Two novel variants in the ATM gene causing ataxia‐telangiectasia,including a duplication of 90 kb: Utility of targeted next‐generation sequencing in detection of copy number variation

Ataxia‐telangiectasia (A‐T) is a rare autosomal recessive neurodegenerative disorder characterized by progressive cerebellar ataxia, ocular apraxia, immunodeficiency, telangiectasia, elevated serum α‐fetoprotein concentration, radiosensitivity and cancer predisposition. Classical A‐T is caused by biallelic variants on ATM (ataxia telangiectasia mutated) gene, leading to a loss of function of the protein kinase ATM, involved in DNA damage repair. Atypical presentations can be found in A‐T‐like disease or in Nijmegen breakage syndrome, caused by deficiency of mre11 or nibrin proteins, respectively. In this report, we present the genetic characterization of a 4‐year‐old female with clinical diagnosis of A‐T. Next‐generation sequencing (NGS) revealed two novel heterozygous mutations in the ATM gene: a single‐nucleotide variant (SNV) at exon 47 (NM_000051.3:c.6899G > C; p.Trp2300Ser) and ～90 kb genomic duplication spanning exons 17–61, NG_009830.1:g.(41245_49339)_(137044_147250)dup. These findings were validated by Sanger sequencing and MLPA (multiplex ligation‐dependent probe amplification) analysis respectively. Familial segregation study confirmed that the two variants are inherited, and the infant is a compound heterozygote. Thus, our study expands the spectrum of ATM pathogenic variants and demonstrates the utility of targeted NGS in the detection of copy number variation.     

A child with autism,behavioral issues,and dysmorphic features found to have a tandem duplication within CTNND2 by mate‐pair sequencing

We describe a 5‐year‐old male with developmental delay, behavioral problems, and dysmorphic features who was found by microarray to have a 93‐kb duplication of uncertain significance that fully encompasses the third exon of CTNND2 (delta catenin). Mate‐pair sequencing was used to determine that the duplication is tandem and is predicted to lead to CTNND2 haploinsufficiency. Haploinsufficiency for CTNND2 has been shown to result in developmental delay and intellectual disability, providing a unifying diagnosis for this patient. His features overlap those associated with the larger cri‐du‐chat deletion of this region, expanding the clinical phenotype of isolated CTNND2 variants. The use of mate‐pair sequencing to determine the orientation of the small duplication was essential to the diagnosis and avoided the use of exome sequencing, which would not have defined the arrangement of the duplication. This is only the second reported patient, to our knowledge, with a single exon duplication of CTNND2.     

Whole exome sequencing is an efficient,sensitive and specific method for determining the genetic cause of short‐rib thoracic dystrophies

Short‐rib thoracic dystrophies (SRTDs) are congenital disorders due to defects in primary cilium function. SRTDs are recessively inherited with mutations identified in 14 genes to date (comprising 398 exons). Conventional mutation detection (usually by iterative Sanger sequencing) is inefficient and expensive, and often not undertaken. Whole exome massive parallel sequencing has been used to identify new genes for SRTD (WDR34, WDR60 and IFT172); however, the clinical utility of whole exome sequencing (WES) has not been established. WES was performed in 11 individuals with SRTDs. Compound heterozygous or homozygous mutations were identified in six confirmed SRTD genes in 10 individuals (IFT172, DYNC2H1, TTC21B, WDR60, WDR34 and NEK1), giving overall sensitivity of 90.9%. WES data from 993 unaffected individuals sequenced using similar technology showed two individuals with rare (minor allele frequency <0.005) compound heterozygous variants of unknown significance in SRTD genes (specificity >99%). Costs for consumables, laboratory processing and bioinformatic analysis were <AU$850 per sample. WES is sensitive, specific, efficient and cost‐effective for mutation screening as well as gene discovery in SRTDs and can be considered a first‐line methodology for mutation identification in affected individuals.     

Novel biallelic SZT2 mutations in 3 cases of early‐onset epileptic encephalopathy

The seizure threshold 2 (SZT2) gene encodes a large, highly conserved protein that is associated with epileptogenesis. In mice, Szt2 is abundantly expressed in the central nervous system. Recently, biallelic SZT2 mutations were found in 7 patients (from 5 families) presenting with epileptic encephalopathy with dysmorphic features and/or non‐syndromic intellectual disabilities. In this study, we identified by whole‐exome sequencing compound heterozygous SZT2 mutations in 3 patients with early‐onset epileptic encephalopathies. Six novel SZT2 mutations were found, including 3 truncating, 1 splice site and 2 missense mutations. The splice‐site mutation resulted in skipping of exon 20 and was associated with a premature stop codon. All individuals presented with seizures, severe developmental delay and intellectual disabilities with high variability. Brain MRIs revealed a characteristic thick and short corpus callosum or a persistent cavum septum pellucidum in each of the 2 cases. Interestingly, in the third case, born to consanguineous parents, had unexpected compound heterozygous missense mutations. She showed microcephaly despite the other case and previous ones presenting with macrocephaly, suggesting that SZT2 mutations might affect head size.     

Compound heterozygosity for loss‐of‐function FARSB variants in a patient with classic features of recessive aminoacyl‐tRNA synthetase‐related disease

Aminoacyl‐tRNA synthetases (ARSs) are ubiquitously expressed enzymes that ligate amino acids onto tRNA molecules. Genes encoding ARSs have been implicated in phenotypically diverse dominant and recessive human diseases. The charging of tRNA^PHE with phenylalanine is performed by a tetrameric enzyme that contains two alpha (FARSA) and two beta (FARSB) subunits. To date, mutations in the genes encoding these subunits (FARSA and FARSB) have not been implicated in any human disease. Here, we describe a patient with a severe, lethal, multisystem, developmental phenotype who was compound heterozygous for FARSB variants: p.Thr256Met and p.His496Lysfs*14. Expression studies using fibroblasts isolated from the proband revealed a severe depletion of both FARSB and FARSA protein levels. These data indicate that the FARSB variants destabilize total phenylalanyl‐tRNA synthetase levels, thus causing a loss‐of‐function effect. Importantly, our patient shows strong phenotypic overlap with patients that have recessive diseases associated with other ARS loci; these observations strongly support the pathogenicity of the identified FARSB variants and are consistent with the essential function of phenylalanyl‐tRNA synthetase in human cells. In sum, our clinical, genetic, and functional analyses revealed the first FARSB variants associated with a human disease phenotype and expand the locus heterogeneity of ARS‐related human disease.     

Female‐restricted syndromic intellectual disability in a patient from Thailand

Female‐restricted syndromic intellectual disability (ID) is a neurodevelopmental disorder with developmental delay (DD)/ID, facial dysmorphism, and diverse congenital anomalies comprising heart defects, anal anomalies, choanal atresia, postaxial polydactyly, scoliosis, and brain abnormalities. Loss‐of‐function mutations in the USP9X gene inherited as X‐linked dominance were identified as its etiology in females of different ethnic groups. Here, we report a 15‐year‐old Thai girl harboring a novel de novo heterozygous one‐base pair deletion (c.3508delG, p.Val1170TrpfsX9) in exon 23 of USP9X. Her profound DD, dysmorphic face including attached earlobes, short stature, and congenital malformations including s‐shaped thoracolumbar scoliosis, hip dislocation, and generalized brain atrophy shared common characteristics of X‐linked syndromic ID. We have observed severely malformed oro‐dental organs and a choledochal cyst, which have never been reported. Our study presents the first patient from Thailand expanding the phenotypic and mutational spectra of the syndrome.     

Novel LDLR Variants in Patients with Familial Hypercholesterolemia: In Silico Analysis as a Tool to Predict Pathogenic Variants in Children and Their Families

Familial hypercholesterolemia (FH) is an autosomal dominant disease with a frequency of 1:500 in its heterozygous form. To date, mutations in the low‐density lipoprotein receptor gene (LDLR) are the only identified causes of FH in the Greek population, causing high levels of low‐density lipoprotein (LDL) and total cholesterol and premature atherosclerosis. The Greek FH population is genetically homogeneous, but most previous studies screened for the most common mutations only. The study aimed to characterize and assess novel LDLR variants. LDLR was examined by whole‐gene DNA sequencing in 561 FH patients from 262 families of Greek origin. Novel LDLR variants were analyzed in silico using various software predicting pathogenicity and changes in protein stability. Twelve novel LDLR variants were identified, six of which are putative disease‐causing variants: c.977C>G in exon 7, c.1124A>C in exon 8, c.1381G>T in exon 10, c.628_643dup{636del}, c.661–673dup in exon 4, and 13 c.1987+1_+33del in intron 13. All six putative variants were confirmed in the hypercholesterolemic members of the family. The results show that in silico analysis is a valuable tool to predict potential pathogenicity of novel variants, especially for populations that have not been extensively studied. The identification of novel pathogenic variants will facilitate the molecular diagnosis of FH from early childhood.     

Prenatal presentation and diagnosis of Baraitser‐Winter syndrome using exome sequencing

Baraitser‐Winter cerebrofrontofacial syndrome (BWCFF) is a rare autosomal dominant developmental disorder associated with missense mutations in the genes ACTB or ACTG1. The classic presentation of BWCFF is discerned by the combination of unique craniofacial characteristics including ocular coloboma, intellectual disability, and hypertelorism. Congenital contractures and organ malformations are often present, including structural defects in the brain, heart, renal, and musculoskeletal system. However, there is limited documentation regarding its prenatal presentation that may encourage healthcare providers to be aware of this disorder when presented throughout pregnancy. Herein we describe a case of a pregnancy with large cystic hygroma and omphalocele. Whole exome sequencing (WES) was performed and a de novo, heterozygous, likely pathogenic mutation in ACTB was detected, c.1004G>A (p.Arg335His), conferring a diagnosis of BWCFF.     

Novel,Compound Heterozygous,Single‐Nucleotide Variants in MARS2 Associated with Developmental Delay,Poor Growth,and Sensorineural Hearing Loss

Novel, single‐nucleotide mutations were identified in the mitochondrial methionyl amino‐acyl tRNA synthetase gene (MARS2) via whole exome sequencing in two affected siblings with developmental delay, poor growth, and sensorineural hearing loss.We show that compound heterozygous mutations c.550C>T:p.Gln 184* and c.424C>T:p.Arg142Trp in MARS2 lead to decreased MARS2 protein levels in patient lymphoblasts. Analysis of respiratory complex enzyme activities in patient fibroblasts revealed decreased complex I and IV activities. Immunoblotting of patient fibroblast and lymphoblast samples revealed reduced protein levels of NDUFB8 and COXII, representing complex I and IV, respectively. Additionally, overexpression of wild‐type MARS2 in patient fibroblasts increased NDUFB8 and COXII protein levels. These findings suggest that recessive single‐nucleotide mutations in MARS2 are causative for a new mitochondrial translation deficiency disorder with a primary phenotype including developmental delay and hypotonia. Identification of additional patients with single‐nucleotide mutations in MARS2 is necessary to determine if pectus carinatum is also a consistent feature of this syndrome.     

Gain‐of‐function pathogenic variants in SMAD4 are associated with neoplasia in Myhre syndrome

Myhre syndrome is an increasingly diagnosed rare syndrome that is caused by one of two specific heterozygous gain‐of‐function pathogenic variants in SMAD4. The phenotype includes short stature, characteristic facial appearance, hearing loss, laryngotracheal stenosis, arthritis, skeletal abnormalities, learning and social challenges, distinctive cardiovascular defects, and a striking fibroproliferative response in the ear canals, airways, and serosal cavities (peritoneum, pleura, pericardium). Confirmation of the clinical diagnosis is usually prompted by the characteristic appearance with developmental delay and autistic‐like behavior using targeted gene sequencing or by whole exome sequencing. We describe six patients (two not previously reported) with molecularly confirmed Myhre syndrome and neoplasia. Loss‐of‐function pathogenic variants in SMAD4 cause juvenile polyposis syndrome and we hypothesize that the gain‐of‐function pathogenic variants observed in Myhre syndrome may contribute to neoplasia in the patients reported herein. The frequency of neoplasia (9.8%, 6/61) in this series (two new, four reported patients) and endometrial cancer (8.8%, 3/34, mean age 40 years) in patients with Myhre syndrome, raises the possibility of cancer susceptibility in these patients. We alert clinicians to the possibility of detecting this syndrome when cancer screening panels are used. We propose that patients with Myhre syndrome are more susceptible to neoplasia, encourage increased awareness, and suggest enhanced clinical monitoring.     

Deletions Overlapping VCAN Exon 8 Are New Molecular Defects for Wagner Disease

Wagner disease is a rare nonsyndromic autosomal‐dominant vitreoretinopathy, associated with splice mutations specifically targeting VCAN exon 8. We report the extensive genetic analysis of two Wagner probands, previously found negative for disease‐associated splice mutations. Next‐generation sequencing (NGS), quantitative real‐time PCR, and long‐range PCR identified two deletions (3.4 and 10.5 kb) removing at least one exon–intron boundary of exon 8, and both correlating with an imbalance of VCAN mRNA isoforms. We showed that the 10.5‐kb deletion occurred de novo, causing somatic mosaicism in the proband's mother who had an unusually mild asymmetrical phenotype. Therefore, exon 8 deletions are novel VCAN genetic defects responsible for Wagner disease, and VCAN mosaic mutations may be involved in the pathogenesis of Wagner disease with attenuated phenotype. NGS is then an effective screening tool for genetic diagnosis of Wagner disease, improving the chance of identifying all disease‐causative variants as well as mosaic mutations in VCAN.     

Long‐read nanopore sequencing resolves a TMEM231 gene conversion event causing Meckel–Gruber syndrome

The diagnostic deployment of massively parallel short‐read next‐generation sequencing (NGS) has greatly improved genetic test availability, speed, and diagnostic yield, particularly for rare inherited disorders. Nonetheless, diagnostic approaches based on short‐read sequencing have a poor ability to accurately detect gene conversion events. We report on the genetic analysis of a family in which 3 fetuses had clinical features consistent with the autosomal recessive disorder Meckel–Gruber syndrome (MKS). Targeted NGS of 29 known MKS‐associated genes revealed a heterozygous TMEM231 splice donor variant c.929+1A>G. Comparative read‐depth analysis, performed to identify a second pathogenic allele, revealed an apparent heterozygous deletion of TMEM231 exon 4. To verify this result we performed single‐molecule long‐read sequencing of a long‐range polymerase chain reaction product spanning this locus. We identified four missense variants that were absent from the short‐read dataset due to the preferential mapping of variant‐containing reads to a downstream TMEM231 pseudogene. Consistent with the parental segregation analysis, we demonstrate that the single‐molecule long reads could be used to show that the variants are arranged in trans. Our experience shows that robust validation of apparent dosage variants remains essential to avoid the pitfalls of short‐read sequencing and that new third‐generation long‐read sequencing technologies can already aid routine clinical care.