Michael P. Kwint

Genome sequencing identifies major causes of severe intellectual disability

Severe intellectual disability (ID) occurs in 0.5% of newborns and is thought to be largely genetic in origin. The extensive genetic heterogeneity of this disorder requires a genome-wide detection of all types of genetic variation. Microarray studies and, more recently, exome sequencing have demonstrated the importance of de novo copy number variations (CNVs) and single-nucleotide variations (SNVs) in ID, but the majority of cases remain undiagnosed. Here we applied whole-genome sequencing to 50 patients with severe ID and their unaffected parents. All patients included had not received a molecular diagnosis after extensive genetic prescreening, including microarray-based CNV studies and exome sequencing. Notwithstanding this prescreening, 84 de novo SNVs affecting the coding region were identified, which showed a statistically significant enrichment of loss-of-function mutations as well as an enrichment for genes previously implicated in ID-related disorders. In addition, we identified eight de novo CNVs, including single-exon and intra-exonic deletions, as well as interchromosomal duplications. These CNVs affected known ID genes more frequently than expected. On the basis of diagnostic interpretation of all de novo variants, a conclusive genetic diagnosis was reached in 20 patients. Together with one compound heterozygous CNV causing disease in a recessive mode, this results in a diagnostic yield of 42% in this extensively studied cohort, and 62% as a cumulative estimate in an unselected cohort. These results suggest that de novo SNVs and CNVs affecting the coding region are a major cause of severe ID. Genome sequencing can be applied as a single genetic test to reliably identify and characterize the comprehensive spectrum of genetic variation, providing a genetic diagnosis in the majority of patients with severe ID.

Next-generation genetic testing for retinitis pigmentosa

Molecular diagnostics for patients with retinitis pigmentosa (RP) has been hampered by extreme genetic and clinical heterogeneity, with 52 causative genes known to date. Here, we developed a comprehensive next‐generation sequencing (NGS) approach for the clinical molecular diagnostics of RP. All known inherited retinal disease genes (n = 111) were captured and simultaneously analyzed using NGS in 100 RP patients without a molecular diagnosis. A systematic data analysis pipeline was developed and validated to prioritize and predict the pathogenicity of all genetic variants identified in each patient, which enabled us to reduce the number of potential pathogenic variants from approximately 1,200 to zero to nine per patient. Subsequent segregation analysis and in silico predictions of pathogenicity resulted in a molecular diagnosis in 36 RP patients, comprising 27 recessive, six dominant, and three X‐linked cases. Intriguingly, De novo mutations were present in at least three out of 28 isolated cases with causative mutations. This study demonstrates the enormous potential and clinical utility of NGS in molecular diagnosis of genetically heterogeneous diseases such as RP. De novo dominant mutations appear to play a significant role in patients with isolated RP, having major implications for genetic counselling. Hum Mutat 33:963–972, 2012.

Targeted next-generation sequencing of a 12.5 Mb homozygous region reveals ANO10 mutations in patients with autosomal-recessive cerebellar ataxia.

Autosomal-recessive cerebellar ataxias comprise a clinically and genetically heterogeneous group of neurodegenerative disorders. In contrast to their dominant counterparts, unraveling the molecular background of these ataxias has proven to be more complicated and the currently known mutations provide incomplete coverage for genotyping of patients. By combining SNP array-based linkage analysis and targeted resequencing of relevant sequences in the linkage interval with the use of next-generation sequencing technology, we identified a mutation in a gene and have shown its association with autosomal-recessive cerebellar ataxia. In a Dutch consanguineous family with three affected siblings a homozygous 12.5 Mb region on chromosome 3 was targeted by array-based sequence capture. Prioritization of all detected sequence variants led to four candidate genes, one of which contained a variant with a high base pair conservation score (phyloP score: 5.26). This variant was a leucine-to-arginine substitution in the DUF 590 domain of a 16K transmembrane protein, a putative calcium-activated chloride channel encoded by anoctamin 10 (ANO10). The analysis of ANO10 by Sanger sequencing revealed three additional mutations: a homozygous mutation (c.1150_1151del [p.Leu384fs]) in a Serbian family and a compound-heterozygous splice-site mutation (c.1476+1G>T) and a frameshift mutation (c.1604del [p.Leu535X]) in a French family. This illustrates the power of using initial homozygosity mapping with next-generation sequencing technology to identify genes involved in autosomal-recessive diseases. Moreover, identifying a putative calcium-dependent chloride channel involved in cerebellar ataxia adds another pathway to the list of pathophysiological mechanisms that may cause cerebellar ataxia.

Pyrosequencing of 16S rRNA gene amplicons to study the microbiota in the gastrointestinal tract of carp (Cyprinus carpio L.)

The microbes in the gastrointestinal (GI) tract are of high importance for the health of the host. In this study, Roche 454 pyrosequencing was applied to a pooled set of different 16S rRNA gene amplicons obtained from GI content of common carp (Cyprinus carpio) to make an inventory of the diversity of the microbiota in the GI tract. Compared to other studies, our culture-independent investigation reveals an impressive diversity of the microbial flora of the carp GI tract. The major group of obtained sequences belonged to the phylum Fusobacteria. Bacteroidetes, Planctomycetes and Gammaproteobacteria were other well represented groups of micro-organisms. Verrucomicrobiae, Clostridia and Bacilli (the latter two belonging to the phylum Firmicutes) had fewer representatives among the analyzed sequences. Many of these bacteria might be of high physiological relevance for carp as these groups have been implicated in vitamin production, nitrogen cycling and (cellulose) fermentation.

Ultra‐deep pyrosequencing of pmoA amplicons confirms the prevalence of Methylomonas and Methylocystis in Sphagnum mosses from a Dutch peat bog

Sphagnum peatlands are important ecosystems in the methane cycle. Methanotrophs in these ecosystems have been shown to reduce methane emissions and provide additional carbon to Sphagnum mosses. However, little is known about the diversity and identity of the methanotrophs present in and on Sphagnum mosses in these peatlands. In this study, we applied a pmoA microarray and high-throughput 454 pyrosequencing to pmoA PCR products obtained from total DNA from Sphagnum mosses from a Dutch peat bog to investigate the presence of methanotrophs and to compare the two different methods. Both techniques showed comparable results and revealed an abundance of Methylomonas and Methylocystis species in the Sphagnum mosses. The advantage of the microarray analysis is that it is fast and cost-effective, especially when many samples have to be screened. Pyrosequencing is superior in providing pmoA sequences of many unknown or uncultivated methanotrophs present in the Sphagnum mosses and, thus, provided much more detailed and quantitative insight into the microbial diversity.

Detection of clinically relevant copy-number variants by exome sequencing in a large cohort of genetic disorders.

PURPOSE Copy-number variation is a common source of genomic variation and an important genetic cause of disease. Microarray-based analysis of copy-number variants (CNVs) has become a first-tier diagnostic test for patients with neurodevelopmental disorders, with a diagnostic yield of 10-20%. However, for most other genetic disorders, the role of CNVs is less clear and most diagnostic genetic studies are generally limited to the study of single-nucleotide variants (SNVs) and other small variants. With the introduction of exome and genome sequencing, it is now possible to detect both SNVs and CNVs using an exome- or genome-wide approach with a single test. METHODS We performed exome-based read-depth CNV screening on data from 2,603 patients affected by a range of genetic disorders for which exome sequencing was performed in a diagnostic setting. RESULTS In total, 123 clinically relevant CNVs ranging in size from 727 bp to 15.3 Mb were detected, which resulted in 51 conclusive diagnoses and an overall increase in diagnostic yield of ~2% (ranging from 0 to -5.8% per disorder). CONCLUSIONS This study shows that CNVs play an important role in a broad range of genetic disorders and that detection via exome-based CNV profiling results in an increase in the diagnostic yield without additional testing, bringing us closer to single-test genomics.Genet Med advance online publication 27 October 2016Genetics in Medicine (2016); doi:10.1038/gim.2016.163.Purpose:Copy-number variation is a common source of genomic variation and an important genetic cause of disease. Microarray-based analysis of copy-number variants (CNVs) has become a first-tier diagnostic test for patients with neurodevelopmental disorders, with a diagnostic yield of 10–20%. However, for most other genetic disorders, the role of CNVs is less clear and most diagnostic genetic studies are generally limited to the study of single-nucleotide variants (SNVs) and other small variants. With the introduction of exome and genome sequencing, it is now possible to detect both SNVs and CNVs using an exome- or genome-wide approach with a single test.Methods:We performed exome-based read-depth CNV screening on data from 2,603 patients affected by a range of genetic disorders for which exome sequencing was performed in a diagnostic setting.Results:In total, 123 clinically relevant CNVs ranging in size from 727 bp to 15.3 Mb were detected, which resulted in 51 conclusive diagnoses and an overall increase in diagnostic yield of ~2% (ranging from 0 to –5.8% per disorder).Conclusions:This study shows that CNVs play an important role in a broad range of genetic disorders and that detection via exome-based CNV profiling results in an increase in the diagnostic yield without additional testing, bringing us closer to single-test genomics.Genet Med advance online publication 27 October 2016

Targeted Next Generation Sequencing Reveals a Novel Intragenic Deletion of the TPO Gene in a Family with Intellectual Disability

BACKGROUNDS AND AIMS Next generation sequencing (NGS) approaches have revolutionized the identification of mutations underlying genetic disorders. This technology is particularly useful for the identification of mutations in known and new genes for conditions with extensive genetic heterogeneity. In the present study we investigated a consanguineous Pakistani family with intellectual disability (ID). METHODS Genotyping was carried out using 250k and 6k SNP microarrays in order to perform homozygosity mapping and copy number variation (CNV) analysis. Targeted NGS was performed to identify the genetic defect in this family. qPCR was performed to validate and confirm the NGS result. RESULTS Homozygosity mapping positioned the causative defect on chromosome 2p25.3-p25.2. Subsequent targeted NGS revealed an intragenic deletion of five exons of the gene TPO. CONCLUSIONS NGS is a powerful method to uncover submicroscopic structural variations. This result demonstrates that an unbiased screening approach such as NGS can help to identify even unexpected disease-causing mutations.

Homozygous mutation of STXBP5L explains an autosomal recessive infantile-onset neurodegenerative disorder

We report siblings of consanguineous parents with an infantile-onset neurodegenerative disorder manifesting a predominant sensorimotor axonal neuropathy, optic atrophy and cognitive deficit. We used homozygosity mapping to identify an ∼12-Mbp interval identical by descent (IBD) between the affected individuals on chromosome 3q13.13-21.1 with an LOD score of 2.31. We combined family-based whole-exome and whole-genome sequencing of parents and affected siblings and, after filtering of likely non-pathogenic variants, identified a unique missense variant in syntaxin-binding protein 5-like (STXBP5L c.3127G>A, p.Val1043Ile [CCDS43137.1]) in the IBD interval. Considering other modes of inheritance, we also found compound heterozygous variants in FMNL3 (c.114G>C, p.Phe38Leu and c.1372T>G, p.Ile458Leu [CCDS44874.1]) located on chromosome 12. STXBP5L (or Tomosyn-2) is expressed in the central and peripheral nervous system and is known to inhibit neurotransmitter release through inhibition of the formation of the SNARE complexes between synaptic vesicles and the plasma membrane. FMNL3 is expressed more widely and is a formin family protein that is involved in the regulation of cell morphology and cytoskeletal organization. The STXBP5L p.Val1043Ile variant enhanced inhibition of exocytosis in comparison with wild-type (WT) STXBP5L. Furthermore, WT STXBP5L, but not variant STXBP5L, promoted axonal outgrowth in manipulated mouse primary hippocampal neurons. However, the FMNL3 p.Phe38Leu and p.Ile458Leu variants showed minimal effects in these cells. Collectively, our clinical, genetic and molecular data suggest that the IBD variant in STXBP5L is the likely cause of the disorder.

Erratum to Next generation genetic testing for retinitis pigmentosa [Human Mutation 33 (2013) 97-102]

Kornelia Neveling, Rob W.J. Collin, Christian Gilissen, Ramon A.C. van Huet, Linda Visser, Michael P. Kwint, Sabine J. Gijsen, Marijke N. Zonneveld, Nienke Wieskamp, Joep de Ligt, Anna M. Siemiatkowska, Lies H. Hoefsloot, Michael F. Buckley, Ulrich Kellner, Kari E. Branham, Anneke I. den Hollander, Alexander Hoischen, Carel Hoyng, B. Jeroen Klevering, L. Ingeborgh van den Born, Joris A. Veltman, Frans P.M. Cremers, and Hans Scheffer

Identifying long indels in exome sequencing data of patients with intellectual disability

Exome sequencing is a powerful tool for detecting both single and multiple nucleotide variation genome wide. However long indels, in the size range 20 – 200bp, remain difficult to accurately detect. By assessing a set of common exonic long indels, we estimate the sensitivity of long indel detection in exome sequencing data to be 92%. To clarify the role of pathogenic long indels in patients with intellectual disability (ID), we analysed exome sequencing data from 820 patients using two variant callers, Pindel and Platypus. We identified three indels explaining the patients’ clinical phenotype by disrupting the UBE3A, PGAP3 and MECP2 genes. Comparison of different tools demonstrated the importance of both correct genotyping and annotation variants. In conclusion, specialized long indel detection can improve diagnostic yield in ID patients.