Progressive myoclonic epilepsy-associated gene KCTD7 is a regulator of potassium conductance in neurons.

The potassium channel tetramerization domain-containing protein 7 (KCTD7) was named after the structural homology of its predicted N-terminal broad complex, tramtrack and bric à brac/poxvirus and zinc finger domain with the T1 domain of the Kv potassium channel, but its expression profile and cellular function are still largely unknown. We have recently reported a homozygous nonsense mutation of KCTD7 in patients with a novel form of autosomal recessive progressive myoclonic epilepsy. Here, we show that KCTD7 expression hyperpolarizes the cell membrane and reduces the excitability of transfected neurons in patch clamp experiments. We found the expression of KCTD7 in the hippocampal and Purkinje cells of the murine brain, an expression profile consistent with our patients' phenotype. The effect on the plasma membrane resting potential is possibly mediated by Cullin-3, as we demonstrated direct molecular interaction of KCTD7 with Cullin-3 in co-immunoprecipitation assays. Our data link progressive myoclonic epilepsy to an inherited defect of the neuron plasma membrane's resting potential in the brain.

Mutation of a potassium channel-related gene in progressive myoclonic epilepsy.

OBJECTIVE: We investigated a large consanguineous Moroccan family with progressive myoclonic epilepsy (PME) consistent with autosomal recessive inheritance, to describe the phenotype and identify the causal gene. METHODS: We recorded the clinical course of the disease and the response to drug therapy, whereas carefully excluding known causes of progressive myoclonic epilepsy. We then linked the disease by homozygosity mapping using microsatellite markers and single nucleotide polymorphism microarrays (11K GeneChip), and studied candidate genes in the critical linkage region. RESULTS: Epilepsy started between 16 and 24 months of age after normal initial development. Seizures were multifocal myoclonus aggravated by movements, and generalized tonic-clonic seizures were experienced by two patients. Electroencephalogram showed slow dysrhythmia, multifocal and occasionally generalized epileptiform discharges, and photosensitivity. Brain magnetic resonance images were normal. All patients were demented. Two had refractory epilepsy and a severe course. Seizures were controlled in the third patient, whose disease course was less severe. Linkage analyses identified a new locus on 7q11.2, with a maximum multipoint logarithm of odds of 4.0 at D7S663. In the critical linkage region, we found a C to T mutation in exon 2 of the potassium channel tetramerization domain containing 7 gene (KCTD7). The mutation affected a highly conserved segment of the predicted protein, changing an arginine codon into a stop codon (R99X). INTERPRETATION: Neurodegeneration in progressive myoclonic epilepsy presented by our patients paralleled the refractoriness of epilepsy. The disease was transmitted as an autosomal recessive trait linked to a novel locus at 7q11.2, where we identified a mutation in KCTD7.

Borate transporter SLC4A11 mutations cause both Harboyan syndrome and non-syndromic corneal endothelial dystrophy.

Harboyan syndrome, or corneal dystrophy and perceptive deafness (CDPD), consists of congenital corneal endothelial dystrophy and progressive perceptive deafness, and is transmitted as an autosomal recessive trait. CDPD and autosomal recessive, non-syndromic congenital hereditary endothelial corneal dystrophy (CHED2) both map at overlapping loci at 20p13, and mutations of SLC4A11 were reported recently in CHED2. A genotype study on six families with CDPD and on one family with either CHED or CDPD, from various ethnic backgrounds (in the seventh family, hearing loss could not be assessed because of the proband's young age), is reported here. Novel SLC4A11 mutations were found in all patients. Why some mutations cause hearing loss in addition to corneal dystrophy is presently unclear. These findings extend the implication of the SLC4A11 borate transporter beyond corneal dystrophy to perceptive deafness.

Mutations of the TGF-beta type II receptor BMPR2 in pulmonary arterial hypertension.

Pulmonary arterial hypertension (PAH) is clinically characterized by a sustained elevation in mean pulmonary artery pressure leading to significant morbidity and mortality. The disorder is typically sporadic, and in such cases the term idiopathic PAH (IPAH) is used. However, cases that occur within families (familial PAH (FPAH)) display similar clinical and histopathological features, suggesting a common etiology. Heterozygous mutations of a type II member of the TGF-beta cell signaling superfamily known as BMPR2 on chromosome 2q33 have been identified in many kindreds with FPAH, yet display both reduced penetrance and sex bias. This report presents the compilation of data for 144 distinct mutations that alter the coding sequence of the BMPR2 gene identified in 210 independent PAH subjects. This large data set characterizes the extent of sequence variation and reveals that the majority (71%) of mutations in FPAH and IPAH comprise nonsense, frameshift, and splice-site defects, and gene rearrangements. These predict premature termination of the transcript with likely loss through the process of nonsense-mediated decay (NMD). A total of 44 missense mutations were identified that substitute amino acid residues at highly conserved sites within recognized functional domains of the mature receptor. We assess this category of mutations in the context of their heterogeneous effects on cell signaling when assayed by in vitro cell-based systems. Disease-causing mutation hot-spots within BMPR2 are summarized. Taken together, these observations are likely to aid in the development of targeted mutation detection strategies relevant for patient management. Finally, we examine the age- and sex-dependent reduced penetrance of BMPR2 mutations by reviewing bmpr2 animal models and the requirement for additional genetic and/or environmental modifiers of disease. In conclusion, these data provide compelling genetic evidence that haploinsufficiency is the predominant molecular mechanism underlying disease predisposition, and support the concept of a critical threshold of signaling activity below which disease may be precipitated.

GCMB mutation in familial isolated hypoparathyroidism with residual secretion of parathyroid hormone.

Isolated hypoparathyroidism is an uncommon metabolic disorder characterized by hypocalcemia and hyperphosphatemia, with absent or low levels of PTH. It may present as an apparently sporadic disorder or may be transmitted in families as a genetic trait. Mutations of the calcium-sensing receptor gene and of the preproPTH gene have been reported in occasional cases, and a mutation of the parathyroid-specific transcription factor GCMB gene has been reported in one familial case. We report a second family with isolated hypoparathyroidism and a GCMB mutation. The patients were two siblings from asymptomatic, first-cousin parents, indicating autosomal recessive inheritance. The mutation consisted of the substitution of a glycine residue with a serine at position 63 (G63S) in the DNA-binding GCM domain of GCMB. Functional studies in transfected cells showed that the mutation caused loss of GCMB function, as it abolished transactivation capacity, despite normal subcellular localization, protein stability, and DNA-binding specificity. Contrary to the previously reported family, our patients displayed low but clearly detectable levels of PTH in plasma. This residual hormone secretion probably results from a very small residual activity of the G63S mutant GCMB.

Congenital stationary night blindness: report of an autosomal recessive family and linkage analysis.

Congenital stationary night blindness (CSNB) is a group of rare, non-progressive conditions of the retina characterized by abnormal rod function causing impaired night vision. Among them, the Schubert-Bornschein subgroup, itself divided into a complete and an incomplete form, is characterized by a specific electrophysiological pattern. Complete, Schubert-Bornschein CSNB is usually transmitted as a monogenic trait, and most familial cases result from mutations of the NYX gene located on the X chromosome. We report a very rare family with consanguineous, first-cousin parents, where a son and a daughter are affected with this condition, indicating autosomal recessive inheritance. As the family was too small for genome-wide linkage, we considered several candidate loci, including the sidekick SDK1 and SDK2 genes. The latter determine lamina-specific connectivity in the retina, a histological substrate of the ON pathway implicated in complete, Schubert-Bornschein CSNB. Although linkage was excluded in our family, observations like the present one may lead to the identification of a new molecular cause for this condition.

The gene for paroxysmal non-kinesigenic dyskinesia encodes an enzyme in a stress response pathway.

Paroxysmal non-kinesigenic dyskinesia (PNKD) is characterized by spontaneous hyperkinetic attacks that are precipitated by alcohol, coffee, stress and fatigue. We report mutations in the myofibrillogenesis regulator 1 (MR-1) gene causing PNKD in 50 individuals from eight families. The mutations cause changes (Ala to Val) in the N-terminal region of two MR-1 isoforms. The MR-1L isoform is specifically expressed in brain and is localized to the cell membrane while the MR-1S isoform is ubiquitously expressed and shows diffuse cytoplasmic and nuclear localization. Bioinformatic analysis reveals that the MR-1 gene is homologous to the hydroxyacylglutathione hydrolase (HAGH) gene. HAGH functions in a pathway to detoxify methylglyoxal, a compound present in coffee and alcoholic beverages and produced as a by-product of oxidative stress. Our results suggest a mechanism whereby alcohol, coffee and stress may act as precipitants of attacks in PNKD. Stress response pathways will be important areas for elucidation of episodic disease genetics where stress is a common precipitant of many common disorders like epilepsy, migraine and cardiac arrhythmias.

TM4SF10 gene sequencing in XLMR patients identifies common polymorphisms but no disease-associated mutation.

BACKGROUND: The TM4SF10 gene encodes a putative four-transmembrane domains protein of unknown function termed Brain Cell Membrane Protein 1 (BCMP1), and is abundantly expressed in the brain. This gene is located on the short arm of human chromosome X at p21.1. The hypothesis that mutations in the TM4SF10 gene are associated with impaired brain function was investigated by sequencing the gene in individuals with hereditary X-linked mental retardation (XLMR). METHODS: The coding region (543 bp) of TM4SF10, including intronic junctions, and the long 3' untranslated region (3 233 bp), that has been conserved during evolution, were sequenced in 16 male XLMR patients from 14 unrelated families with definite, or suggestive, linkage to the TM4SF10 gene locus, and in 5 normal males. RESULTS: Five sequence changes were identified but none was found to be associated with the disease. Two of these changes correspond to previously known SNPs, while three other were novel SNPs in the TM4SF10 gene. CONCLUSION: We have investigated the majority of the known MRX families linked to the TM4SF10 gene region. In the absence of mutations detected, our study indicates that alterations of TM4SF10 are not a frequent cause of XLMR.

A translocation breakpoint disrupts the ASPM gene in a patient with primary microcephaly.

Primary microcephaly (microcephalia vera) is a developmental abnormality resulting in a small brain, with mental retardation. It is usually transmitted as an autosomal recessive trait, and six loci have been reported to date. We analyzed a translocation breakpoint previously reported in a patient with apparently sporadic primary microcephaly, at 1q31, where locus MCPH5 maps. The patient was lost to follow-up, and we sampled a maternal aunt who carried the familial translocation. FISH analyses showed that the insert of BAC clone RP11-32D17 spanned the breakpoint. The breakpoint was further located within a fragment of this insert corresponding to intron 17 of the ASPM gene, resulting in a predicted transcript truncated of more than half of its coding sequence. It is very likely that the proband carried a second ASPM mutation in trans, but he was not available for sampling and hence we could not confirm this hypothesis. Our observation adds to the mutation spectrum of ASPM in primary microcephaly, and is to our knowledge the second example of a constitutional, reciprocal translocation responsible for a bona fide autosomal recessive phenotype.

Serotonin 5-HT(2B) receptor loss of function mutation in a patient with fenfluramine-associated primary pulmonary hypertension.

OBJECTIVE: Appetite-suppressant drug fenfluramine is implicated in primary pulmonary hypertension (PPH) but the molecular pathways that mediate this effect are unknown. A mouse model incriminates the serotonin 5-HT(2B) receptor but contrasts with other models where this receptor has been shown to mediate pulmonary arterial relaxation via nitric oxide production. METHODS: We analyzed the human 5-HT(2B) gene in 10 patients with appetite-suppressant drug-associated PPH. RESULTS: A mutation causing premature truncation of the protein product was found in one patient. The mutation was not found in 80 control subjects and no 5-HT(2B) mutation was found in 18 PPH patients not associated with appetite-suppressants. Functional analysis of the transfected receptor expressed either transiently in COS cells or stably in CHO cells demonstrated that the mutated receptor fails to activate the second messenger inositol-phosphates cascade and subsequent intracellular calcium release, in spite of normal expression at the cell membrane. The mutated receptor had no constitutive activity, and produced no dominant negative effect on the wild-type receptor. CONCLUSION: Loss of serotonin 5-HT(2B) receptor function may predispose to fenfluramine-associated PPH in man.

Protein-truncating mutations in ASPM cause variable reduction in brain size.

Mutations in the ASPM gene at the MCPH5 locus are expected to be the most common cause of human autosomal recessive primary microcephaly (MCPH), a condition in which there is a failure of normal fetal brain development, resulting in congenital microcephaly and mental retardation. We have performed the first comprehensive mutation screen of the 10.4-kb ASPM gene, identifying all 19 mutations in a cohort of 23 consanguineous families. Mutations occurred throughout the ASPM gene and were all predicted to be protein truncating. Phenotypic variation in the 51 affected individuals occurred in the degree of microcephaly (5-11 SDs below normal) and of mental retardation (mild to severe) but appeared independent of mutation position.