De novo variants in HK1 associated with neurodevelopmental abnormalities and visual impairment.

Hexokinase 1 (HK1) phosphorylates glucose to glucose-6-phosphate, the first rate-limiting step in glycolysis. Homozygous and heterozygous variants in HK1 have been shown to cause autosomal recessive non-spherocytic hemolytic anemia, autosomal recessive Russe type hereditary motor and sensory neuropathy, and autosomal dominant retinitis pigmentosa (adRP). We report seven patients from six unrelated families with a neurodevelopmental disorder associated with developmental delay, intellectual disability, structural brain abnormality, and visual impairments in whom we identified four novel, de novo missense variants in the N-terminal half of HK1. Hexokinase activity in red blood cells of two patients was normal, suggesting that the disease mechanism is not due to loss of hexokinase enzymatic activity.

Predicting the pathogenicity of aminoacyl-tRNA synthetase mutations.

Aminoacyl-tRNA synthetases (ARSs) are ubiquitously expressed, essential enzymes responsible for charging tRNA with cognate amino acids-the first step in protein synthesis. ARSs are required for protein translation in the cytoplasm and mitochondria of all cells. Surprisingly, mutations in 28 of the 37 nuclear-encoded human ARS genes have been linked to a variety of recessive and dominant tissue-specific disorders. Current data indicate that impaired enzyme function is a robust predictor of the pathogenicity of ARS mutations. However, experimental model systems that distinguish between pathogenic and non-pathogenic ARS variants are required for implicating newly identified ARS mutations in disease. Here, we outline strategies to assist in predicting the pathogenicity of ARS variants and urge cautious evaluation of genetic and functional data prior to linking an ARS mutation to a human disease phenotype.

Mendelian disorders of PI metabolizing enzymes.

More than twenty different genetic diseases have been described that are caused by mutations in phosphoinositide metabolizing enzymes, mostly in phosphoinositide phosphatases. Although generally ubiquitously expressed, mutations in these enzymes, which are mainly loss-of-function, result in tissue-restricted clinical manifestations through mechanisms that are not completely understood. Here we analyze selected disorders of phosphoinositide metabolism grouped according to the principle tissue affected: the nervous system, muscle, kidney, the osteoskeletal system, the eye, and the immune system. We will highlight what has been learnt so far from the study of these disorders about not only the cellular and molecular pathways that are involved or are governed by phosphoinositides, but also the many gaps that remain to be filled to gain a full understanding of the pathophysiological mechanisms underlying the clinical manifestations of this steadily growing class of diseases, most of which still remain orphan in terms of treatment. This article is part of a Special Issue entitled Phosphoinositides.

Mitochondrial aminoacyl-tRNA synthetases in human disease.

Mitochondrial aminoacyl-tRNA synthetases (mtARSs) are essential in the process of transferring genetic information from mitochondrial DNA to the complexes of the oxidative phosphorylation system. These synthetases perform an integral step in the initiation of mitochondrial protein synthesis by charging tRNAs with their cognate amino acids. All mtARSs are encoded by nuclear genes, nine of which have recently been described as disease genes for mitochondrial disorders. Unexpectedly, the clinical presentations of these diseases are highly specific to the affected synthetase. Encephalopathy is the most common manifestation but again with gene-specific outcomes. Other clinical presentations include myopathy with anemia, cardiomyopathy, tubulopathy and hearing loss with female ovarian dysgenesis. Here we review the described mutation types and the associated patient phenotypes. The identified mutation spectrum suggests that only mutation types that allow some residual tRNA-charging activity can result in the described mtARS diseases but the molecular mechanisms behind the selective tissue involvement are not currently understood.

Loss-of-function mutations in HINT1 cause axonal neuropathy with neuromyotonia.

Inherited peripheral neuropathies are frequent neuromuscular disorders known for their clinical and genetic heterogeneity. In 33 families, we identified 8 mutations in HINT1 (encoding histidine triad nucleotide-binding protein 1) by combining linkage analyses with next-generation sequencing and subsequent cohort screening of affected individuals. Our study provides evidence that loss of functional HINT1 protein results in a distinct phenotype of autosomal recessive axonal neuropathy with neuromyotonia.

A mutation in an alternative untranslated exon of hexokinase 1 associated with hereditary motor and sensory neuropathy -- Russe (HMSNR).

Hereditary Motor and Sensory Neuropathy -- Russe (HMSNR) is a severe autosomal recessive disorder, identified in the Gypsy population. Our previous studies mapped the gene to 10q22-q23 and refined the gene region to approximately 70 kb. Here we report the comprehensive sequencing analysis and fine mapping of this region, reducing it to approximately 26 kb of fully characterised sequence spanning the upstream exons of Hexokinase 1 (HK1). We identified two sequence variants in complete linkage disequilibrium, a G>C in a novel alternative untranslated exon (AltT2) and a G>A in the adjacent intron, segregating with the disease in affected families and present in the heterozygote state in only 5/790 population controls. Sequence conservation of the AltT2 exon in 16 species with invariable preservation of the G allele at the mutated site, strongly favour the exonic change as the pathogenic mutation. Analysis of the Hk1 upstream region in mouse mRNA from testis and neural tissues showed an abundance of AltT2-containing transcripts generated by extensive, developmentally regulated alternative splicing. Expression is very low compared with ubiquitous Hk1 and all transcripts skip exon1, which encodes the protein domain responsible for binding to the outer mitochondrial membrane, and regulation of energy production and apoptosis. Hexokinase activity measurement and immunohistochemistry of the peripheral nerve showed no difference between patients and controls. The mutational mechanism and functional effects remain unknown and could involve disrupted translational regulation leading to increased anti-apoptotic activity (suggested by the profuse regenerative activity in affected nerves), or impairment of an unknown HK1 function in the peripheral nervous system (PNS).

Mutations in the gene encoding peroxisomal alpha-methylacyl-CoA racemase cause adult-onset sensory motor neuropathy.

Sensory motor neuropathy is associated with various inherited disorders including Charcot-Marie-Tooth disease, X-linked adrenoleukodystrophy/adrenomyeloneuropathy and Refsum disease. In the latter two, the neuropathy is thought to result from the accumulation of specific fatty acids. We describe here three patients with elevated plasma concentrations of pristanic acid (a branched-chain fatty acid) and C27-bile-acid intermediates. Two of the patients suffered from adult-onset sensory motor neuropathy. One patient also had pigmentary retinopathy, suggesting Refsum disease, whereas the other patient had upper motor neuron signs in the legs, suggesting adrenomyeloneuropathy. The third patient was a child without neuropathy. In all three patients we discovered a deficiency of alpha-methylacyl-CoA racemase (AMACR). This enzyme is responsible for the conversion of pristanoyl-CoA and C27-bile acyl-CoAs to their (S)-stereoisomers, which are the only stereoisomers that can be degraded via peroxisomal beta-oxidation. Sequence analysis of AMACR cDNA from the patients identified two different mutations that are likely to cause disease, based on analysis in Escherichia coli. Our findings have implications for the diagnosis of adult-onset neuropathies of unknown aetiology.

Clinical similarities of hereditary progressive/dopa responsive dystonia caused by different types of mutations in the GTP cyclohydrolase I gene.

OBJECTIVE: Hereditary progressive dystonia with pronounced diurnal fluctuation [(HPD)/dopa responsive dystonia (DRD)] is a childhood onset dystonia which responds to levodopa. Various clinical signs and symptoms of HPD/DRD have been recognised to date. Mutations in the GTP cyclohydrolase I (GTP-CH-I) gene were recently identified as the cause of HPD/ DRD. In the present study, the GTP-CH-I gene and the clinical features of eight HPD/DRD patients from six families were analysed to determine the correlations between clinical expression and the mutations in the GTP-CH-I gene. METHODS: The exons, exon-intron junctions, and an indispensable part of the 5' flanking region of the GTP-CH-I gene were sequenced in the eight clinically diagnosed patients with HPD/DRD and their asymptomatic parents. RESULTS: Three independent mutations in the GTP-CH-I gene were found in three patients. One of the patients and her asymptomatic mother were heterozygous for a novel mutation at the initiation codon. The three patients with dissimilar GTP-CH-I mutations exhibited similar clinical features. The other five patients with normal sequences presented several features not manifested by the three patients with the mutations. No mutation was found in the 5' flanking region of any patients or their parents. CONCLUSIONS: A novel initiation codon mutation was found in a Japanese patient with HPD/DRD. The clinical manifestations common to the patients with HPD/ DRD with a mutated GTP-CH-I gene were also identified. Although focal manifestations of HPD/DRD associated with the mutations of this gene will be broadened, it is inferred that these clinical features are fundamental to HPD/DRD caused by mutations in this gene.

Genomic structure and expression of the human heme A:farnesyltransferase (COX10) gene.

Charcot-Marie-Tooth disease type 1A (CMT1A) is associated with a 1.5-Mb tandem DNA duplication in chromosome 17p11.2-p12, while hereditary neuropathy with liability to pressure palsies (HNPP) is associated with a 1.5-Mb deletion at this locus. The 1.5-Mb CMT1A monomer unit duplicated in CMT1A and deleted in HNPP is flanked by two 24-kb direct repeats termed the CMT1A-REPs. Recently, sequence analysis of the CMT1A-REPs revealed that they contain an internal exon of the COX10 gene. To characterize COX10, encoding human heme A:farnesyltransferase, the genomic region was isolated and the gene structure and expression profile were determined. COX10 spans approximately 135 kb and consists of seven exons. Exons I-V are telomeric to the 1.5-Mb CMT1A monomer unit, whereas exon VII is located within this 1.5-Mb region. Exon VI is contained within the distal CMT1A-REP. All splice sites conform to the GT/AG rule. Analysis of the putative promoter region of the COX10 gene indicates that it lacks conventional TATA and CAAT boxes, but it does have several potential transcription factor-binding sites. This gene is expressed in multiple tissues with highest expression observed in the heart, skeletal muscle, and testis.

Familial adrenal insufficiency, achalasia, alacrima, peripheral neuropathy, microcephaly, normal plasma very long chain fatty acids, and normal muscle mitochondrial respiratory chain enzymes.

Adrenal insufficiency has been associated with adrenoleukodystrophy and adrenomyeloneuropathy. In these diseases, plasma very long chain fatty acids are elevated. Peripheral neuropathy is frequently seen in adults with adrenomyeloneuropathy. We encountered two first cousins with adrenal insufficiency, who also developed peripheral neuropathy, achalasia, alacrima, and microcephaly. However, plasma very long chain fatty acids, pipecolic acid, phytanic acid, and cranial computed tomographic scan were normal. Muscle mitochondrial respiratory chain enzymes were also normal. This syndrome of adrenal insufficiency, achalasia, alacrima, microcephaly, and peripheral neuropathy is different from either adrenomyeloneuropathy or adrenoleukodystrophy.

Rhodanese isozymes in three subjects with Leber's optic neuropathy.

Previous studies have reported an association between Leber's optic neuropathy and deficiency of rhodanese activity in liver and rectal mucosa. We have studied the rhodanese isozymes in liver biopsies from three subjects with Leber's optic neuropathy. The rhodanese isozyme patterns were indistinguishable from controls both in relative intensity and position on the isoelectric focusing pH gradient. No new rhodanese isozymes were observed and there was no evidence of deficiency in any of the cases.