Additionally we demonstrate decreased steady-state levels of mt-tRNAAsn in fibroblasts from the Leigh syndrome patients. In these cells we show that a decrease in oxygen consumption rates (OCR) and electron transport chain (ETC) activity can be rescued by overexpression of wild type NARS2. However, overexpression of the hearing loss associated p.Val213Phe mutant protein in these fibroblasts cannot complement the OCR and ETC defects. Our findings establish lesions in NARS2 as a new cause for nonsyndromic hearing loss and Leigh syndrome

<i>NARS2</i> mutations identified in two unrelated families.

<p>(A) Pedigree of the LS06 family. Filled symbols represent affected individuals and small circles represent carrier individual. The pedigree shows autosomal recessive inheritance of compound heterozygous NARS2 variants [c.969T>A; p.Tyr323*] and [c.1142A>G; p.Asn381Ser]. (B) SDS PAGE and Western blot of control and patient II.1 muscle homogenates (10μg and 20μg of protein), samples were probed for mitochondrial respiratory chain complexes via MitoProfile total OXPHOS human WB antibody cocktail. The result showed significantly decreased amounts of mitochondrial respiratory complex I and IV. (C) SDS PAGE and Western blot of fibroblast lysates from both affected probands (II.1, II.3), their parents (I.1, I.2) and controls using anti-NARS2 antibody and anti-GAPDH antibody as loading control. The expected position of a truncated NARS2 protein product (Δ154aa) stemming from the p.Tyr323* allele is indicated with a black arrow. (D) Pedigree of the PKDF406 family. Filled symbols represent affected individuals, and a double horizontal line represents a consanguineous marriage. Alleles forming the risk haplotypes are boxed. The short tandem repeat (STR) markers, their relative map positions (Mb) according to UCSC Genome Bioinformatics build GRCh37 (hg19), and their genetic positions (cM) based on the Marshfield genetic map are shown next to the pedigree. A haplotype analysis revealed a linkage region delimited by a proximal meiotic recombination at marker D11S911 in individual IV:4 (arrowhead) and distal recombination at marker D11S4082 in individuals IV:8 and IV:9 (arrowhead).</p

NARS2 homodimerization and RNA level: effect of the p.Val213Phe and p.Asn381Ser mutations.

<p>(A) Immunoprecipitates (IP) with anti-GFP antibodies from HEK293T cells transiently transfected with GFP-tagged (arrowhead) and HA-tagged NARS2 (arrow) constructs. Precipitates were immunoblotted with antibodies to the GFP and HA tags. NARS2 homodimerizes, and the p.Val213Phe mutation does not affect the dimerization process. No dimerization was detected with p.Asn381Ser NARS2 construct. (B) Steady state level for mt-tRNA^Asn was assessed by Northern blot and the results were validated by two independent laboratories. 5S-rRNA probe was used as a loading control on the same membrane. In fibroblasts of patient II.1, from LS06 family, the level of mt-tRNA^Asn is decreased compared to his parents and a control sample. Due to high passage number, we could not measure the mt-tRNA^Asn levels in the fibroblast of patient II.3.</p