a-novel-strategy-combining-array-cgh-whole-exome-sequencing-utero

A Novel Strategy Combining Array-CGH, Whole-exome Sequencing and <em>In Utero</em> Electroporation in Rodents to Identify Causative Genes for Brain Malformations

Birth defects that involve the cerebral cortex &#8212; also known as malformations of cortical development (MCD) &#8212; are important causes of ...

Wellcome Trust Centre for Human Genetics

The development of next generation sequencing (NGS) has radically transformed the scientific landscape, making it possible to sequence the exome of any given individual in a cost-effective way. The power of this approach has been demonstrated by a number of groups who have identified pathogenic mutations in small pedigrees that have been resistant to traditional genetic mapping. Recently it has become clear that exome sequencing has great potential with respect to sporadic disease and the identification of de novo mutations. This is highlighted by studies reporting whole-exome sequencing of patient-parental trios affected by learning disability, autism and schizophrenia. It is widely anticipated that the introduction of this technique into a clinical setting will revolutionise genetic diagnosis. However, the sensitivity of NGS exome sequencing is currently unclear. Here, we describe the exome sequencing of DNA samples from a patient with double cortex syndrome and her parents, resulting in the detection of a mosaic splicing mutation in LIS1. This variant was found at an allele frequency of just 18%, demonstrating that NGS methods have the capacity to identify pathogenic mosaic mutations present at a low level.

Exome sequencing can detect pathogenic mosaic mutations present at low allele frequencies.

Inherited retinal degeneration (IRD) is a common cause of visual impairment (prevalence ∼1/3500). There is considerable phenotype and genotype heterogeneity, making a specific diagnosis very difficult without molecular testing. We investigated targeted capture combined with next-generation sequencing using Nimblegen 12plex arrays and the Roche 454 sequencing platform to explore its potential for clinical diagnostics in two common types of IRD, retinitis pigmentosa and cone-rod dystrophy. 50 patients (36 unknowns and 14 positive controls) were screened, and pathogenic mutations were identified in 25% of patients in the unknown, with 53% in the early-onset cases. All patients with new mutations detected had an age of onset <21 years and 44% had a family history. Thirty-one percent of mutations detected were novel. A de novo mutation in rhodopsin was identified in one early-onset case without a family history. Bioinformatic pipelines were developed to identify likely pathogenic mutations and stringent criteria were used for assignment of pathogenicity. Analysis of sequencing metrics revealed significant variability in capture efficiency and depth of coverage. We conclude that targeted capture and next-generation sequencing are likely to be very useful in a diagnostic setting, but patients with earlier onset of disease are more likely to benefit from using this strategy. The mutation-detection rate suggests that many patients are likely to have mutations in novel genes.

Next-generation sequencing (NGS) as a diagnostic tool for retinal degeneration reveals a much higher detection rate in early-onset disease.

β-III spectrin is present in the brain and is known to be important in the function of the cerebellum. Heterozygous mutations in SPTBN2, the gene encoding β-III spectrin, cause Spinocerebellar Ataxia Type 5 (SCA5), an adult-onset, slowly progressive, autosomal-dominant pure cerebellar ataxia. SCA5 is sometimes known as "Lincoln ataxia," because the largest known family is descended from relatives of the United States President Abraham Lincoln. Using targeted capture and next-generation sequencing, we identified a homozygous stop codon in SPTBN2 in a consanguineous family in which childhood developmental ataxia co-segregates with cognitive impairment. The cognitive impairment could result from mutations in a second gene, but further analysis using whole-genome sequencing combined with SNP array analysis did not reveal any evidence of other mutations. We also examined a mouse knockout of β-III spectrin in which ataxia and progressive degeneration of cerebellar Purkinje cells has been previously reported and found morphological abnormalities in neurons from prefrontal cortex and deficits in object recognition tasks, consistent with the human cognitive phenotype. These data provide the first evidence that β-III spectrin plays an important role in cortical brain development and cognition, in addition to its function in the cerebellum; and we conclude that cognitive impairment is an integral part of this novel recessive ataxic syndrome, Spectrin-associated Autosomal Recessive Cerebellar Ataxia type 1 (SPARCA1). In addition, the identification of SPARCA1 and normal heterozygous carriers of the stop codon in SPTBN2 provides insights into the mechanism of molecular dominance in SCA5 and demonstrates that the cell-specific repertoire of spectrin subunits underlies a novel group of disorders, the neuronal spectrinopathies, which includes SCA5, SPARCA1, and a form of West syndrome.

Recessive mutations in SPTBN2 implicate β-III spectrin in both cognitive and motor development.

To assess factors influencing the success of whole-genome sequencing for mainstream clinical diagnosis, we sequenced 217 individuals from 156 independent cases or families across a broad spectrum of disorders in whom previous screening had identified no pathogenic variants. We quantified the number of candidate variants identified using different strategies for variant calling, filtering, annotation and prioritization. We found that jointly calling variants across samples, filtering against both local and external databases, deploying multiple annotation tools and using familial transmission above biological plausibility contributed to accuracy. Overall, we identified disease-causing variants in 21% of cases, with the proportion increasing to 34% (23/68) for mendelian disorders and 57% (8/14) in family trios. We also discovered 32 potentially clinically actionable variants in 18 genes unrelated to the referral disorder, although only 4 were ultimately considered reportable. Our results demonstrate the value of genome sequencing for routine clinical diagnosis but also highlight many outstanding challenges.

Factors influencing success of clinical genome sequencing across a broad spectrum of disorders.

Current screening methods for ovarian cancer can only detect advanced disease. Earlier detection has proved difficult because the molecular precursors involved in the natural history of the disease are unknown. To identify early driver mutations in ovarian cancer cells, we used dense whole genome sequencing of micrometastases and microscopic residual disease collected at three time points over three years from a single patient during treatment for high-grade serous ovarian cancer (HGSOC). The functional and clinical significance of the identified mutations was examined using a combination of population-based whole genome sequencing, targeted deep sequencing, multi-center analysis of protein expression, loss of function experiments in an in-vivo reporter assay and mammalian models, and gain of function experiments in primary cultured fallopian tube epithelial (FTE) cells. We identified frequent mutations involving a 40kb distal repressor region for the key stem cell differentiation gene SOX2. In the apparently normal FTE, the region was also mutated. This was associated with a profound increase in SOX2 expression (p<2(-16)), which was not found in patients without cancer (n=108). Importantly, we show that SOX2 overexpression in FTE is nearly ubiquitous in patients with HGSOCs (n=100), and common in BRCA1-BRCA2 mutation carriers (n=71) who underwent prophylactic salpingo-oophorectomy. We propose that the finding of SOX2 overexpression in FTE could be exploited to develop biomarkers for detecting disease at a premalignant stage, which would reduce mortality from this devastating disease.

Premalignant SOX2 overexpression in the fallopian tubes of ovarian cancer patients: Discovery and validation studies.

The aim of this study was to identify the causative mutation in a family with an unusual presentation of autosomal dominant osteopetrosis (OPT), proximal renal tubular acidosis (RTA), renal stones, epilepsy, and blindness, a combination of features not previously reported. We undertook exome sequencing of one affected and one unaffected family member, followed by targeted analysis of known candidate genes to identify the causative mutation. This identified a missense mutation (c.643G>A; p.Gly215Arg) in the gene encoding the chloride/proton antiporter 7 (gene CLCN7, protein CLC-7), which was confirmed by amplification refractory mutation system (ARMS)-PCR, and to be present in the three available patients. CLC-7 mutations are known to cause autosomal dominant OPT type 2, also called Albers-Schonberg disease, which is characterized by osteosclerosis, predominantly of the spine, pelvis and skull base, resulting in bone fragility and fractures. Albers-Schonberg disease is not reported to be associated with RTA, but autosomal recessive OPT type 3 (OPTB3) with RTA is associated with carbonic anhydrase type 2 (CA2) mutations. No mutations were detected in CA2 or any other genes known to cause proximal RTA. Neither CLCN7 nor CA2 mutations have previously been reported to be associated with renal stones or epilepsy. Thus, we identified a CLCN7 mutation in a family with autosomal dominant osteopetrosis, RTA, renal stones, epilepsy, and blindness. © 2016 Wiley Periodicals, Inc.

Autosomal dominant osteopetrosis associated with renal tubular acidosis is due to a CLCN7 mutation.

Identifying variants from RNA-seq (transcriptome sequencing) data is a cost-effective and versatile alternative to whole-genome sequencing. However, current variant callers do not generally behave well with RNA-seq data due to reads encompassing intronic regions. We have developed a software programme called Opossum to address this problem. Opossum pre-processes RNA-seq reads prior to variant calling, and although it has been designed to work specifically with Platypus, it can be used equally well with other variant callers such as GATK HaplotypeCaller. In this work, we show that using Opossum in conjunction with either Platypus or GATK HaplotypeCaller maintains precision and improves the sensitivity for SNP detection compared to the GATK Best Practices pipeline. In addition, using it in combination with Platypus offers a substantial reduction in run times compared to the GATK pipeline so it is ideal when there are only limited time or computational resources available.

Stefano Lise

Wellcome Trust Centre for Human Genetics

A Novel Strategy Combining Array-CGH, Whole-exome Sequencing and In Utero Electroporation in Rodents to Identify Causative Genes for Brain Malformations