Rare Missense Variants in KCNJ10 Are Associated with Paroxysmal Kinesigenic Dyskinesia

BACKGROUND

Although the group of paroxysmal kinesigenic dyskinesia (PKD) genes is expanding, the molecular cause remains elusive in more than 50% of cases.

OBJECTIVE

The aim is to identify the missing genetic causes of PKD.

METHODS

Phenotypic characterization, whole exome sequencing and association test were performed among 53 PKD cases.

RESULTS

We identified four causative variants in KCNJ10, already associated with EAST syndrome (epilepsy, cerebellar ataxia, sensorineural hearing impairment and renal tubulopathy). Homozygous p.(Ile209Thr) variant was found in two brothers from a single autosomal recessive PKD family, whereas heterozygous p.(Cys294Tyr) and p.(Thr178Ile) variants were found in six patients from two autosomal dominant PKD families. Heterozygous p.(Arg180His) variant was identified in one additional sporadic PKD case. Compared to the Genome Aggregation Database v2.1.1, our PKD cohort was significantly enriched in both rare heterozygous (odds ratio, 21.6; P = 9.7 × 10-8) and rare homozygous (odds ratio, 2047; P = 1.65 × 10-6) missense variants in KCNJ10.

CONCLUSIONS

We demonstrated that both rare monoallelic and biallelic missense variants in KCNJ10 are associated with PKD. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Does Spinocerebellar ataxia 27B mimic cerebellar multiple system atrophy?

BACKGROUND

Whether spinocerebellar ataxia 27B (SCA27B) may present as a cerebellar multiple system atrophy (MSA-C) mimic remains undetermined.

OBJECTIVES

To assess the prevalence of FGF14 (GAA)≥250 expansions in patients with MSA-C, to compare SCA27B and MSA-C clinical presentation and natural history.

METHODS

FGF14 expansion screening combined with longitudinal deep-phenotyping in a prospective cohort of 195 patients with sporadic late-onset cerebellar ataxia.

RESULTS

After a mean disease duration of 6.4 years, 111 patients were not meeting criteria for MSA-C while 24 and 60 patients had a final diagnosis of possible and probable MSA-C, respectively. 16 patients carried an FGF14 (GAA)≥250 expansion in the group not meeting MSA-C criteria (14.4%), 3 patients in the possible MSA-C group (12.5%), but none among probable MSA-C cases. SCA27B patients were evolving more slowly than probable MSA-C patients.

CONCLUSIONS

FGF14 (GAA)≥250 expansion may account for MSA look-alike cases and should be screened among slow progressors.

Recurrent familial case of early childhood sudden death: Complex post mortem genetic investigations

INTRODUCTION

Sudden Unexplained Death in Childhood (SUDC) needs to be fully assessed considering its impact on the family, parents and siblings. Inborn Errors of Metabolism (IEM) such as Medium-Chain Acyl-CoA Dehydrogenase Deficiency (MCADD) should be taken into consideration when SUDC occurres. Our aim is to present a family with two successive SUDC and to discuss the post-mortem genetics investigations revealing an IEM implication.

CASES REPORT

A complete autopsy with genetic testing was performed when the proband, a 4-year-old girl, died. A few years previously, her older brother had died at the same age and off the same condition. Years later, his exhumation was necessary in order to perform a post-mortem diagnosis.The two siblings were revealed to have had the same pathogenic genotype of the ACADM gene, heterozygous substitutions in ACADM (NM_000016.5): c.985 A>G p.(Lys329Glu) and c.347 G>A p.(Cys116Tyr). In addition, they also both carried a VUS in TECRL, a gene implicated in Catecholaminergic Polymorphic Tachycardia Ventricular (CPVT) and SUDC.

CONCLUSION

We illustrate the importance of exome analyses for investigating unexplained sudden death, especially in children, with the possible impact for genetic counselling in the family. The finding of the implication of ACADM gene in this case, raises likely responsibility of the public health system in countries such as France, who delayed implementation of new born screening for these conditions. Exome analyses in this case detected unexpected complexity in interpretation linked to the identification of a second candidate gene for SUDC.

Functional analysis of novel variants identified in cis in the PCCB gene in a patient with propionic acidemia

Next-generation sequencing has improved the diagnosis of inborn errors of metabolism, allowing rapid confirmation of cases detected by clinical/biochemical studies or newborn screening. The challenge, however, remains for establishing the pathogenicity of the identified variants, especially for novel missense changes or small in-frame deletions. In this work we report a propionic acidemia patient exhibiting a severe neonatal form with coma and hyperammonaemia. Genetic analysis identified the previously described pathogenic PCCB variant p.R512C in the maternal allele and two novel PCCB variants in cis in the paternal allele, p.G246del and p.S322F. Expression analysis in a eukaryotic system confirmed the deleterious effect of the novel missense variant and of the one amino acid deletion, as they both exhibited reduced protein levels and reduced or null PCC activity compared to the wild-type construct. Accordingly, the double mutant resulted in no residual activity. This study increases the knowledge of the genotype-phenotype correlations in the rare disease propionic acidemia and highlights the necessity of functional analysis of novel variants to understand their contribution to disease severity and to accurately classify their pathogenic status. In conclusion, two novel PCCB pathogenic variants have been identified, expanding the current mutational spectrum of propionic acidemia.

Natural History and Phenotypic Spectrum of GAA-FGF14 Sporadic Late-Onset Cerebellar Ataxia (SCA27B)

BACKGROUND

Heterozygous GAA expansions in the FGF14 gene have been related to autosomal dominant cerebellar ataxia (SCA27B-MIM:620174). Whether they represent a common cause of sporadic late-onset cerebellar ataxia (SLOCA) remains to be established.

OBJECTIVES

To estimate the prevalence, characterize the phenotypic spectrum, identify discriminative features, and model longitudinal progression of SCA27B in a prospective cohort of SLOCA patients.

METHODS

FGF14 expansions screening combined with longitudinal deep-phenotyping in a prospective cohort of 118 SLOCA patients (onset >40 years of age, no family history of cerebellar ataxia) without a definite diagnosis.

RESULTS

Prevalence of SCA27B was 12.7% (15/118). Higher age of onset, higher Spinocerebellar Degeneration Functional Score, presence of vertigo, diplopia, nystagmus, orthostatic hypotension absence, and sensorimotor neuropathy were significantly associated with SCA27B. Ataxia progression was ≈0.4 points per year on the Scale for Assessment and Rating of Ataxia.

CONCLUSIONS

FGF14 expansion is a major cause of SLOCA. Our natural history data will inform future FGF14 clinical trials. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Recessive NUP54 Variants Underlie Early-Onset Dystonia with Striatal Lesions

Infantile striatonigral degeneration is caused by a homozygous variant of the nuclear-pore complex (NPC) gene NUP62, involved in nucleo-cytoplasmic trafficking. By querying sequencing-datasets of patients with dystonia and/or Leigh(-like) syndromes, we identified 3 unrelated individuals with biallelic variants in NUP54. All variants clustered in the C-terminal protein region that interacts with NUP62. Associated phenotypes were similar to those of NUP62-related disease, including early-onset dystonia with dysphagia, choreoathetosis, and T2-hyperintense lesions in striatum. In silico and protein-biochemical studies gave further evidence for the argument that the variants were pathogenic. We expand the spectrum of NPC component-associated dystonic conditions with localized basal-ganglia abnormalities. ANN NEUROL 2023;93:330-335.

Highlighting the Dystonic Phenotype Related to GNAO1

BACKGROUND

Most reported patients carrying GNAO1 mutations showed a severe phenotype characterized by early-onset epileptic encephalopathy and/or chorea.

OBJECTIVE

The aim was to characterize the clinical and genetic features of patients with mild GNAO1-related phenotype with prominent movement disorders.

METHODS

We included patients diagnosed with GNAO1-related movement disorders of delayed onset (>2 years). Patients experiencing either severe or profound intellectual disability or early-onset epileptic encephalopathy were excluded.

RESULTS

Twenty-four patients and 1 asymptomatic subject were included. All patients showed dystonia as prominent movement disorder. Dystonia was focal in 1, segmental in 6, multifocal in 4, and generalized in 13. Six patients showed adolescence or adulthood-onset dystonia. Seven patients presented with parkinsonism and 3 with myoclonus. Dysarthria was observed in 19 patients. Mild and moderate ID were present in 10 and 2 patients, respectively.

CONCLUSION

We highlighted a mild GNAO1-related phenotype, including adolescent-onset dystonia, broadening the clinical spectrum of this condition. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Heterogeneous clinical features in Cockayne syndrome patients and siblings carrying the same CSA mutations

Background

Cockayne syndrome (CS) is a rare autosomal recessive disorder caused by mutations in ERCC6/CSB or ERCC8/CSA that participate in the transcription-coupled nucleotide excision repair (TC-NER) of UV-induced DNA damage. CS patients display a large heterogeneity of clinical symptoms and severities, the reason of which is not fully understood, and that cannot be anticipated in the diagnostic phase. In addition, little data is available for affected siblings, and this disease is largely undiagnosed in North Africa.

Methods

We report here the clinical description as well as genetic and functional characterization of eight Tunisian CS patients, including siblings. These patients, who belonged to six unrelated families, underwent complete clinical examination and biochemical analyses. Sanger sequencing was performed for the recurrent mutation in five families, and targeted gene sequencing was done for one patient of the sixth family. We also performed Recovery RNA Synthesis (RRS) to confirm the functional impairment of DNA repair in patient-derived fibroblasts.

Results

Six out of eight patients carried a homozygous indel mutation (c.598_600delinsAA) in exon 7 of ERCC8, and displayed a variable clinical spectrum including between siblings sharing the same mutation. The other two patients were siblings who carried a homozygous splice-site variant in ERCC8 (c.843+1G>C). This last pair presented more severe clinical manifestations, which are rarely associated with CSA mutations, leading to gastrostomy and hepatic damage. Impaired TC-NER was confirmed by RRS in six tested patients.

Conclusions

This study provides the first deep characterization of case series of CS patients carrying CSA mutations in North Africa. These mutations have been described only in this region and in the Middle-East. We also provide the largest characterization of multiple unrelated patients, as well as siblings, carrying the same mutation, providing a framework for dissecting elusive genotype–phenotype correlations in CS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02257-1.

Statistical Approach of the Role of the Conserved CSB-PiggyBac Transposase Fusion Protein (CSB-PGBD3) in Genotype-Phenotype Correlation in Cockayne Syndrome Type B

Cockayne syndrome is a rare condition that encompasses a very wide spectrum of clinical severity. Mutations upstream of a transposon called PiggyBac Transposable Element Derived 3 in intron 5 of the CSB/ERCC6 gene could bring about less severe forms than mutations located downstream of that transposon insertion. Our aim was to study genotype-phenotype correlation by determining whether the position of each mutation of the CSB/ERCC6 gene has an impact on the phenotype. A hundred and forty-seven Cockayne patients, who had two pathogenic mutations in the CSB/ERCC6 gene and for whom clinical data was available, were retrospectively selected and included in the study. Data analysis was performed under the Bayesian paradigm. Analysis of the proportion of the different subtypes of Cockayne syndrome according to the position of the mutations was done using an ordinal logistic regression model. Using a vague prior, the risk of developing a more severe subtype when exposed to 2 mutations downstream compared to 2 mutations upstream was 2.0 [0.9–4.5]. Estimations varied through the sensitivity analysis. We could reasonably conclude that a relationship between the number of downstream mutations and the Cockayne syndrome clinical expression exists but it is still difficult to give a precise estimate of this relationship. The real effect could be more complex that the one described in the initial model and other genetic factors might be taken into consideration together with the mutation site to better explain clinical variability.

Identification and Characterization of a Novel Recurrent ERCC6 Variant in Patients with a Severe Form of Cockayne Syndrome B

Cockayne syndrome (CS) is a rare disease caused by mutations in ERCC6/CSB or ERCC8/CSA. We report here the clinical, genetic, and functional analyses of three unrelated patients mutated in ERCC6/CSB with a severe phenotype. After clinical examination, two patients were investigated via next generation sequencing, targeting seventeen Nucleotide Excision Repair (NER) genes. All three patients harbored a novel, c.3156dup, homozygous mutation located in exon 18 of ERCC6/CSB that affects the C-terminal region of the protein. Sanger sequencing confirmed the mutation and the parental segregation in the three families, and Western blots showed a lack of the full-length protein. NER functional impairment was shown by reduced recovery of RNA synthesis with proficient unscheduled DNA synthesis after UV-C radiations in patient-derived fibroblasts. Despite sharing the same mutation, the clinical spectrum was heterogeneous among the three patients, and only two patients displayed clinical photosensitivity. This novel ERCC6 variant in Tunisian patients suggests a founder effect and has implications for setting-up prenatal diagnosis/genetic counselling in North Africa, where this disease is largely undiagnosed. This study reveals one of the rare cases of CS clinical heterogeneity despite the same mutation. Moreover, the occurrence of an identical homozygous mutation, which either results in clinical photosensitivity or does not, strongly suggests that this classic CS symptom relies on multiple factors.

Diagnostic and severity scores for Cockayne syndrome

Background

Cockayne syndrome is a progressive multisystem genetic disorder linked to defective DNA repair and transcription. This rare condition encompasses a very wide spectrum of clinical severity levels ranging from severe prenatal onset to mild adult-onset subtypes. The rarity, complexity and variability of the disease make early diagnosis and severity assessment difficult. Based on similar approaches in other neurodegenerative disorders, we propose to validate diagnostic and severity scores for Cockayne syndrome.

Methods

Clinical, imaging and genetic data were retrospectively collected from 69 molecularly confirmed CS patients. A clinical diagnostic score and a clinical-radiological diagnostic score for CS were built using a multivariable logistic regression model with a stepwise variable selection procedure. A severity score for CS was designed on five items (head circumference, growth failure, neurosensorial signs, motor autonomy, communication skills) and validated by comparison with classical predefined severity subtypes of CS.

Results

Short stature, enophtalmos, hearing loss, cataracts, cutaneous photosensitivity, frequent dental caries, enamel hypoplasia, morphological abnormalities of the teeth, areflexia and spasticity were included in the clinical diagnostic score as being the most statistically relevant criteria. Appropriate weights and thresholds were assigned to obtain optimal sensitivity and specificity (95.7% and 86.4% respectively). The severity score was shown to be able to quantitatively differentiate classical predefined subtypes of CS and confirmed the continuous distribution of the clinical presentations in CS. Longitudinal follow-up of the severity score was able to reflect the natural course of the disease.

Conclusion

The diagnostic and severity scores for CS will facilitate early diagnosis and longitudinal evaluation of future therapeutic interventions. Prospective studies will be needed to confirm these findings.

Expanding the clinical spectrum of STIP1 homology and U-box containing protein 1-associated ataxia

BACKGROUND

STUB1 has been first associated with autosomal recessive (SCAR16, MIM# 615768) and later with dominant forms of ataxia (SCA48, MIM# 618093). Pathogenic variations in STUB1 are now considered a frequent cause of cerebellar ataxia.

OBJECTIVE

We aimed to improve the clinical, radiological, and molecular delineation of SCAR16 and SCA48.

METHODS

Retrospective collection of patients with SCAR16 or SCA48 diagnosed in three French genetic centers (Montpellier, Strasbourg and Nancy).

RESULTS

Here, we report four SCAR16 and nine SCA48 patients from two SCAR16 and five SCA48 unrelated French families. All presented with slowly progressive cerebellar ataxia. Additional findings included cognitive decline, dystonia, parkinsonism and swallowing difficulties. The age at onset was highly variable, ranging from 14 to 76 years. Brain MRI showed marked cerebellar atrophy in all patients. Phenotypic findings associated with STUB1 pathogenic variations cover a broad spectrum, ranging from isolated slowly progressive ataxia to severe encephalopathy, and include extrapyramidal features. We described five new pathogenic variations, two previously reported pathogenic variations, and two rare variants of unknown significance in association with STUB1-related disorders. We also report the first pathogenic variation associated with both dominant and recessive forms of inheritance (SCAR16 and SCA48).

CONCLUSION

Even though differences are observed between the recessive and dominant forms, it appears that a continuum exists between these two entities. While adding new symptoms associated with STUB1 pathogenic variations, we insist on the difficulty of genetic counselling in STUB1-related pathologies. Finally, we underscore the usefulness of DAT-scan as an additional clue for diagnosis.

Growth charts in Cockayne syndrome type 1 and type 2

Cockayne syndrome (CS) is a multisystem degenerative disorder divided in 3 overlapping subtypes, with a continuous phenotypic spectrum: CS2 being the most severe form, CS1 the classical form and CS3 the late-onset form. Failure to thrive and growth difficulties are among the most consistent features of CS, leaving affected individuals vulnerable to numerous medical complications, including adverse effects of undernutrition, abrupt overhydration and overfeeding. There is thus a significant need for specific growth charts. We retrospectively collected growth parameters from genetically-confirmed CS1 and CS2 patients, used the GAMLSS package to construct specific CS growth charts compared to healthy children from WHO and CDC databases. Growth data were obtained from 88 CS patients with a total of 1626 individual growth data points. 49 patients were classified as CS1 and 39 as CS2 with confirmed mutations in CSB/ERCC6, CSA/ERCC8 or ERCC1 genes. Individuals with CS1 initially have normal growth parameters; microcephaly occurs from 2 months whereas onset of weight and height restrictions appear later, between 5 and 22 months. In CS2, growth parameters are already below standard references at birth or drop below the 5th percentile before 3 months. Microcephaly is the first parameter to show a delay, appearing around 2 months in CS1 and at birth in CS2. Height and head circumference are more severely affected in CS2 compared to CS1 whereas weight curves are similar in CS1 and CS2 patients. These new growth charts will serve as a practical tool to improve the nutritional management of children with CS.

X-linked muscular dystrophy in a Labrador Retriever strain: phenotypic and molecular characterisation

Background

Canine models of Duchenne muscular dystrophy (DMD) are a valuable tool to evaluate potential therapies because they faithfully reproduce the human disease. Several cases of dystrophinopathies have been described in canines, but the Golden Retriever muscular dystrophy (GRMD) model remains the most used in preclinical studies. Here, we report a new spontaneous dystrophinopathy in a Labrador Retriever strain, named Labrador Retriever muscular dystrophy (LRMD).

Methods

A colony of LRMD dogs was established from spontaneous cases. Fourteen LRMD dogs were followed-up and compared to the GRMD standard using several functional tests. The disease causing mutation was studied by several molecular techniques and identified using RNA-sequencing.

Results

The main clinical features of the GRMD disease were found in LRMD dogs; the functional tests provided data roughly overlapping with those measured in GRMD dogs, with similar inter-individual heterogeneity. The LRMD causal mutation was shown to be a 2.2-Mb inversion disrupting the DMD gene within intron 20 and involving the TMEM47 gene. In skeletal muscle, the Dp71 isoform was ectopically expressed, probably as a consequence of the mutation. We found no evidence of polymorphism in either of the two described modifier genes LTBP4 and Jagged1. No differences were found in Pitpna mRNA expression levels that would explain the inter-individual variability.

Conclusions

This study provides a full comparative description of a new spontaneous canine model of dystrophinopathy, found to be phenotypically equivalent to the GRMD model. We report a novel large DNA mutation within the DMD gene and provide evidence that LRMD is a relevant model to pinpoint additional DMD modifier genes.

Early-onset nucleotide excision repair disorders with neurological impairment: Clues for early diagnosis and prognostic counseling

Nucleotide excision repair associated diseases comprise overlapping phenotypes and a wide range of outcomes. The early stages still remain under-investigated and underdiagnosed, even although an early recognition of the first symptoms is of utmost importance for appropriate care and genetic counseling. We systematically collected clinical and molecular data from the literature and from newly diagnosed NER patients with neurological impairment, presenting clinical symptoms before the age of 12 months, including foetal cases. One hundred and eighty-five patients were included, 13 with specific symptoms during foetal life. Arthrogryposis, microcephaly, cataracts, and skin anomalies are the most frequently reported signs in early subtypes. Non ERCC6/CSB or ERCC8/CSA genes are overrepresented compared to later onset cohorts: 19% patients of this cohort presented variants in ERCC1, ERCC2/XPD, ERCC3/XPB or ERCC5/XPG. ERCC5/XPG is even the most frequently involved gene in foetal cases (10/13 cases, [4/7 families]). In this cohort, the mutated gene, the age of onset, the type of disease, severe global developmental delay, IUGR and skin anomalies were associated with earlier death. This large survey focuses on specific symptoms that should attract the attention of clinicians towards early-onset NER diagnosis in foetal and neonatal period, without waiting for the completeness of classical criteria.

The Iberian legacy into a young genetic xeroderma pigmentosum cluster in central Brazil

In central Brazil, in the municipality of Faina (state of Goiás), the small and isolated village of Araras comprises a genetic cluster of xeroderma pigmentosum (XP) patients. The high level of consanguinity and the geographical isolation gave rise to a high frequency of XP patients. Recently, two founder events were identified affecting that community, with two independent mutations at the POLH gene, c.764 + 1 G > A (intron 6) and c.907 C > T; p.Arg303* (exon 8). These deleterious mutations lead to the xeroderma pigmentosum variant syndrome (XP-V). Previous reports identified both mutations in other countries: the intron 6 mutation in six patients (four families) from Northern Spain (Basque Country and Cantabria) and the exon 8 mutation in two patients from different families in Europe, one of them from Kosovo. In order to investigate the ancestry of the XP patients and the age for these mutations at Araras, we generated genotyping information for 22 XP-V patients from Brazil (16), Spain (6) and Kosovo (1). The local genomic ancestry and the shared haplotype segments among the patients showed that the intron 6 mutation at Araras is associated with an Iberian genetic legacy. All patients from Goiás, homozygotes for intron 6 mutation, share with the Spanish patients identical-by-descent (IBD) genomic segments comprising the mutation. The entrance date for the Iberian haplotype at the village was calculated to be approximately 200 years old. This result is in agreement with the historical arrival of Iberian individuals at the Goiás state (BR). Patients from Goiás and the three families from Spain share 1.8 cM (family 14), 1.7 cM (family 15), and a more significant segment of 4.7 cM within family 13. On the other hand, the patients carrying the exon 8 mutation do not share any specific genetic segment, indicating an old genetic distance between them or even no common ancestry.

Prenatal diagnosis of cerebro-oculo-facio-skeletal syndrome: Report of three fetuses and review of the literature

Cerebro-oculo-facio-skeletal syndrome (COFS) is a rare autosomal recessive neurodegenerative disease belonging to the family of DNA repair disorders, characterized by microcephaly, congenital cataracts, facial dysmorphism and arthrogryposis. Here, we describe the detailed morphological and microscopic phenotype of three fetuses from two families harboring ERCC5/XPG likely pathogenic variants, and review the five previously reported fetal cases. In addition to the classical features of COFS, the fetuses display thymus hyperplasia, splenomegaly and increased hematopoiesis. Microencephaly is present in the three fetuses with delayed development of the gyri, but normal microscopic anatomy at the supratentorial level. Microscopic anomalies reminiscent of pontocerebellar hypoplasia are present at the infratentorial level. In conclusion, COFS syndrome should be considered in fetuses when intrauterine growth retardation is associated with microcephaly, arthrogryposis and ocular anomalies. Further studies are needed to better understand XPG functions during human development.

Renal disease in Cockayne syndrome

BACKGROUND

Cockayne Syndrome (CS) is a rare autosomal recessive multi-systemic disorder, characterized; by developmental delay, microcephaly, severe growth failure and sensorial impairment. Renal complications have been reported but remain underinvestigated. The objective of this study was to perform a review of renal disease in a cohort of CS patients.

METHODS

We retrospectively collected relevant clinical, biochemical and genetic data from a cohort of 136 genetically confirmed CS patients. Blood pressure (BP), proteinuria, albuminemia, uric acid, creatinine clearance, renal ultrasounds and renal biopsy result were analysed.

RESULTS

Thirty-two patients had a renal investigation. We found that 69% of investigated patients had a renal disorder and/or an elevated BP. Fifteen out of 21 patients (71% of investigated patients) had an increased BP, 10 out of 16 patients (62% of investigated patients) presented with proteinuria and 4 of them had a nephrotic syndrome. Thirteen patients out of 29 (45%) had a decreased Glomerular Filtration Rate (GFR), 18 out of 25 patients (72%) had a hyperuricemia. No correlation with the genetic background or clinical types of CS was found, except for the renal clearance.

CONCLUSIONS

Renal disease, increased blood pressure and hyperuricemia were highly prevalent in our study. We believe that CS patients should benefit from a nephrological follow-up and that anti-uric acid drug and Angiotensin-converting enzyme (ACE) inhibitor should be discussed in these patients.

Uncommon nucleotide excision repair phenotypes revealed by targeted high-throughput sequencing

BACKGROUND

Deficient nucleotide excision repair (NER) activity causes a variety of autosomal recessive diseases including xeroderma pigmentosum (XP) a disorder which pre-disposes to skin cancer, and the severe multisystem condition known as Cockayne syndrome (CS). In view of the clinical overlap between NER-related disorders, as well as the existence of multiple phenotypes and the numerous genes involved, we developed a new diagnostic approach based on the enrichment of 16 NER-related genes by multiplex amplification coupled with next-generation sequencing (NGS).

METHODS

Our test cohort consisted of 11 DNA samples, all with known mutations and/or non pathogenic SNPs in two of the tested genes. We then used the same technique to analyse samples from a prospective cohort of 40 patients. Multiplex amplification and sequencing were performed using AmpliSeq protocol on the Ion Torrent PGM (Life Technologies).

RESULTS

We identified causative mutations in 17 out of the 40 patients (43%). Four patients showed biallelic mutations in the ERCC6(CSB) gene, five in the ERCC8(CSA) gene: most of them had classical CS features but some had very mild and incomplete phenotypes. A small cohort of 4 unrelated classic XP patients from the Basque country (Northern Spain) revealed a common splicing mutation in POLH (XP-variant), demonstrating a new founder effect in this population. Interestingly, our results also found ERCC2(XPD), ERCC3(XPB) or ERCC5(XPG) mutations in two cases of UV-sensitive syndrome and in two cases with mixed XP/CS phenotypes.

CONCLUSIONS

Our study confirms that NGS is an efficient technique for the analysis of NER-related disorders on a molecular level. It is particularly useful for phenotypes with combined features or unusually mild symptoms. Targeted NGS used in conjunction with DNA repair functional tests and precise clinical evaluation permits rapid and cost-effective diagnosis in patients with NER-defects.

Progressive demyelinating neuropathy correlates with clinical severity in Cockayne syndrome

OBJECTIVE

Cockayne syndrome (CS) is characterized by postnatal growth failure and progressive multi-organ dysfunctions. CSA and CSB gene mutations account for the majority of cases and three degrees of severity are delineated. A peripheral neuropathy is known to be associated with CS but the type, severity and correlation of the nerve involvement with CS subtypes remain unknown in genetically identified patients.

METHODS

Clinical and nerve conduction studies (NCS) in 25 CS patients with CSA (n=13) CSB (n=12) mutations.

RESULTS

NCS show a widespread decrease in motor and sensory conduction velocities (CV) in all severe and classical form of CS. In one patient, CV were normal at age 8months but severe slowing was detected at 2years. Conduction block and/or temporal dispersion were observed in 68% of patients.

CONCLUSIONS

CS is associated with a progressive sensory and motor neuropathy. Signs of segmental demyelination, including conduction blocks, may not be obvious before the age of 2years. CV slowing is correlated with the CS clinical severity.

SIGNIFICANCE

NCS should be performed in patients with suspected CS as an additional tool to guide the diagnosis before molecular studies. Further studies focused on NCS course are required in order to assess its relevance as a biomarker in research therapy projects.

Homozygous SMN1 exons 1-6 deletion: pitfalls in genetic counseling and general recommendations for spinal muscular atrophy molecular diagnosis

We report on a rare homozygous intragenic deletion encompassing exons 1-6 of the SMN1 gene in a patient with spinal muscular atrophy (SMA) born into a consanguineous family. This exceptional configuration induced misinterpretation of the molecular defect involved in this patient, who was first reported as having a classic SMN1 exon 7 deletion. This case points out the possible pitfalls in molecular diagnosis of SMA in affected patients and their relatives: exploration of the SMN1 exon 7 (c.840C/T alleles) may be disturbed by several non-pathological or pathological variants around the SMN1 exon 7. In order to accurately describe the molecular defect in an SMA-affected patient, we propose to apply the Human Genome Variation Society nomenclature. This widely accepted nomenclature would improve the reporting of the molecular defect observed in SMA patients and thus would avoid the commonly used but imprecise terminology "absence of SMN1 exon 7."

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) >or= 40 kg m(-2) or BMI standard deviation score >or= 4; P = 6.4 x 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.

Limitations in a frataxin knockdown cell model for Friedreich ataxia in a high-throughput drug screen

BACKGROUND

Pharmacological high-throughput screening (HTS) represents a powerful strategy for drug discovery in genetic diseases, particularly when the full spectrum of pathological dysfunctions remains unclear, such as in Friedreich ataxia (FRDA). FRDA, the most common recessive ataxia, results from a generalized deficiency of mitochondrial and cytosolic iron-sulfur cluster (ISC) proteins activity, due to a partial loss of frataxin function, a mitochondrial protein proposed to function as an iron-chaperone for ISC biosynthesis. In the absence of measurable catalytic function for frataxin, a cell-based assay is required for HTS assay.

METHODS

Using a targeted ribozyme strategy in murine fibroblasts, we have developed a cellular model with strongly reduced levels of frataxin. We have used this model to screen the Prestwick Chemical Library, a collection of one thousand off-patent drugs, for potential molecules for FRDA.

RESULTS

The frataxin deficient cell lines exhibit a proliferation defect, associated with an ISC enzyme deficit. Using the growth defect as end-point criteria, we screened the Prestwick Chemical Library. However no molecule presented a significant and reproducible effect on the proliferation rate of frataxin deficient cells. Moreover over numerous passages, the antisense ribozyme fibroblast cell lines revealed an increase in frataxin residual level associated with the normalization of ISC enzyme activities. However, the ribozyme cell lines and FRDA patient cells presented an increase in Mthfd2 transcript, a mitochondrial enzyme that was previously shown to be upregulated at very early stages of the pathogenesis in the cardiac mouse model.

CONCLUSION

Although no active hit has been identified, the present study demonstrates the feasibility of using a cell-based approach to HTS for FRDA. Furthermore, it highlights the difficulty in the development of a stable frataxin-deficient cell model, an essential condition for productive HTS in the future.

The first cellular models based on frataxin missense mutations that reproduce spontaneously the defects associated with Friedreich ataxia

BACKGROUND

Friedreich ataxia (FRDA), the most common form of recessive ataxia, is due to reduced levels of frataxin, a highly conserved mitochondrial iron-chaperone involved in iron-sulfur cluster (ISC) biogenesis. Most patients are homozygous for a (GAA)(n) expansion within the first intron of the frataxin gene. A few patients, either with typical or atypical clinical presentation, are compound heterozygous for the GAA expansion and a micromutation.

METHODOLOGY

We have developed a new strategy to generate murine cellular models for FRDA: cell lines carrying a frataxin conditional allele were used in combination with an EGFP-Cre recombinase to create murine cellular models depleted for endogenous frataxin and expressing missense-mutated human frataxin. We showed that complete absence of murine frataxin in fibroblasts inhibits cell division and leads to cell death. This lethal phenotype was rescued through transgenic expression of human wild type as well as mutant (hFXN(G130V) and hFXN(I154F)) frataxin. Interestingly, cells expressing the mutated frataxin presented a FRDA-like biochemical phenotype. Though both mutations affected mitochondrial ISC enzymes activities and mitochondria ultrastructure, the hFXN(I154F) mutant presented a more severe phenotype with affected cytosolic and nuclear ISC enzyme activities, mitochondrial iron accumulation and an increased sensitivity to oxidative stress. The differential phenotype correlates with disease severity observed in FRDA patients.

CONCLUSIONS

These new cellular models, which are the first to spontaneously reproduce all the biochemical phenotypes associated with FRDA, are important tools to gain new insights into the in vivo consequences of pathological missense mutations as well as for large-scale pharmacological screening aimed at compensating frataxin deficiency.