Looking beyond the exome: a phenotype-first approach to molecular diagnostic resolution in rare and undiagnosed diseases

PurposeTo describe examples of missed pathogenic variants on whole-exome sequencing (WES) and the importance of deep phenotyping for further diagnostic testing.MethodsGuided by phenotypic information, three children with negative WES underwent targeted single-gene testing.ResultsIndividual 1 had a clinical diagnosis consistent with infantile systemic hyalinosis, although WES and a next-generation sequencing (NGS)-based ANTXR2 test were negative. Sanger sequencing of ANTXR2 revealed a homozygous single base pair insertion, previously missed by the WES variant caller software. Individual 2 had neurodevelopmental regression and cerebellar atrophy, with no diagnosis on WES. New clinical findings prompted Sanger sequencing and copy number testing of PLA2G6. A novel homozygous deletion of the noncoding exon 1 (not included in the WES capture kit) was detected, with extension into the promoter, confirming the clinical suspicion of infantile neuroaxonal dystrophy. Individual 3 had progressive ataxia, spasticity, and magnetic resonance image changes of vanishing white matter leukoencephalopathy. An NGS leukodystrophy gene panel and WES showed a heterozygous pathogenic variant in EIF2B5; no deletions/duplications were detected. Sanger sequencing of EIF2B5 showed a frameshift indel, probably missed owing to failure of alignment.ConclusionThese cases illustrate potential pitfalls of WES/NGS testing and the importance of phenotype-guided molecular testing in yielding diagnoses.     

Speech and language delay in a patient with WDR4 mutations

Primordial dwarfism (PD) is mainly characterized by growth deficiency with heterogeneous phenotypes. A group of genes are known to be associated with PD or PD-related syndrome. WD repeat domain 4 (WDR4) is recently reported to be responsible for PD. Here we report a 6-year-old boy from a non-consanguineous couple with motor and speech delay as well as intellectual disability. Whole exome sequencing (WES) identified a missense mutation (NM_033661.4:c.491A > C; p.(Asp164Ala)) and a small insertion (NM_033661.4:c.940dupC; p.(Leu314Profs*16)) of WDR4 in this patient. Two novel mutations confirmed by Sanger sequencing are from father and mother respectively according to a recessive inheritance pattern. Asp164Ala located in functional region is predicted to be deleterious by two kinds of algorithm. The small insertion causing a frameshift mutation leads to truncated protein. In this study, we present two novel WDR4 mutations responsible for PD in a 6-year-old patient, expanding the molecular and phenotype spectrum of WDR4-related PD.     

Sedaghatian-type spondylometaphyseal dysplasia: Whole exome sequencing in neonatal dry blood spots enabled identification of a novel variant in GPX4

Accumulation of lipid peroxides causes membrane damage and cell death. Glutathione peroxidase 4 (GPX4) acts as a hydroperoxidase which prevents accumulation of toxic oxidized lipids and blocks ferroptosis, an iron-dependent, non-apoptotic mode of cell death. GPX4 deficiency causes Sedaghatian-type spondylo-metaphyseal dysplasia (SSMD), a lethal autosomal recessive disorder, featuring skeletal dysplasia, cardiac arrhythmia and brain anomalies with only three pathogenic GPX4 variants reported in two SSMD patients. Our objective was to identify the underlying genetic cause of neonatal death of two siblings presenting with hypotonia, cardiorespiratory failure and SSMD. Whole exome sequencing (WES) was performed in DNA samples from two siblings and their parents. Since “critical samples” were not available from the patients, DNA was extracted from dry blood spots (DBS) retrieved from the Israeli newborn-screening center. Sanger sequencing and segregation analysis followed the WES. Homozygous novel GPX4 variant, c.153_160del; p.His52fs*1 causing premature truncation of GPX4 was detected in both siblings; their parents were heterozygotes. Segregation analysis confirmed autosomal recessive inheritance. This report underscores the importance of DBS WES in identifying the genes and mutations causing devastating rare diseases. Obtaining critical samples from a dying patient is crucial for enabling genetic diagnosis.     

A novel missense variant in GPT2 causes non-syndromic autosomal recessive intellectual disability in a consanguineous Iranian family

Intellectual disability (ID) affects 1–3% of the general population worldwide. Genetic factors play an undeniable role in the etiology of Non-Syndromic Intellectual disability (NS-ID). Nowadays, whole-exome sequencing (WES) technique is used frequently to identify the causative genes in such heterogeneous diseases. Herein, we subjected four patients with initial diagnostics of NS-ID in a consanguineous Iranian family. To find the possible genetic cause(s), Trio-WES was performed on the proband and his both healthy parents. Sanger sequencing was performed to confirm the identified variant by WES and also investigate whether it co-segregates with the patients’ phenotype in the family. Using several online in-silico predictors, the probable impacts of the variant on structure and function of GPT2 protein were predicted. A novel variant, c.266A>G; p.(Glu89Gly), in exon 3 of GPT2 (NM_133443.3) was identified using Trio-WES. The candidate variant was also verified by Sanger sequencing. All affected members showed the common clinical features suffering from a non-progressive mild-to-severe ID. Also, different clinical observations compared to previously reported cases such as no facial features, no obvious structural malformations, ability to speak but with difficulty, and lack of any morphological defects were noted for the first time in this family. The c.266A>G; p.(Glu89Gly) variant reported here is the sixth variant identified up to now in the GPT2 gene, to be associated with NS-ID. Our data support the potential malfunction of the substituted GPT2 protein resulted from the novel variant, however, we strongly suggest confirming this finding more by doing functional analysis.     

Identification of Two Homozygous Sequence Variants in the COL7A1 Gene Underlying Dystrophic Epidermolysis Bullosa by Whole‐Exome Analysis in a Consanguineous Family

Dystrophic epidermolysis bullosa (DEB) is an inherited skin disorder with variable severity and heterogeneous genetic involvement. Diagnostic approaches for this condition include clinical evaluations and electron microscopy of patients’ skin biopsies, followed by Sanger sequencing (SS) of a large gene (118 exons) that encodes the alpha chain of type VII collagen (COL7A1) located on Chromosome 3p21.1. However, the use of SS may hinder diagnostic efficiency and lead to delays because it is costly and time‐consuming. We evaluated a 5‐generation consanguineous family with 3 affected individuals presenting the severe generalised DEB phenotype. Human whole‐exome sequencing (WES) revealed 2 homozygous sequence variants: the previously reported variant p.Arg578* in exon 13 and a novel variant p.Arg2063Gln in exon 74 of the COL7A1 gene. Validation by SS, performed on all family members, confirmed the cosegregation of the 2 variants with the disease phenotype. To the best of our knowledge, 2 homozygous COL7A1 variants have never been simultaneously reported in DEB patients; however, the upstream protein truncation variant is more likely to be disease‐causing than the novel missense variant. WES can be used as an efficient molecular diagnostic tool for evaluating autosomal recessive forms of DEB.     

Severe neurodevelopmental disorder with intractable seizures due to a novel SLC1A4 homozygous variant

IntroductionBiallelic variants in the SLC1A4 gene have been so far identified as a very rare cause of neurodevelopmental disorders with or without epilepsy and almost exclusively described in the Ashkenazi-Jewish population.Patients and methodsHere we present Czech patient with microcephaly, severe global developmental delay and intractable seizures whose condition remained undiagnosed despite access to clinical experience and standard diagnostic methods including examination with an epilepsy targeted NGS gene panel.ResultsWhole exome sequencing revealed a novel variant NM_003038.4:c.1370G > A p.(Arg457Gln) of the SLC1A4 gene in a homozygous state in the patient, and afterwards Sanger sequencing in both parents confirmed the biallelic origin of the variant. A variant in the same codon, but with a different amino acid exchange, was described previously in a patient that had a very similar phenotype, however, without epilepsy.ConclusionOur data suggest that the SLC1A4 gene should be considered in the diagnosis of patients with severe, early onset neurodevelopmental impairment with epilepsy and encourage the analysis of SLC1A4 gene variants via targeted NGS gene panel or whole exome sequencing.     

Missense variant in CCDC22 causes X-linked recessive intellectual disability with features of Ritscher-Schinzel/3C syndrome

Ritscher-Schinzel syndrome (RSS)/3C (cranio-cerebro-cardiac) syndrome (OMIM#220210) is a rare and clinically heterogeneous developmental disorder characterized by intellectual disability, cerebellar brain malformations, congenital heart defects, and craniofacial abnormalities. A recent study of a Canadian cohort identified homozygous sequence variants in the KIAA0196 gene, which encodes the WASH complex subunit strumpellin, as a cause for a form of RSS/3C syndrome. We have searched for genetic causes of a phenotype similar to RSS/3C syndrome in an Austrian family with two affected sons. To search for disease-causing variants, whole-exome sequencing (WES) was performed on samples from two affected male children and their parents. Before WES, CGH array comparative genomic hybridization was applied. Validation of WES and segregation studies was done using routine Sanger sequencing. Exome sequencing detected a missense variant (c.1670A>G; p.(Tyr557Cys)) in exon 15 of the CCDC22 gene, which maps to chromosome Xp11.23. Western blots of immortalized lymphoblastoid cell lines (LCLs) from the affected individual showed decreased expression of CCDC22 and an increased expression of WASH1 but a normal expression of strumpellin and FAM21 in the patients cells. We identified a variant in CCDC22 gene as the cause of an X-linked phenotype similar to RSS/3C syndrome in the family described here. A hypomorphic variant in CCDC22 was previously reported in association with a familial case of syndromic X-linked intellectual disability, which shows phenotypic overlap with RSS/3C syndrome. Thus, different inactivating variants affecting CCDC22 are associated with a phenotype similar to RSS/3C syndrome.     

Expansion of the phenotypic and mutational spectrum of Carpenter syndrome

Carpenter syndrome 1 (CRPT1) is an acrocephalopolysyndactyly (ACPS) disorder characterized by craniosynostosis, polysyndactyly, obesity, and other malformations. It is caused by mutations in the gene RAB23. We are reporting on two patients from two unrelated consanguineous Egyptian families. Patient 1 presented with an atypical clinical presentation of Carpenter syndrome including overgrowth with advanced bone age, epileptogenic changes on electroencephalogram and autistic features. Patient 2 presented with typical clinical features suggestive of Carpenter syndrome. Therefore, Patient 1 was subjected to whole exome sequencing (WES) to find an explanation for his unusual features and Patient 2 was subjected to Sanger sequencing of the coding exons of theRAB23 gene to confirm the diagnosis. We identified a novel homozygous missense RAB23 variant (NM_001278668:c.T416C:p.Leu139Pro) in Patient 1 and a novel homozygous splicing variant (NM_016277.5:c.398+1G > A) in Patient 2. We suggest that the overgrowth with advanced bone age, electroencephalogram epileptogenic changes, and autistic features seen in Patient 1 are an expansion of the Carpenter phenotype and could be due to the novel missense RAB23 variant. Additionally, the novel identified RAB23 variants in Patient 1 and 2 broaden the spectrum of variants associated with Carpenter syndrome.     

一例Sifrim-Hitz-Weiss综合征患儿的临床与CHD4基因变异分析

目的探讨一例Sifrim-Hitz-Weiss综合征(Sifrim-Hitz-Weiss syndrome,SIHIWES)患儿的基因型与表型的对应关系。方法收集先证者及其父母的外周血样,提取基因组DNA,采用全外显子测序对先证者进行检测,并应用Sanger测序对先证者及父母进行验证。结果先证者为一例2岁女性患儿,表现为全面发育迟缓、智力障碍、特殊面容等。其产前表现为NT增厚,颅面部异常和胎动减少等。全外显子测序发现患儿CHD4基因发生了致病性变异NM_001273:c.2989C>G(p.Leu997Val)(GRCh37/hg19)。与以往报道的SIHIWES的临床表型相比,该患儿产前临床特征未有描述,而产后某些新的表型包括眼底检查结果和某些特殊面容如左右耳不对称、眼睑下垂、长人中和嘴角下斜等也呈现出来。结论本文研究结果扩展了CHD4基因变异谱,并丰富了SIHIWES的临床表型谱。     

An AP4B1 frameshift mutation in siblings with intellectual disability and spastic tetraplegia further delineates the AP-4 deficiency syndrome

The recently proposed adaptor protein 4 (AP-4) deficiency syndrome comprises a group of congenital neurological disorders characterized by severe intellectual disability (ID), delayed or absent speech, hereditary spastic paraplegia, and growth retardation. AP-4 is a heterotetrameric protein complex with important functions in vesicle trafficking. Mutations in genes affecting different subunits of AP-4, including AP4B1, AP4E1, AP4S1, and AP4M1, have been reported in patients with the AP-4 deficiency phenotype. We describe two siblings from a non-consanguineous couple who presented with severe ID, absent speech, microcephaly, growth retardation, and progressive spastic tetraplegia. Whole-exome sequencing in the two patients identified the novel homozygous 2-bp deletion c.1160_1161delCA (p.(Thr387Argfs*30)) in AP4B1. Sanger sequencing confirmed the mutation in the siblings and revealed it in the heterozygous state in both parents. The AP4B1-associated phenotype has previously been assigned to spastic paraplegia-47. Identification of a novel AP4B1 alteration in two patients with clinical manifestations highly similar to other individuals with mutations affecting one of the four AP-4 subunits further supports the observation that loss of AP-4 assembly or functionality underlies the common clinical features in these patients and underscores the existence of the clinically recognizable AP-4 deficiency syndrome.     

A novel homozygous missense variant in NECTIN4 (PVRL4) causing ectodermal dysplasia cutaneous syndactyly syndrome

Ectodermal dysplasia syndactyly syndrome 1 (EDSS1) is a rare form of ectodermal dysplasia including anomalies of hair, nails, and teeth along with bilateral cutaneous syndactyly of hands and feet. In the present report, we performed a clinical and genetic characterization of a consanguineous Pakistani family with four individuals affected by EDSS1. We performed exome sequencing using DNA of one affected individual. Exome data analysis identified a novel homozygous missense variant (c.242T>C; p.(Leu81Pro)) in NECTIN4 (PVRL4). Sanger sequencing validated this variant and confirmed its cosegregation with the disease phenotype in the family members. Thus, our report adds a novel variant to the NECTIN4 mutation spectrum and contributes to the NECTIN4‐related clinical characterization.     

The third patient of ACACA-related acetyl-CoA carboxylase deficiency with seizure and literature review

Pathogenic variants in ACACA are the cause of acetyl-CoA carboxylase deficiency with an autosomal recessive inheritance that is identified by hypotonia, motor, and intellectual developmental delay. In this article, we describe a seven-year-old boy who is the child of consanguineous parents with a homozygous variant in ACACA (NM_198834.3:c.6641C > A, p.P2214H) that was detected by Whole-Exome Sequencing and confirmed by Sanger sequencing. This is the first reported patient of acetyl-CoA carboxylase deficiency that results from a homozygous pathogenic variant in the ACACA gene in the Iranian family. The proband presents with motor and intellectual developmental delay, muscle weakness, language disorder, facial dysmorphism, and poor growth. The patient discussed here is similar to other patients that were previously published; however, we were able to identify seizure that has hitherto not been reported. This paper describes the third person with a novel variant in the ACACA gene in the world that accounts for acetyl-CoA carboxylase deficiency and implicates the clinical spectrum of the disease. Finally, we describe an individual-based review of the symptoms associated with acetyl-CoA carboxylase deficiency. So far, only two acetyl-CoA carboxylase deficiency patients have been reviewed in the literature.     

一例 DIS3L2基因变异所致Perlman综合征患儿的临床表型及遗传学分析

目的:分析1例Perlman综合征患儿的临床表型及 DIS3L2基因变异,了解其可能的致病原因。 方法:抽取患儿及其父母外周血,对患儿进行全外显子测序分析,根据筛出致病基因变异对患儿及其父母进行Sanger测序验证。按照美国医学遗传学与基因组学学会(American College of Medi...     

心-面-皮肤综合征MAP2K1基因新发变异一例

目的探讨MAP2K1基因变异所致的心-面-皮肤综合征(cardio-facio-cutaneous syndrome,CFCS)基因型与表型的对应关系。方法收集先证者及其父母的外周血样,提取DNA,采用全外显子组测序对先证者进行检测,用Sanger测序对先证者及父母进行验证。结果先证者为1岁8个月的男性,临床表现为身材矮小、精神运动发育迟缓、大头畸形、特殊面容、先天性心脏病等多发异常。全外显子组测序发现其携带MAP2K1基因c.389A>G(p.Tyr130Cys)杂合错义变异,Sanger测序证实其为新发变异。根据ACMG/AMP指南,判定为致病性变异。结论本例患儿存在明显的行为异常、食欲佳、三尖瓣返流,有别于既往报道的病例,因此丰富了MAP2K1基因变异所导致的CFCS的临床表型谱。     

Colitis susceptibility in mice with reactive oxygen species deficiency is mediated by mucus barrier and immune defense defects

Reactive oxygen species (ROS) generated by NADPH oxidases (NOX/DUOX) provide antimicrobial defense, redox signaling, and gut barrier maintenance. Inactivating NOX variants are associated with comorbid intestinal inflammation in chronic granulomatous disease (CGD; NOX2) and pediatric inflammatory bowel disease (IBD; NOX1); however Nox-deficient mice do not reflect human disease susceptibility. Here we assessed if a hypomorphic patient-relevant CGD mutation will increase the risk for intestinal inflammation in mice. Cyba (p22^phox) mutant mice generated low intestinal ROS, while maintaining Nox4 function. The Cyba variant caused profound mucus layer disruption with bacterial penetration into crypts, dysbiosis, and a compromised innate immune response to invading microbes, leading to mortality. Approaches used in treatment-resistant CGD or pediatric IBD such as bone marrow transplantation or oral antibiotic treatment ameliorated or prevented disease in mice. The Cyba mutant mouse phenotype implicates loss of both mucus barrier and efficient innate immune defense in the pathogenesis of intestinal inflammation due to ROS deficiency, supporting a combined-hit model where a single disease variant compromises different cellular functions in interdependent compartments.     

Identification of the first homozygous 1‐bp deletion in GDF9 gene leading to primary ovarian insufficiency by using targeted massively parallel sequencing

Targeted massively parallel sequencing (TMPS) has been used in genetic diagnosis for Mendelian disorders. In the past few years, the TMPS has identified new and already described genes associated with primary ovarian insufficiency (POI) phenotype. Here, we performed a targeted gene sequencing to find a genetic diagnosis in idiopathic cases of Brazilian POI cohort. A custom SureSelect^XT DNA target enrichment panel was designed and the sequencing was performed on Illumina NextSeq sequencer. We identified 1 homozygous 1‐bp deletion variant (c.783delC) in the GDF9 gene in 1 patient with POI. The variant was confirmed and segregated using Sanger sequencing. The c.783delC GDF9 variant changed an amino acid creating a premature termination codon (p.Ser262Hisfs*2). This variant was not present in all public databases (ExAC/gnomAD, NHLBI/EVS and 1000Genomes). Moreover, it was absent in 400 alleles from fertile Brazilian women screened by Sanger sequencing. The patient's mother and her unaffected sister carried the c.783delC variant in a heterozygous state, as expected for an autosomal recessive inheritance. Here, the TMPS identified the first homozygous 1‐bp deletion variant in GDF9. This finding reveals a novel inheritance pattern of pathogenic variant in GDF9 associated with POI, thus improving the genetic diagnosis of this disorder.     

Molecular genetic study of 59 Chinese Oculocutaneous albinism families

Oculocutaneous albinism is an autosomal recessive disorder characterized by either a complete lack of or reduction in melanin biosynthesis in the skin, hair, and eyes. The aim of the present study was to identify the molecular basis for 59 Chinese OCA families. In this study, compound heterozygous or homozygous pathogenic variants were found in 53 families, 4 families possessed only one heterozygous variant, and the pathogenic variants of 2 families remain undiscovered by using Sanger sequencing, whole exome sequencing and multiplex ligation-dependent probe amplification. We have identified a total of 55 variants including 21 novel variants in TYR, OCA2, SLC45A2, SLC24A5, and HPS1. The 21 novel variants include 11 missense changes, 4 nonsense changes, 2 splice site changes, 1 frameshift and 3 gross deletions. Forty-six variants including 14 novel variants were segregated with the phenotype in 37 families. We conducted RT-PCR of the novel splicing site variant (c.399-14G > A) of HPS1 and verified that the variant would result in the inclusion of 12 bp of intronic material in exon 6 of HPS1. The results of platelet whole mount electron microscopy further confirmed the diagnosis of HPS1. These novel variants identified in our study expand the mutational spectrum of the disease, which contributes to prenatal diagnosis and genetic counselling.     

一例Bainbridge-Ropers综合征患儿的基因变异分析

目的分析1例Bainbridge-Ropers综合征患儿及其父母ASXL3基因的变异类型,明确其可能的遗传学病因,为其临床诊断和遗传咨询提供依据。方法采集患儿和父母的外周血样,应用全外显子测序的方法对患儿基因进行检测,并采用Sanger测序的方法对变异位点进行验证。结果全外显子测序结果显示,患儿的ASXL3基因c.3106C>T杂合变异(p.Arg1036*),父母外周血ASXL3基因该位点未检测到c.3106C>T变异。该变异为无义变异,会产生截短蛋白。根据美国医学遗传学与基因组学学会变异分类标准与指南,c.3106C>T变异为致病性变异(PVS1+PS2+PP4)。结论ASXL3基因c.3106C>T杂合变异可能为患儿的致病原因,通过ASXL3基因变异分析,可以为其临床诊断和遗传咨询提供依据。