Expanding the genotype-phenotype spectrum in SCN8A-related disorders.

BACKGROUND: SCN8A-related disorders are a group of variable conditions caused by pathogenic variations in SCN8A. Online Mendelian Inheritance in Man (OMIM) terms them as developmental and epileptic encephalopathy 13, benign familial infantile seizures 5 or cognitive impairment with or without cerebellar ataxia. METHODS: In this study, we describe clinical and genetic results on eight individuals from six families with SCN8A pathogenic variants identified via exome sequencing. RESULTS: Clinical findings ranged from normal development with well-controlled epilepsy to significant developmental delay with treatment-resistant epilepsy. Three novel and three reported variants were observed in SCN8A. Electrophysiological analysis in transfected cells revealed a loss-of-function variant in Patient 4. CONCLUSIONS: This work expands the clinical and genotypic spectrum of SCN8A-related disorders and provides electrophysiological results on a novel loss-of-function SCN8A variant.

Biallelic start loss variant, c.1A > G in GCSH is associated with variant nonketotic hyperglycinemia.

The glycine cleavage system H protein (GCSH) is an integral part of the glycine cleavage system with its additional involvement in the synthesis and transport of lipoic acid. We hypothesize that pathogenic variants in GCSH can cause variant nonketotic hyperglycinemia (NKH), a heterogeneous group of disorders with findings resembling a combination of severe NKH (elevated levels of glycine in plasma and CSF, progressive lethargy, seizures, severe hypotonia, no developmental progress, early death) and mitochondriopathies (lactic acidosis, leukoencephalopathy and Leigh-like lesions on MRI). We herein report three individuals from two unrelated Indian families with clinical, biochemical, and radiological findings of variant NKH, harboring a biallelic start loss variant, c.1A > G in GCSH.

Clinical and genetic spectrum of 104 Indian families with central nervous system white matter abnormalities.

Genetic disorders with predominant central nervous system white matter abnormalities (CNS WMAs), also called leukodystrophies, are heterogeneous entities. We ascertained 117 individuals with CNS WMAs from 104 unrelated families. Targeted genetic testing was carried out in 16 families and 13 of them received a diagnosis. Chromosomal microarray (CMA) was performed for three families and one received a diagnosis. Mendeliome sequencing was used for testing 11 families and all received a diagnosis. Whole exome sequencing (WES) was performed in 80 families and was diagnostic in 52 (65%). Singleton WES was diagnostic for 50/75 (66.67%) families. Overall, genetic diagnoses were obtained in 77 families (74.03%). Twenty-two of 47 distinct disorders observed in this cohort have not been reported in Indian individuals previously. Notably, disorders of nuclear mitochondrial pathology were most frequent (9 disorders in 20 families). Thirty-seven of 75 (49.33%) disease-causing variants are novel. To sum up, the present cohort describes the phenotypic and genotypic spectrum of genetic disorders with CNS WMAs in our population. It demonstrates WES, especially singleton WES, as an efficient tool in the diagnosis of these heterogeneous entities. It also highlights possible founder events and recurrent disease-causing variants in our population and their implications on the testing strategy.

Biallelic Loss of Proprioception-Related PIEZO2 Causes Muscular Atrophy with Perinatal Respiratory Distress, Arthrogryposis, and Scoliosis.

We report ten individuals of four independent consanguineous families from Turkey, India, Libya, and Pakistan with a variable clinical phenotype that comprises arthrogryposis, spontaneously resolving respiratory insufficiency at birth, muscular atrophy predominantly of the distal lower limbs, scoliosis, and mild distal sensory involvement. Using whole-exome sequencing, SNPchip-based linkage analysis, DNA microarray, and Sanger sequencing, we identified three independent homozygous frameshift mutations and a homozygous deletion of two exons in PIEZO2 that segregated in all affected individuals of the respective family. The mutations are localized in the N-terminal and central region of the gene, leading to nonsense-mediated transcript decay and consequently to lack of PIEZO2 protein. In contrast, heterozygous gain-of-function missense mutations, mainly localized at the C terminus, cause dominant distal arthrogryposis 3 (DA3), distal arthrogryposis 5 (DA5), or Marden-Walker syndrome (MWKS), which encompass contractures of hands and feet, scoliosis, ophthalmoplegia, and ptosis. PIEZO2 encodes a mechanosensitive ion channel that plays a major role in light-touch mechanosensation and has recently been identified as the principal mechanotransduction channel for proprioception. Mice ubiquitously depleted of PIEZO2 are postnatally lethal. However, individuals lacking PIEZO2 develop a not life-threatening, slowly progressive disorder, which is likely due to loss of PIEZO2 protein in afferent neurons leading to disturbed proprioception causing aberrant muscle development and function. Here we report a recessively inherited PIEZO2-related disease and demonstrate that depending on the type of mutation and the mode of inheritance, PIEZO2 causes clinically distinguishable phenotypes.

Locus and allelic heterogeneity in five families with hereditary spastic paraplegia.

Hereditary spastic paraplegias are a group of genetically heterogeneous neurological disorders characterized by progressive weakness and spasticity of lower limbs. We ascertained five families with eight individuals with hereditary spastic paraplegia. Pathogenic variants were identified by exome sequencing of index cases. The cohort consists of three families with spastic paraplegia type 47 (AP4B1) with a common mutation in two families, a family with spastic paraplegia type 50 (AP4M1), and two male siblings with X-linked spastic paraplegia 2 (PLP1). This work illustrates locus and allelic heterogeneity in five families with hereditary spastic paraplegia.

A biallelic 36-bp insertion in PIBF1 is associated with Joubert syndrome.

Biallelic pathogenic variants in PIBF1 have been identified as one of the genetic etiologies of Joubert syndrome. We report a two-year-old girl with global developmental delay, facial dysmorphism, hypotonia, enlarged cystic kidneys, molar tooth sign, and thinning of corpus callosum. A novel homozygous 36-bp insertion in PIBF1 (c.1181_1182ins36) was identified by exome sequencing as the likely cause of her condition. This is the second publication demonstrating the cause and effect relationship between PIBF1 and Joubert syndrome.

Homozygosity for a nonsense variant in AIMP2 is associated with a progressive neurodevelopmental disorder with microcephaly, seizures, and spastic quadriparesis.

We ascertained two unrelated consanguineous families with two affected children each having microcephaly, refractory seizures, intellectual disability, and spastic quadriparesis. Magnetic resonance imaging showed atrophy of cerebrum, cerebellum and spinal cord, prominent cisterna magna, symmetric T2 hypo-intensities in the bilateral basal ganglia and thinning of corpus callosum. Whole-exome sequencing of three affected individuals revealed c.105C>A [p.(Tyr35Ter)] variant in AIMP2. The variant lies in a common homozygous region of 940 kb on chromosome 7 and is likely to have been inherited from a common ancestor. The phenotype noted in our subjects' shares marked similarity with that of hypomyelinating leukodystrophy-3 caused by mutations in closely related gene AIMP1. We hereby report the first human disease associated with deleterious mutations in AIMP2.

A neurodegenerative mitochondrial disease phenotype due to biallelic loss-of-function variants in PNPLA8 encoding calcium-independent phospholipase A2γ.

Animal studies have demonstrated the critical roles of the patatin-like protein family plays in cellular growth, lipid homeostasis, and second messenger signaling the nervous system. Of the nine known calcium-independent phospholipase A2γ, only PNPLA2, PNLPA6, PNPLA9 and most recently a single patient with PNPLA8 are associated with mitochondrial-related neurodegeneration. Using whole exome sequencing, we report two unrelated individuals with variable but similar clinical features of microcephaly, severe global developmental delay, spasticity, lactic acidosis, and progressive cerebellar atrophy with biallelic loss-of-function variants in PNPLA8.

p.Arg69Trp in RNASEH2C is a founder variant in three Indian families with Aicardi-Goutières syndrome.

Aicardi-Goutières syndrome is an early-onset severe neurological disorder characterized by intracranial calcification, white matter abnormalities, hepatosplenomegaly, cerebrospinal fluid lymphocytosis, and elevated interferon-α levels, thus mimicking congenital viral infections. It is a genetically heterogeneous condition and autosomal recessive and autosomal dominant forms with variations in seven genes known till date. Variations in RNASEH2C cause an autosomal recessive form of AGS. Here we report three Indian families with variant, c.205C>T (NM_032193.3, p.Arg69Trp) in RNASEH2C gene identified by whole-exome sequencing and targeted molecular testing of the variant. Review of literature and our data suggest this is likely to be a founder variant in Asians and it would be a good initial variant to screen in patients with Aicardi-Goutières syndrome in Indians.

Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome.

The iron-sulfur (Fe-S) cluster (ISC) biogenesis pathway is indispensable for many fundamental biological processes and pathogenic variations in genes encoding several components of the Fe-S biogenesis machinery, such as NFU1, BOLA3, IBA57 and ISCA2 are already implicated in causing four types of multiple mitochondrial dysfunctions syndromes (MMDS). We report on two unrelated families, with two affected children each with early onset neurological deterioration, seizures, extensive white matter abnormalities, cortical migrational abnormalities, lactic acidosis and early demise. Exome sequencing of two affected individuals, one from each family, revealed a homozygous c.259G>A [p.(Glu87Lys)] variant in ISCA1 and Mendelian segregation was confirmed in both families. The ISCA1 variant lies in the only shared region of homozygosity between the two families suggesting the possibility of a founder effect. In silico functional analyses and structural modeling of the protein predict the identified ISCA1 variant to be detrimental to protein stability and function. Notably the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS. Our findings suggest association of a pathogenic variant in ISCA1 with another MMDS.

Identification of a novel LRRK1 mutation in a family with osteosclerotic metaphyseal dysplasia.

Osteosclerotic metaphyseal dysplasia (OSMD) is a rare skeletal dysplasia characterized by osteosclerotic metaphyses with osteopenic diaphyses of the long tubular bones. Our previous study identified a homozygous elongation mutation in leucine-rich repeat kinase 1 gene (LRRK1) in a patient with OSMD and showed that Lrrk1 knockout mice exhibited phenotypic similarity with OSMD. Here we report a second LRRK1 mutation in Indian sibs with OSMD. They had homozygous mutation (c.5971_5972insG) that produces an elongated mutant protein (p.A1991Gfs*31) similar to the first case. The sibs had normal stature, normal intelligence and recurrent fractures. The common radiographic feature was asymmetric and variable sclerosis of vertebral end plates, pelvic margin and metaphyses of tubular bones. One of the sibs had facial dysmorphisms, dentine abnormalities and acro-osteolysis. A comparison between the three OSMD cases with LRRK1 mutations with different ages suggested that the sclerotic lesions resolved with age. Our findings further support that LRRK1 would cause a subset of OSMD cases.

Phenotypic variability in patients with interstitial 6q21-q22 microdeletion and Acro-Cardio-Facial syndrome.

Deletions of 6q are known to be associated with variable clinical phenotypes including facial dysmorphism, hand malformations, heart defects, microcephaly, intellectual disability, epilepsy, and other neurodevelopmental and neuropsychiatric conditions. Here, we report a 7-year-old boy evaluated for facial dysmorphism, trigonocephaly, microcephaly, global developmental delay, and behavioral abnormalities. Molecular karyotyping revealed a 13-Mb deletion within 6q21-q22.31, (chr6:105,771,520-119,130,805; hg19, GRch37) comprising 81 genes. Review of 15 cases with interstitial 6q21-q22.3 deletion from the literature showed that facial dysmorphism, intellectual disability, and corpus callosum abnormalities are the most consistent clinical features in these individuals. Deleted genes and breakpoints in the 6q21-q22 region of the patient reported here are similar to two earlier reported cases with the clinical diagnosis of Acro-Cardio-Facial syndrome. However, the present case lacks characteristic clinical findings of Acro-Cardio-Facial syndrome. We discuss, the considerable phenotypic variability seen in individuals with 6q21-q22 microdeletion and emphasize the need for further scrutiny into the hypothesis of Acro-Cardio-Facial syndrome being a microdeletion syndrome. © 2016 Wiley Periodicals, Inc.

TREX tetramer disruption alters RNA processing necessary for corticogenesis in THOC6 Intellectual Disability Syndrome.

THOC6 variants are the genetic basis of autosomal recessive THOC6 Intellectual Disability Syndrome (TIDS). THOC6 is critical for mammalian Transcription Export complex (TREX) tetramer formation, which is composed of four six-subunit THO monomers. The TREX tetramer facilitates mammalian RNA processing, in addition to the nuclear mRNA export functions of the TREX dimer conserved through yeast. Human and mouse TIDS model systems revealed novel THOC6-dependent, species-specific TREX tetramer functions. Germline biallelic Thoc6 loss-of-function (LOF) variants result in mouse embryonic lethality. Biallelic THOC6 LOF variants reduce the binding affinity of ALYREF to THOC5 without affecting the protein expression of TREX members, implicating impaired TREX tetramer formation. Defects in RNA nuclear export functions were not detected in biallelic THOC6 LOF human neural cells. Instead, mis-splicing was detected in human and mouse neural tissue, revealing novel THOC6-mediated TREX coordination of mRNA processing. We demonstrate that THOC6 is required for key signaling pathways known to regulate the transition from proliferative to neurogenic divisions during human corticogenesis. Together, these findings implicate altered RNA processing in the developmental biology of TIDS neuropathology.

Expanding the genotype-phenotype spectrum in SCN8A-related disorders.

Background: SCN8A-related disorders are a group of variable conditions caused by pathogenic variations in SCN8A. Online Mendelian Inheritance in Man (OMIM) terms them as developmental and epileptic encephalopathy 13, benign familial infantile seizures 5 or cognitive impairment with or without cerebellar ataxia. Methods: In this study, we describe clinical and genetic results on eight individuals from six families with SCN8A pathogenic variants identified via exome sequencing. Results: Clinical findings ranged from normal development with well-controlled epilepsy to significant developmental delay with treatment-resistant epilepsy. Three novel and three reported variants were observed in SCN8A. Electrophysiological analysis in transfected cells revealed a loss-of-function variant in Patient 4. Conclusions: This work expands the clinical and genotypic spectrum of SCN8A-related disorders and provides electrophysiological results on a novel loss-of-function SCN8A variant.

Mechanisms of mRNA processing defects in inherited THOC6 intellectual disability syndrome.

THOC6 is the genetic basis of autosomal recessive THOC6 Intellectual Disability Syndrome (TIDS). THOC6 facilitates the formation of the Transcription Export complex (TREX) tetramer, composed of four THO monomers. The TREX tetramer supports mammalian mRNA processing that is distinct from yeast TREX dimer functions. Human and mouse TIDS model systems allow novel THOC6-dependent TREX tetramer functions to be investigated. Biallelic loss-of-functon(LOF) THOC6 variants do not influence the expression and localization of TREX members in human cells, but our data suggests reduced binding affinity of ALYREF. Impairment of TREX nuclear export functions were not detected in cells with biallelic THOC6 LOF. Instead, mRNA mis-splicing was observed in human and mouse neural tissue, revealing novel insights into THOC6-mediated TREX coordination of mRNA processing. We demonstrate that THOC6 is required for regulation of key signaling pathways in human corticogenesis that dictate the transition from proliferative to neurogenic divisions that may inform TIDS neuropathology.

Diagnostic Utility of Genome-wide DNA Methylation Analysis in Genetically Unsolved Developmental and Epileptic Encephalopathies and Refinement of a CHD2 Episignature.

Sequence-based genetic testing currently identifies causative genetic variants in ∼50% of individuals with developmental and epileptic encephalopathies (DEEs). Aberrant changes in DNA methylation are implicated in various neurodevelopmental disorders but remain unstudied in DEEs. Rare epigenetic variations ("epivariants") can drive disease by modulating gene expression at single loci, whereas genome-wide DNA methylation changes can result in distinct "episignature" biomarkers for monogenic disorders in a growing number of rare diseases. Here, we interrogate the diagnostic utility of genome-wide DNA methylation array analysis on peripheral blood samples from 516 individuals with genetically unsolved DEEs who had previously undergone extensive genetic testing. We identified rare differentially methylated regions (DMRs) and explanatory episignatures to discover causative and candidate genetic etiologies in 10 individuals. We then used long-read sequencing to identify DNA variants underlying rare DMRs, including one balanced translocation, three CG-rich repeat expansions, and two copy number variants. We also identify pathogenic sequence variants associated with episignatures; some had been missed by previous exome sequencing. Although most DEE genes lack known episignatures, the increase in diagnostic yield for DNA methylation analysis in DEEs is comparable to the added yield of genome sequencing. Finally, we refine an episignature for CHD2 using an 850K methylation array which was further refined at higher CpG resolution using bisulfite sequencing to investigate potential insights into CHD2 pathophysiology. Our study demonstrates the diagnostic yield of genome-wide DNA methylation analysis to identify causal and candidate genetic causes as ∼2% (10/516) for unsolved DEE cases.

Recent advances in epilepsy genomics and genetic testing.

Developmental and epileptic encephalopathies (DEEs) are a group of severe, early onset epilepsies characterized by refractory seizures, developmental delay or regression associated with ongoing epileptic activity, and generally poor prognosis. DEE is genetically and phenotypically heterogeneous, and there is a plethora of genetic testing options to investigate the rapidly growing list of epilepsy genes. However, more than 50% of patients with DEE remain without a genetic diagnosis despite state-of-the-art genetic testing. In this review, we discuss the major advances in epilepsy genomics that have surfaced in recent years. The goal of this review is to reach a larger audience and build a better understanding of pathogenesis and genetic testing options in DEE.