The Huntington's disease (HD) gene has been mapped in 4~16.3 but has eluded identification. We have used haplotype analysis of linkage disequilibrium to spotlight a small segment of 4~16.3 as the likely location of the defect. A new gene, IT15, isolated using cloned trapped exons from the target area contains a polymorphic trinucleotide repeat that is expanded and unstable on HD chromosomes. A (CAG), repeat longer than the normal range was observed on HD chromosomes from all 75 disease families examined, comprising a variety of ethnic backgrounds and 4~16.3 haplotypes. The GAG), repeat appears to be located within the coding sequence of a predicted-346 kd protein that is widely expressed but unrelated to any known gene. Thus, the HD mutation involves an unstable DNA segment, similar to those described in fragile X syndrome, spino-bulbar muscular atrophy, and myotonic dystrophy, acting in the context of a novel 4~16.3 gene to produce a dominant phenotype.

No abstract

Thanatophoric dysplasia (TD), the most common neonatal lethal skeletal dysplasia, affects one out of 20,000 live births. Affected individuals display features similar to those seen in homozygous achondroplasia. Mutations causing achondroplasia are in FGFR3, suggesting that mutations in this gene may cause TD. A sporadic mutation causing a Lys650Glu change in the tyrosine kinase domain of FGFR3 was found in 16 of 16 individuals with one type of TD. Of 39 individuals with a second type of TD, 22 had a mutation causing an Arg248Cys change and one had a Ser371Cys substitution, both in the extracellular region of the protein. None of these mutations were found in 50 controls showing that mutations affecting different functional domains of FGFR3 cause different forms of this lethal disorder.

Hereditary hyperekplexia, or familial startle disease (STHE), is an autosomal dominant neurologic disorder characterized by marked muscle rigidity of central nervous system origin and an exaggerated startle response to unexpected acoustic or tactile stimuli. Linkage analyses in several large families provided evidence for locus homogeneity and showed the disease gene was linked to DNA markers on the long arm of chromosome 5. Here we describe the identification of point mutations in the gene encoding the alpha 1 subunit of the glycine receptor (GLRA1) in STHE patients from four different families. All mutations occur in the same base pair of exon 6 and result in the substitution of an uncharged amino acid (leucine or glutamine) for Arg271 in the mature protein.

A single missense mutation was identified in a novel, highly conserved zinc-finger gene, ZCD2, in three consanguineous families of Jordanian descent with Wolfram syndrome (WFS). It had been shown that these families did not have mutations in the WFS1 gene (WFS1) but were mapped to the WFS2 locus at 4q22-25. A G-->C transversion at nucleotide 109 predicts an amino acid change from glutamic acid to glutamine (E37Q). Although the amino acid is conserved and the mutation is nonsynonymous, the pathogenesis for the disorder is because the mutation also causes aberrant splicing. The mutation was found to disrupt messenger RNA splicing by eliminating exon 2, and it results in the introduction of a premature stop codon. Mutations in WFS1 have also been found to cause low-frequency nonsyndromic hearing loss, progressive hearing loss, and isolated optic atrophy associated with hearing loss. Screening of 377 probands with hearing loss did not identify mutations in the WFS2 gene. The WFS1-encoded protein, Wolframin, is known to localize to the endoplasmic reticulum and plays a role in calcium homeostasis. The ZCD2-encoded protein, ERIS (endoplasmic reticulum intermembrane small protein), is also shown to localize to the endoplasmic reticulum but does not interact directly with Wolframin. Lymphoblastoid cells from affected individuals show a significantly greater rise in intracellular calcium when stimulated with thapsigargin, compared with controls, although no difference was observed in resting concentrations of intracellular calcium.

Hyperekplexia is a human neurological disorder characterized by an excessive startle response and is typically caused by missense and nonsense mutations in the gene encoding the inhibitory glycine receptor (GlyR) α1 subunit (GLRA1) [1][2][3] . Genetic heterogeneity has been confirmed in isolated sporadic cases with mutations in other postsynaptic glycinergic proteins including the GlyR β subunit (GLRB) 4 , gephyrin (GPHN) 5 and RhoGEF collybistin (ARHGEF9) 6 . However, many sporadic patients diagnosed with hyperekplexia do not carry mutations in these genes 2-7 . Here we reveal that missense, nonsense and frameshift mutations in the presynaptic glycine transporter 2 (GlyT2) gene (SLC6A5) 8 also cause hyperekplexia. Patients harbouring mutations in SLC6A5 presented with hypertonia, an exaggerated startle response to tactile or acoustic stimuli, and life-threatening neonatal apnoea episodes. GlyT2 mutations result in defective subcellular localisation and/or decreased glycine uptake, with selected mutations affecting predicted glycine and Na + binding sites. Our results demonstrate that SLC6A5 is a major gene for hyperekplexia and define the first neurological disorder linked to mutations in a Na + /Cl − -dependent transporter for a classical fast neurotransmitter. By analogy, we suggest that in other human disorders where Correspondence and requests for materials (subject to a Material Transfer Agreement) should be addressed to R.J.H. (robert.harvey@pharmacy.ac.uk) or M. I.R. (m.i.rees@swansea.ac.uk).. † these authors contributed equally to this work. COMPETING INTERESTS STATEMENT:The authors declare that they have no competing financial interests. Europe PMC Funders GroupAuthor Manuscript Nat Genet. Author manuscript; available in PMC 2011 October 31. Glycine transporters (GlyTs) are members of the Na + /Cl − -dependent neurotransmitter transporter superfamily 9,10 , integral membrane proteins that utilise electrochemical gradients to control the concentration of neurotransmitters at central synapses. This superfamily also includes transporters for GABA, biogenic amines (norepinephrine, dopamine, serotonin, proline), betaine, taurine and creatine. GlyTs have dual functions at both inhibitory and excitatory synapses, resulting from the differential localisation of two distinct transporters 9,10 , GlyT1 and GlyT2. GlyT1 is predominantly expressed in glial cells 9,10 , exhibits a 2 Na + /1 Cl − /1 glycine stoichiometry and bi-directional glycine transport 11 . These properties are appropriate for the control of extracellular glycine concentrations in the submicromolar range for modulation of N-methyl-D-aspartate receptors 12 , and also for lowering extracellular glycine levels at inhibitory glycinergic synapses 13,14 . By contrast, GlyT2 is found in glycinergic axons, exhibits a 3 Na + /1 Cl − /1 glycine stoichiometry and does not display reverse uptake 11 , reflecting an essential role for GlyT2 in maintaining a high presynaptic pool of neurotransmitter at glycinergic synapses 15 . Na + /Cl − -dependent tr...

Chondrocalcinosis (CC) is a common cause of joint pain and arthritis that is caused by the deposition of calcium-containing crystals within articular cartilage. Although most cases are sporadic, rare familial forms have been linked to human chromosomes 8 (CCAL1) or 5p (CCAL2) (Baldwin et al. 1995; Hughes et al. 1995; Andrew et al. 1999). Here, we show that two previously described families with CCAL2 have mutations in the human homolog of the mouse progressive ankylosis gene (ANKH). One of the human mutations results in the substitution of a highly conserved amino acid residue within a predicted transmembrane segment. The other creates a new ATG start site that adds four additional residues to the ANKH protein. Both mutations segregate completely with disease status and are not found in control subjects. In addition, 1 of 95 U.K. patients with sporadic CC showed a deletion of a single codon in the ANKH gene. The same change was found in a sister who had bilateral knee replacement for osteoarthritis. Each of the three human mutations was reconstructed in a full-length ANK expression construct previously shown to regulate pyrophosphate levels in cultured cells in vitro. All three of the human mutations showed significantly more activity than a previously described nonsense mutation that causes severe hydroxyapatite mineral deposition and widespread joint ankylosis in mice. These results suggest that small sequence changes in ANKH are one cause of CC and joint disease in humans. Increased ANK activity may explain the different types of crystals commonly deposited in human CCAL2 families and mutant mice and may provide a useful pharmacological target for treating some forms of human CC.

Hyperekplexia is a rare, but potentially fatal, neuromotor disorder characterized by exaggerated startle reflexes and hypertonia in response to sudden, unexpected auditory or tactile stimuli. This disorder is primarily caused by inherited mutations in the genes encoding the glycine receptor (GlyR) ␣1 subunit (GLRA1) and the presynaptic glycine transporter GlyT2 (SLC6A5). In this study, systematic DNA sequencing of GLRA1 in 88 new unrelated human hyperekplexia patients revealed 19 sequence variants in 30 index cases, of which 21 cases were inherited in recessive or compound heterozygote modes. This indicates that recessive hyperekplexia is far more prevalent than previous estimates. From the 19 GLRA1 sequence variants, we have investigated the functional effects of 11 novel and 2 recurrent mutations. The expression levels and functional properties of these hyperekplexia mutants were analyzed using a high-content imaging system and patch-clamp electrophysiology. When expressed in HEK293 cells, either as homomeric ␣1 or heteromeric ␣1␤ GlyRs, subcellular localization defects were the major mechanism underlying recessive mutations. However, mutants without trafficking defects typically showed alterations in the glycine sensitivity suggestive of disrupted receptor function. This study also reports the first hyperekplexia mutation associated with a GlyR leak conductance, suggesting tonic channel opening as a new mechanism in neuronal ligand-gated ion channels.