Valérie Serre

Interleukin-36-Receptor Antagonist Deficiency and Generalized Pustular Psoriasis

BACKGROUND Generalized pustular psoriasis is a life-threatening disease of unknown cause. It is characterized by sudden, repeated episodes of high-grade fever, generalized rash, and disseminated pustules, with hyperleukocytosis and elevated serum levels of C-reactive protein, which may be associated with plaque-type psoriasis. METHODS We performed homozygosity mapping and direct sequencing in nine Tunisian multiplex families with autosomal recessive generalized pustular psoriasis. We assessed the effect of mutations on protein expression and conformation, stability, and function. RESULTS We identified significant linkage to an interval of 1.2 megabases on chromosome 2q13-q14.1 and a homozygous missense mutation in IL36RN, encoding an interleukin-36-receptor antagonist (interleukin-36Ra), an antiinflammatory cytokine. This mutation predicts the substitution of a proline residue for leucine at amino acid position 27 (L27P). Homology-based structural modeling of human interleukin-36Ra suggests that the proline at position 27 affects both the stability of interleukin-36Ra and its interaction with its receptor, interleukin-1 receptor-like 2 (interleukin-1 receptor-related protein 2). Biochemical analyses showed that the L27P variant was poorly expressed and less potent than the nonvariant interleukin-36Ra in inhibiting a cytokine-induced response in an interleukin-8 reporter assay, leading to enhanced production of inflammatory cytokines (interleukin-8 in particular) by keratinocytes from the patients. CONCLUSIONS Aberrant interleukin-36Ra structure and function lead to unregulated secretion of inflammatory cytokines and generalized pustular psoriasis. (Funded by Agence Nationale de la Recherche and Société Française de Dermatologie.).

Mutation of RRM2B , encoding p53-controlled ribonucleotide reductase (p53R2), causes severe mitochondrial DNA depletion

Mitochondrial DNA (mtDNA) depletion syndrome (MDS; MIM 251880) is a prevalent cause of oxidative phosphorylation disorders characterized by a reduction in mtDNA copy number. The hitherto recognized disease mechanisms alter either mtDNA replication (POLG (ref. 1)) or the salvage pathway of mitochondrial deoxyribonucleosides 5′-triphosphates (dNTPs) for mtDNA synthesis (DGUOK (ref. 2), TK2 (ref. 3) and SUCLA2 (ref. 4)). A last gene, MPV17 (ref. 5), has no known function. Yet the majority of cases remain unexplained. Studying seven cases of profound mtDNA depletion (1–2% residual mtDNA in muscle) in four unrelated families, we have found nonsense, missense and splice-site mutations and in-frame deletions of the RRM2B gene, encoding the cytosolic p53-inducible ribonucleotide reductase small subunit. Accordingly, severe mtDNA depletion was found in various tissues of the Rrm2b−/− mouse. The mtDNA depletion triggered by p53R2 alterations in both human and mouse implies that p53R2 has a crucial role in dNTP supply for mtDNA synthesis.

CABC1 Gene Mutations Cause Ubiquinone Deficiency with Cerebellar Ataxia and Seizures

Coenzyme Q(10) (CoQ(10)) plays a pivotal role in oxidative phosphorylation (OXPHOS) in that it distributes electrons between the various dehydrogenases and the cytochrome segments of the respiratory chain. Primary coenzyme Q(10) deficiency represents a clinically heterogeneous condition suggestive of genetic heterogeneity, and several disease genes have been previously identified. The CABC1 gene, also called COQ8 or ADCK3, is the human homolog of the yeast ABC1/COQ8 gene, one of the numerous genes involved in the ubiquinone biosynthesis pathway. The exact function of the Abc1/Coq8 protein is as yet unknown, but this protein is classified as a putative protein kinase. We report here CABC1 gene mutations in four ubiquinone-deficient patients in three distinct families. These patients presented a similar progressive neurological disorder with cerebellar atrophy and seizures. In all cases, enzymological studies pointed to ubiquinone deficiency. CoQ(10) deficiency was confirmed by decreased content of ubiquinone in muscle. Various missense mutations (R213W, G272V, G272D, and E551K) modifying highly conserved amino acids of the protein and a 1 bp frameshift insertion c.[1812_1813insG] were identified. The missense mutations were introduced into the yeast ABC1/COQ8 gene and expressed in a Saccharomyces cerevisiae strain in which the ABC1/COQ8 gene was deleted. All the missense mutations resulted in a respiratory phenotype with no or decreased growth on glycerol medium and a severe reduction in ubiquinone synthesis, demonstrating that these mutations alter the protein function.

A mitochondrial origin for frontotemporal dementia and amyotrophic lateral sclerosis through CHCHD10 involvement

Mitochondrial DNA instability disorders are responsible for a large clinical spectrum, among which amyotrophic lateral sclerosis-like symptoms and frontotemporal dementia are extremely rare. We report a large family with a late-onset phenotype including motor neuron disease, cognitive decline resembling frontotemporal dementia, cerebellar ataxia and myopathy. In all patients, muscle biopsy showed ragged-red and cytochrome c oxidase-negative fibres with combined respiratory chain deficiency and abnormal assembly of complex V. The multiple mitochondrial DNA deletions found in skeletal muscle revealed a mitochondrial DNA instability disorder. Patient fibroblasts present with respiratory chain deficiency, mitochondrial ultrastructural alterations and fragmentation of the mitochondrial network. Interestingly, expression of matrix-targeted photoactivatable GFP showed that mitochondrial fusion was not inhibited in patient fibroblasts. Using whole-exome sequencing we identified a missense mutation (c.176C>T; p.Ser59Leu) in the CHCHD10 gene that encodes a coiled-coil helix coiled-coil helix protein, whose function is unknown. We show that CHCHD10 is a mitochondrial protein located in the intermembrane space and enriched at cristae junctions. Overexpression of a CHCHD10 mutant allele in HeLa cells led to fragmentation of the mitochondrial network and ultrastructural major abnormalities including loss, disorganization and dilatation of cristae. The observation of a frontotemporal dementia-amyotrophic lateral sclerosis phenotype in a mitochondrial disease led us to analyse CHCHD10 in a cohort of 21 families with pathologically proven frontotemporal dementia-amyotrophic lateral sclerosis. We identified the same missense p.Ser59Leu mutation in one of these families. This work opens a novel field to explore the pathogenesis of the frontotemporal dementia-amyotrophic lateral sclerosis clinical spectrum by showing that mitochondrial disease may be at the origin of some of these phenotypes.

DYNC2H1 Mutations Cause Asphyxiating Thoracic Dystrophy and Short Rib-Polydactyly Syndrome, Type III

Jeune asphyxiating thoracic dystrophy (ATD) is an autosomal-recessive chondrodysplasia characterized by short ribs and a narrow thorax, short long bones, inconstant polydactyly, and trident acetabular roof. ATD is closely related to the short rib polydactyly syndrome (SRP) type III, which is a more severe condition characterized by early prenatal expression and lethality and variable malformations. We first excluded IFT80 in a series of 26 fetuses and children belonging to 14 families diagnosed with either ATD or SRP type III. Studying a consanguineous family from Morocco, we mapped an ATD gene to chromosome 11q14.3-q23.1 in a 20.4 Mb region and identified homozygous mutations in the cytoplasmic dynein 2 heavy chain 1 (DYNC2H1) gene in the affected children. Compound heterozygosity for DYNC2H1 mutations was also identified in four additional families. Among the five families, 3/5 were diagnosed with ATD and 2/5 included pregnancies terminated for SRP type III. DYNC2H1 is a component of a cytoplasmic dynein complex and is directly involved in the generation and maintenance of cilia. From this study, we conclude that ATD and SRP type III are variants of a single disorder belonging to the ciliopathy group.

Twinkle helicase (PEO1) gene mutation causes mitochondrial DNA depletion

Mitochondrial DNA (mtDNA) depletion syndrome (MDS) is a clinically and genetically heterogeneous group of autosomal recessive diseases characterized by a reduction in mtDNA copy number. Several nuclear genes have been shown to account for these severe oxidative phosphorylation disorders, but the disease‐causing mutations remain largely unknown.

Riboflavin-responsive oxidative phosphorylation complex I deficiency caused by defective ACAD9: new function for an old gene

Mitochondrial complex I deficiency is the most common oxidative phosphorylation defect. Mutations have been detected in mitochondrial and nuclear genes, but the genetics of many patients remain unresolved and new genes are probably involved. In a consanguineous family, patients presented easy fatigability, exercise intolerance and lactic acidosis in blood from early childhood. In muscle, subsarcolemmal mitochondrial proliferation and a severe complex I deficiency were observed. Exercise intolerance and complex I activity was improved by a supplement of riboflavin at high dosage. Homozygosity mapping revealed a candidate region on chromosome three containing six mitochondria-related genes. Four genes were screened for mutations and a homozygous substitution was identified in ACAD9 (c.1594 C>T), changing the highly conserved arginine-532 into tryptophan. This mutation was absent in 188 ethnically matched controls. Protein modelling suggested a functional effect due to the loss of a stabilizing hydrogen bond in an α-helix and a local flexibility change. To test whether the ACAD9 mutation caused the complex I deficiency, we transduced fibroblasts of patients with wild-type and mutant ACAD9. Wild-type, but not mutant, ACAD9 restored complex I activity. An unrelated patient with the same phenotype was compound heterozygous for c.380 G>A and c.1405 C>T, changing arginine-127 into glutamine and arginine-469 into tryptophan, respectively. These amino acids were highly conserved and the substitutions were not present in controls, making them very probably pathogenic. Our data support a new function for ACAD9 in complex I function, making this gene an important new candidate for patients with complex I deficiency, which could be improved by riboflavin treatment.

Exome sequencing identifies MRPL3 mutation in mitochondrial cardiomyopathy.

By combining exome sequencing in conjunction with genetic mapping, we have identified the first mutation in large mitochondrial ribosomal protein MRPL3 in a family of four sibs with hypertrophic cardiomyopathy, psychomotor retardation, and multiple respiratory chain deficiency. Affected sibs were compound heterozygotes for a missense MRPL3 mutation (P317R) and a large‐scale deletion, inherited from the mother and the father, respectively. These mutations were shown to alter ribosome assembly and cause a mitochondrial translation deficiency in cultured skin fibroblasts resulting in an abnormal assembly of several complexes of the respiratory chain. This observation gives support to the view that exome sequencing combined with genetic mapping is a powerful approach for the identification of new genes of mitochondrial disorders. Hum Mutat 32:1225–1231, 2011. ©2011 Wiley Periodicals, Inc.

Mutations in NMNAT1 cause Leber congenital amaurosis with early-onset severe macular and optic atrophy

In addition to its activity in nicotinamide adenine dinucleotide (NAD+) synthesis, the nuclear nicotinamide mononucleotide adenyltransferase NMNAT1 acts as a chaperone that protects against neuronal activity–induced degeneration. Here we report that compound heterozygous and homozygous NMNAT1 mutations cause severe neonatal neurodegeneration of the central retina and early-onset optic atrophy in 22 unrelated individuals. Their clinical presentation is consistent with Leber congenital amaurosis and suggests that the mutations affect neuroprotection of photoreceptor cells.

Mutations at a single codon in Mad homology 2 domain of SMAD4 cause Myhre syndrome

Myhre syndrome (MIM 139210) is a developmental disorder characterized by short stature, short hands and feet, facial dysmorphism, muscular hypertrophy, deafness and cognitive delay. Using exome sequencing of individuals with Myhre syndrome, we identified SMAD4 as a candidate gene that contributes to this syndrome on the basis of its pivotal role in the bone morphogenetic pathway (BMP) and transforming growth factor (TGF)-β signaling. We identified three distinct heterozygous missense SMAD4 mutations affecting the codon for Ile500 in 11 individuals with Myhre syndrome. All three mutations are located in the region of SMAD4 encoding the Mad homology 2 (MH2) domain near the site of monoubiquitination at Lys519, and we found a defect in SMAD4 ubiquitination in fibroblasts from affected individuals. We also observed decreased expression of downstream TGF-β target genes, supporting the idea of impaired TGF-β–mediated transcriptional control in individuals with Myhre syndrome.