Renaud Touraine

A new highly penetrant form of obesity due to deletions on chromosome 16p11.2

Obesity has become a major worldwide challenge to public health, owing to an interaction between the Western ‘obesogenic’ environment and a strong genetic contribution. Recent extensive genome-wide association studies (GWASs) have identified numerous single nucleotide polymorphisms associated with obesity, but these loci together account for only a small fraction of the known heritable component. Thus, the ‘common disease, common variant’ hypothesis is increasingly coming under challenge. Here we report a highly penetrant form of obesity, initially observed in 31 subjects who were heterozygous for deletions of at least 593 kilobases at 16p11.2 and whose ascertainment included cognitive deficits. Nineteen similar deletions were identified from GWAS data in 16,053 individuals from eight European cohorts. These deletions were absent from healthy non-obese controls and accounted for 0.7% of our morbid obesity cases (body mass index (BMI) ≥ 40 kg m-2 or BMI standard deviation score ≥ 4; P = 6.4 × 10-8, odds ratio 43.0), demonstrating the potential importance in common disease of rare variants with strong effects. This highlights a promising strategy for identifying missing heritability in obesity and other complex traits: cohorts with extreme phenotypes are likely to be enriched for rare variants, thereby improving power for their discovery. Subsequent analysis of the loci so identified may well reveal additional rare variants that further contribute to the missing heritability, as recently reported for SIM1 (ref. 3). The most productive approach may therefore be to combine the ‘power of the extreme’ in small, well-phenotyped cohorts, with targeted follow-up in case-control and population cohorts.

Neurological phenotype in Waardenburg syndrome type 4 correlates with novel SOX10 truncating mutations and expression in developing brain

Waardenburg syndrome type 4 (WS4), also called Shah-Waardenburg syndrome, is a rare neurocristopathy that results from the absence of melanocytes and intrinsic ganglion cells of the terminal hindgut. WS4 is inherited as an autosomal recessive trait attributable to EDN3 or EDNRB mutations. It is inherited as an autosomal dominant condition when SOX10 mutations are involved. We report on three unrelated WS4 patients with growth retardation and an as-yet-unreported neurological phenotype with impairment of both the central and autonomous nervous systems and occasionally neonatal hypotonia and arthrogryposis. Each of the three patients was heterozygous for a SOX10 truncating mutation (Y313X in two patients and S251X [corrected] in one patient). The extended spectrum of the WS4 phenotype is relevant to the brain expression of SOX10 during human embryonic and fetal development. Indeed, the expression of SOX10 in human embryo was not restricted to neural-crest-derived cells but also involved fetal brain cells, most likely of glial origin. These data emphasize the important role of SOX10 in early development of both neural-crest-derived tissues, namely melanocytes, autonomic and enteric nervous systems, and glial cells of the central nervous system.

Association of TALS developmental disorder with defect in minor splicing component U4atac snRNA

Mutation in a small nuclear RNA hinders splicing of pre–messenger RNAs and causes the severe malformations of Taybi-Linder syndrome. The spliceosome, a ribonucleoprotein complex that includes proteins and small nuclear RNAs (snRNAs), catalyzes RNA splicing through intron excision and exon ligation to produce mature messenger RNAs, which, in turn serve as templates for protein translation. We identified four point mutations in the U4atac snRNA component of the minor spliceosome in patients with brain and bone malformations and unexplained postnatal death [microcephalic osteodysplastic primordial dwarfism type 1 (MOPD 1) or Taybi-Linder syndrome (TALS); Mendelian Inheritance in Man ID no. 210710]. Expression of a subgroup of genes, possibly linked to the disease phenotype, and minor intron splicing were affected in cell lines derived from TALS patients. Our findings demonstrate a crucial role of the minor spliceosome component U4atac snRNA in early human development and postnatal survival.

Large-Scale Deletions and SMADIP1 Truncating Mutations in Syndromic Hirschsprung Disease with Involvement of Midline Structures

Hirschsprung disease (HSCR) is a common malformation of neural-crest-derived enteric neurons that is frequently associated with other congenital abnormalities. The SMADIP1 gene recently has been recognized as disease causing in some patients with 2q22 chromosomal rearrangement, resulting in syndromic HSCR with mental retardation, with microcephaly, and with facial dysmorphism. We screened 19 patients with HSCR and mental retardation and eventually identified large-scale SMADIP1 deletions or truncating mutations in 8 of 19 patients. These results allow further delineation of the spectrum of malformations ascribed to SMADIP1 haploinsufficiency, which includes frequent features such as hypospadias and agenesis of the corpus callosum. Thus, SMADIP1, which encodes a transcriptional corepressor of Smad target genes, may play a role not only in the patterning of neural-crest-derived cells and of CNS but also in the development of midline structures in humans.

Pelizaeus-Merzbacher-Like disease presentation of MCT8 mutated male subjects.

Pelizaeus–Merzbacher Disease is an X‐linked hypomyelinatiing leukodystrophy. We report mutations in the thyroid hormone transporter gene MCT8 in 11% of 53 families affected by hypomyelinating leukodystrophies of unknown aetiology. The 12 MCT8 mutated patients express initially a Pelizaeus–Merzbacher‐Like disease phenotype with a latter unusual improvement of magnetic resonance imaging white matter signal despite absence of clinical progression. This observation underlines the interest of determining both free T3 and free T4 serum concentrations to screen for MCT8 mutations in young patients (<3 y) with a severe Pelizaeus–Merzbacher‐Like disease presentation or older severe mentally retarded male patients with “hypomyelinated” regions. Ann Neurol 2009;65:114–118

Identification of fifteen novel mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene in European patients with severe hypophosphatasia.

Hypophosphatasia is an inherited disorder characterised by defective bone mineralisation and deficiency of serum and tissue liver/bone/kidney alkaline phosphatase (L/B/K ALP) activity. We report the characterisation of tissue-nonspecific alkaline phosphatase (TNSALP) gene mutations in a series of 13 European families affected by perinatal, infantile or childhood hypophosphatasia. Eighteen distinct mutations were found, only three of which had been reported previously in North American and Japanese populations. Most of the 15 new mutations were missense mutations, but we also found two mutations affecting donor splice sites and a nonsense mutation. A missense mutation in the last codon of the putative signal peptide probably affects the final maturation of the protein. Despite extensive sequencing of the gene and its promotor region, only one mutation was identified in two cases, one of which was compatible with a possible dominant effect of certain mutations and the putative role of polymorphisms of the TNSALP gene. In 12 of the 13 tested families, genetic diagnosis was possible by characterisation of the mutations or by use of polymorphisms as genetic markers. Hypophosphatasia diagnosis was assigned in two families where clinical, laboratory and radiographic data were unclear and prenatal diagnosis was performed in one case. The results also show that severe hypophosphatasia is due to a very large spectrum of mutations in European populations with no prevalent mutation and that genetic diagnosis of the disease must be performed by extensive analysis of the gene.

A Rare SMN2 Variant in a Previously Unrecognized Composite Splicing Regulatory Element Induces Exon 7 Inclusion and Reduces the Clinical Severity of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a common neuromuscular disorder caused by homozygous inactivation of the SMN1 (Survival Motor Neuron 1) gene. The disease severity is mainly influenced by the copy number of SMN2, a nearly identical gene from which only low amounts of full‐length mRNA are produced. This correlation is not absolute, suggesting the existence of yet unknown factors modulating disease progression. We identified and characterized the rare variant c.859G>C (p.Gly287Arg) in exon 7 in both SMN2 copies of a male patient affected with type III SMA, a milder form of the disease rarely associated with only two SMN2 copies. We demonstrated in vivo, in this patient and in a second unrelated patient, and ex vivo, using SMN splicing assays, that the variant induces inclusion of exon 7 into SMN2 mRNA. Moreover, we show that the c.859G>C variation is located in a composite splicing regulatory element in the centre of exon 7. The variation does not affect binding of HTra2â nor creates a novel SF2/ASF enhancer, but disrupts an hnRNP A1 binding site. The natural occurrence of enhanced inclusion of SMN2 exon 7 in milder SMA cases supports the current therapeutic strategies based on splicing modulation in SMA patients.

Spectrum of pontocerebellar hypoplasia in 13 girls and boys with CASK mutations: confirmation of a recognizable phenotype and first description of a male mosaic patient.

BackgroundPontocerebellar hypoplasia (PCH) is a heterogeneous group of diseases characterized by lack of development and/or early neurodegeneration of cerebellum and brainstem. According to clinical features, seven subtypes of PCH have been described, PCH type 2 related to TSEN54 mutations being the most frequent. PCH is most often autosomal recessive though de novo anomalies in the X-linked gene CASK have recently been identified in patients, mostly females, presenting with intellectual disability, microcephaly and PCH (MICPCH).MethodsFourteen patients (12 females and two males; aged 16 months-14 years) presenting with PCH at neuroimaging and with clinical characteristics unsuggestive of PCH1 or PCH2 were included. The CASK gene screening was performed using Array-CGH and sequencing. Clinical and neuroradiological features were collected.ResultsWe observed a high frequency of patients with a CASK mutation (13/14). Ten patients (8 girls and 2 boys) had intragenic mutations and three female patients had a Xp11.4 submicroscopic deletion including the CASK gene. All were de novo mutations. Phenotype was variable in severity but highly similar among the 11 girls and was characterized by psychomotor retardation, severe intellectual disability, progressive microcephaly, dystonia, mild dysmorphism, and scoliosis. Other signs were frequently associated, such as growth retardation, ophthalmologic anomalies (glaucoma, megalocornea and optic atrophy), deafness and epilepsy. As expected in an X-linked disease manifesting mainly in females, the boy hemizygous for a splice mutation had a very severe phenotype with nearly no development and refractory epilepsy. We described a mild phenotype in a boy with a mosaic truncating mutation. We found some degree of correlation between severity of the vermis hypoplasia and clinical phenotype.ConclusionThis study describes a new series of PCH female patients with CASK inactivating mutations and confirms that these patients have a recognizable although variable phenotype consisting of a specific form of pontocerebellar hypoplasia. In addition, we report the second male patient to present with a severe MICPCH phenotype and a de novo CASK mutation and describe for the first time a mildly affected male patient harboring a mosaic mutation. In our reference centre, CASK related PCH is the second most frequent cause of PCH. The identification of a de novo mutation in these patients enables accurate and reassuring genetic counselling.

Natural history of Barth syndrome: a national cohort study of 22 patients

BackgroundThis study describes the natural history of Barth syndrome (BTHS).MethodsThe medical records of all patients with BTHS living in France were identified in multiple sources and reviewed.ResultsWe identified 16 BTHS pedigrees that included 22 patients. TAZ mutations were observed in 15 pedigrees. The estimated incidence of BTHS was 1.5 cases per million births (95%CI: 0.2–2.3). The median age at presentation was 3.1 weeks (range, 0–1.4 years), and the median age at last follow-up was 4.75 years (range, 3–15 years). Eleven patients died at a median age of 5.1 months; 9 deaths were related to cardiomyopathy and 2 to sepsis. The 5-year survival rate was 51%, and no deaths were observed in patients ≥3 years. Fourteen patients presented with cardiomyopathy, and cardiomyopathy was documented in 20 during follow-up. Left ventricular systolic function was very poor during the first year of life and tended to normalize over time. Nineteen patients had neutropenia. Metabolic investigations revealed inconstant moderate 3-methylglutaconic aciduria and plasma arginine levels that were reduced or in the low-normal range. Survival correlated with two prognostic factors: severe neutropenia at diagnosis (<0.5 × 109/L) and birth year. Specifically, the survival rate was 70% for patients born after 2000 and 20% for those born before 2000.ConclusionsThis survey found that BTHS outcome was affected by cardiac events and by a risk of infection that was related to neutropenia. Modern management of heart failure and prevention of infection in infancy may improve the survival of patients with BTHS without the need for heart transplantation.

Identification and functional analysis of SOX10 missense mutations in different subtypes of Waardenburg syndrome.

Waardenburg syndrome (WS) is a rare disorder characterized by pigmentation defects and sensorineural deafness, classified into four clinical subtypes, WS1–S4. Whereas the absence of additional features characterizes WS2, association with Hirschsprung disease defines WS4. WS is genetically heterogeneous, with six genes already identified, including SOX10. About 50 heterozygous SOX10 mutations have been described in patients presenting with WS2 or WS4, with or without myelination defects of the peripheral and central nervous system (PCWH, Peripheral demyelinating neuropathy‐Central dysmyelinating leukodystrophy‐Waardenburg syndrome‐Hirschsprung disease, or PCW, PCWH without HD). The majority are truncating mutations that most often remove the main functional domains of the protein. Only three missense mutations have been thus far reported. In the present study, novel SOX10 missense mutations were found in 11 patients and were examined for effects on SOX10 characteristics and functions. The mutations were associated with various phenotypes, ranging from WS2 to PCWH. All tested mutations were found to be deleterious. Some mutants presented with partial cytoplasmic redistribution, some lost their DNA‐binding and/or transactivation capabilities on various tissue‐specific target genes. Intriguingly, several mutants were redistributed in nuclear foci. Whether this phenomenon is a cause or a consequence of mutation‐associated pathogenicity remains to be determined, but this observation could help to identify new SOX10 modes of action. 32:1436–1449, 2011. ©2011 Wiley Periodicals, Inc.