Paediatrics (RCH) - Research Publications
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ItemNo Preview AvailablePathogenic Variants in GPC4 Cause Keipert Syndrome(CELL PRESS, 2019-05-02)Glypicans are a family of cell-surface heparan sulfate proteoglycans that regulate growth-factor signaling during development and are thought to play a role in the regulation of morphogenesis. Whole-exome sequencing of the Australian family that defined Keipert syndrome (nasodigitoacoustic syndrome) identified a hemizygous truncating variant in the gene encoding glypican 4 (GPC4). This variant, located in the final exon of GPC4, results in premature termination of the protein 51 amino acid residues prior to the stop codon, and in concomitant loss of functionally important N-linked glycosylation (Asn514) and glycosylphosphatidylinositol (GPI) anchor (Ser529) sites. We subsequently identified seven affected males from five additional kindreds with novel and predicted pathogenic variants in GPC4. Segregation analysis and X-inactivation studies in carrier females provided supportive evidence that the GPC4 variants caused the condition. Furthermore, functional studies of recombinant protein suggested that the truncated proteins p.Gln506∗ and p.Glu496∗ were less stable than the wild type. Clinical features of Keipert syndrome included a prominent forehead, a flat midface, hypertelorism, a broad nose, downturned corners of mouth, and digital abnormalities, whereas cognitive impairment and deafness were variable features. Studies of Gpc4 knockout mice showed evidence of the two primary features of Keipert syndrome: craniofacial abnormalities and digital abnormalities. Phylogenetic analysis demonstrated that GPC4 is most closely related to GPC6, which is associated with a bone dysplasia that has a phenotypic overlap with Keipert syndrome. Overall, we have shown that pathogenic variants in GPC4 cause a loss of function that results in Keipert syndrome, making GPC4 the third human glypican to be linked to a genetic syndrome.
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ItemMosaic uniparental disomy results in GM1 gangliosidosis with normal enzyme assay(WILEY, 2018-01)Inherited metabolic disorders are traditionally diagnosed using broad and expensive panels of screening tests, often including invasive skin and muscle biopsy. Proponents of next-generation genetic sequencing have argued that replacing these screening panels with whole exome sequencing (WES) would save money. Here, we present a complex patient in whom WES allowed diagnosis of GM1 gangliosidosis, caused by homozygous GLB1 mutations, resulting in β-galactosidase deficiency. A 10-year-old girl had progressive neurologic deterioration, macular cherry-red spot, and cornea verticillata. She had marked clinical improvement with initiation of the ketogenic diet. Comparative genomic hybridization microarray showed mosaic chromosome 3 paternal uniparental disomy (UPD). GM1 gangliosidosis was suspected, however β-galactosidase assay was normal. Trio WES identified a paternally-inherited pathogenic splice-site GLB1 mutation (c.75+2dupT). The girl had GM1 gangliosidosis; however, enzymatic testing in blood was normal, presumably compensated for by non-UPD cells. Severe neurologic dysfunction occurred due to disruptive effects of UPD brain cells.
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ItemHFE p.C282Y homozygosity predisposes to rapid serum ferritin rise after menopause: A genotype-stratified cohort study of hemochromatosis in Australian women(WILEY, 2017-04)BACKGROUND AND AIM: Women who are homozygous for the p.C282Y mutation in the HFE gene are at much lower risk of iron overload-related disease than p.C282Y homozygous men, presumably because of the iron-depleting effects of menstruation and pregnancy. We used data from a population cohort study to model the impact of menstruation cessation at menopause on serum ferritin (SF) levels in female p.C282Y homozygotes, with p.C282Y/p.H63D simple or compound heterozygotes and those with neither p.C282Y nor p.H63D mutations (HFE wild types) as comparison groups. METHODS: A sample of the Melbourne Collaborative Cohort Study was selected for the "HealthIron" study (n = 1438) including all HFE p.C282Y homozygotes plus a random sample stratified by HFE-genotype (p.C282Y and p.H63D). The relationship between the natural logarithm of SF and time since menopause was examined using linear mixed models incorporating spline smoothing. RESULTS: For p.C282Y homozygotes, SF increased by a factor of 3.6 (95% CI (1.8, 7.0), P < 0.001) during the first 10 years postmenopause, after which SF continued to increase but at less than half the previous rate. In contrast, SF profiles for other HFE genotype groups increase more gradually and did not show a distinction between premenopausal and postmenopausal SF levels. Only p.C282Y homozygotes had predicted SF exceeding 200 μg/L postmenopause, but the projected SF did not increase the risk of iron overload-related disease. CONCLUSIONS: These data provide the first documented evidence that physiological blood loss is a major factor in determining the marked gender difference in expression of p.C282Y homozygosity.
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ItemNo Preview AvailableRecessive variants in ZNF142 cause a complex neurodevelopmental disorder with intellectual disability, speech impairment, seizures, and dystonia(NATURE PUBLISHING GROUP, 2019-11)PURPOSE: The purpose of this study was to expand the genetic architecture of neurodevelopmental disorders, and to characterize the clinical features of a novel cohort of affected individuals with variants in ZNF142, a C2H2 domain-containing transcription factor. METHODS: Four independent research centers used exome sequencing to elucidate the genetic basis of neurodevelopmental phenotypes in four unrelated families. Following bioinformatic filtering, query of control data sets, and secondary variant confirmation, we aggregated findings using an online data sharing platform. We performed in-depth clinical phenotyping in all affected individuals. RESULTS: We identified seven affected females in four pedigrees with likely pathogenic variants in ZNF142 that segregate with recessive disease. Affected cases in three families harbor either nonsense or frameshifting likely pathogenic variants predicted to undergo nonsense mediated decay. One additional trio bears ultrarare missense variants in conserved regions of ZNF142 that are predicted to be damaging to protein function. We performed clinical comparisons across our cohort and noted consistent presence of intellectual disability and speech impairment, with variable manifestation of seizures, tremor, and dystonia. CONCLUSION: Our aggregate data support a role for ZNF142 in nervous system development and add to the emergent list of zinc finger proteins that contribute to neurocognitive disorders.
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ItemInvestigating and Correcting Plasma DNA Sequencing Coverage Bias to Enhance Aneuploidy Discovery(PUBLIC LIBRARY SCIENCE, 2014-01-29)Pregnant women carry a mixture of cell-free DNA fragments from self and fetus (non-self) in their circulation. In recent years multiple independent studies have demonstrated the ability to detect fetal trisomies such as trisomy 21, the cause of Down syndrome, by Next-Generation Sequencing of maternal plasma. The current clinical tests based on this approach show very high sensitivity and specificity, although as yet they have not become the standard diagnostic test. Here we describe improvements to the analysis of the sequencing data by reducing GC bias and better handling of the genomic repeats. We show substantial improvements in the sensitivity of the standard trisomy 21 statistical tests, which we measure by artificially reducing read coverage. We also explore the bias stemming from the natural cleavage of plasma DNA by examining DNA motifs and position specific base distributions. We propose a model to correct this fragmentation bias and observe that incorporating this bias does not lead to any further improvements in the detection of fetal trisomy. The improved bias corrections that we demonstrate in this work can be readily adopted into existing fetal trisomy detection protocols and should also lead to improvements in sub-chromosomal copy number variation detection.
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ItemReducing the exome search space for Mendelian diseases using genetic linkage analysis of exome genotypes(BIOMED CENTRAL LTD, 2011)Many exome sequencing studies of Mendelian disorders fail to optimally exploit family information. Classical genetic linkage analysis is an effective method for eliminating a large fraction of the candidate causal variants discovered, even in small families that lack a unique linkage peak. We demonstrate that accurate genetic linkage mapping can be performed using SNP genotypes extracted from exome data, removing the need for separate array-based genotyping. We provide software to facilitate such analyses.
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ItemWhole exome sequencing combined with linkage analysis identifies a novel 3 bp deletion in NR5A1(SPRINGERNATURE, 2015-04)Disorders of sex development (DSDs) encompass a broad spectrum of conditions affecting the development of the gonads and genitalia. The underlying causes for DSDs include gain or loss of function variants in genes responsible for gonad development or steroidogenesis. Most patients with DSD have an unknown genetic etiology and cannot be given an accurate diagnosis. We used whole exome capture and massively parallel sequencing to analyse a large family with 46,XY DSD and 46,XX premature ovarian insufficiency. In addition, we used a recently developed method for linkage analysis using genotypes extracted from the MPS data. This approach identified a unique linkage peak on chromosome 9 and a novel, 3 bp, in-frame deletion in exon six of NR5A1 (steroidogenic factor-1 or SF1) in all affected individuals. We confirmed that the variant disrupts the SF1 protein and its ability to bind and regulate downstream genes. NR5A1 has key roles at multiple points in gonad development and steroidogenic pathways. The variant described here affects the function of SF1 in early testis development and later ovarian function, ultimately leading to the 46,XY DSD and 46,XX premature ovarian insufficiency phenotypes, respectively. This study shows that even at low coverage, whole exome sequencing, when combined with linkage analysis, can be a powerful tool to identify rapidly the disease-causing variant in large pedigrees.
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ItemCpipe: a shared variant detection pipeline designed for diagnostic settings(BIOMED CENTRAL LTD, 2015-07-10)The benefits of implementing high throughput sequencing in the clinic are quickly becoming apparent. However, few freely available bioinformatics pipelines have been built from the ground up with clinical genomics in mind. Here we present Cpipe, a pipeline designed specifically for clinical genetic disease diagnostics. Cpipe was developed by the Melbourne Genomics Health Alliance, an Australian initiative to promote common approaches to genomics across healthcare institutions. As such, Cpipe has been designed to provide fast, effective and reproducible analysis, while also being highly flexible and customisable to meet the individual needs of diverse clinical settings. Cpipe is being shared with the clinical sequencing community as an open source project and is available at http://cpipeline.org.
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ItemNeuropathology of childhood-onset basal ganglia degeneration caused by mutation of VAC14(WILEY, 2017-12)OBJECTIVE: To characterize the clinical features and neuropathology associated with recessive VAC14 mutations. METHODS: Whole-exome sequencing was used to identify the genetic etiology of a rapidly progressive neurological disease presenting in early childhood in two deceased siblings with distinct neuropathological features on post mortem examination. RESULTS: We identified compound heterozygous variants in VAC14 in two deceased siblings with early childhood onset of severe, progressive dystonia, and neurodegeneration. Their clinical phenotype is consistent with the VAC14-related childhood-onset, striatonigral degeneration recently described in two unrelated children. Post mortem examination demonstrated prominent vacuolation associated with degenerating neurons in the caudate nucleus, putamen, and globus pallidus, similar to previously reported ex vivo vacuoles seen in the late-endosome/lysosome of VAC14-deficient neurons. We identified upregulation of ubiquitinated granules within the cell cytoplasm and lysosomal-associated membrane protein (LAMP2) around the vacuole edge to suggest a process of vacuolation of lysosomal structures associated with active autophagocytic-associated neuronal degeneration. INTERPRETATION: Our findings reveal a distinct clinicopathological phenotype associated with recessive VAC14 mutations.
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ItemRecent advances in the detection of repeat expansions with short-read next-generation sequencing.(F1000 Research Ltd, 2018)Short tandem repeats (STRs), also known as microsatellites, are commonly defined as consisting of tandemly repeated nucleotide motifs of 2-6 base pairs in length. STRs appear throughout the human genome, and about 239,000 are documented in the Simple Repeats Track available from the UCSC (University of California, Santa Cruz) genome browser. STRs vary in size, producing highly polymorphic markers commonly used as genetic markers. A small fraction of STRs (about 30 loci) have been associated with human disease whereby one or both alleles exceed an STR-specific threshold in size, leading to disease. Detection of repeat expansions is currently performed with polymerase chain reaction-based assays or with Southern blots for large expansions. The tests are expensive and time-consuming and are not always conclusive, leading to lengthy diagnostic journeys for patients, potentially including missed diagnoses. The advent of whole exome and whole genome sequencing has identified the genetic cause of many genetic disorders; however, analysis pipelines are focused primarily on the detection of short nucleotide variations and short insertions and deletions (indels). Until recently, repeat expansions, with the exception of the smallest expansion (SCA6), were not detectable in next-generation short-read sequencing datasets and would have been ignored in most analyses. In the last two years, four analysis methods with accompanying software (ExpansionHunter, exSTRa, STRetch, and TREDPARSE) have been released. Although a comprehensive comparative analysis of the performance of these methods across all known repeat expansions is still lacking, it is clear that these methods are a valuable addition to any existing analysis pipeline. Here, we detail how to assess short-read data for evidence of expansions, reviewing all four methods and outlining their strengths and weaknesses. Implementation of these methods should lead to increased diagnostic yield of repeat expansion disorders for known STR loci and has the potential to detect novel repeat expansions.