Undiagnosed rare disease clinic identifies a novel UBE3A variant in two sisters with Angelman syndrome: The end of a diagnostic odyssey.

Angelman syndrome (AS, MIM #105830) is a neurodevelopmental disorder characterized by severe intellectual disability, profound developmental delay, movement or balance problems, an excessively cheerful disposition, and seizures. AS results from inadequate expression of the maternal UBE3A gene (MIM #601623), which encodes an E3 ligase in the ubiquitin-proteasome pathway. Here we present the case of two sisters with features consistent with AS who had negative methylation analyses. An autism/intellectual disability expanded panel revealed a maternally inherited novel UBE3A (NM_001354506.2) variant c.2443C>T p.(Pro815Ser) in both patients that was initially classified as a variant of uncertain significance. The patients were enrolled in Indiana University's Undiagnosed Rare Disease Clinic (URDC) to further investigate the variant. Additional data, including deep phenotyping, familial segregation analysis, and in silico studies, suggest that the variant is likely pathogenic. 3D modeling studies based on the available crystal structure revealed that the Pro815Ser variant can introduce more flexibility into the protein and alter its enzymatic activity. Recent literature confirms the pathogenic nature of the variant. Reanalysis of the UBE3A variant has heightened existing knowledge of AS and has offered this family an end to their diagnostic odyssey.

Further evidence of involvement of ITSN1 in autosomal dominant neurodevelopmental disorder.

A 5-year-old affected male had following phenotypes: autism, motor stereotypy, developmental regression, staring gaze, absent speech, and behavioral abnormality. The biochemical testing was normal and genetic testing identified a de novo pathogenic variant in ITSN1 gene in the proband. To our knowledge, this is the second report that elucidates the role of ITSN1 gene in an autosomal dominant neurodevelopmental disorder.

A case of MBTPS1-related disorder due to compound heterozygous variants in MBTPS1 gene: Genotype-phenotype expansion and the emergence of a novel syndrome.

MBTPS1 (NM_003791.4) encodes Site-1 protease, a serine protease that functions sequentially with Site-2 protease regulating cholesterol homeostasis and endoplasmic reticulum stress response. MBTPS1 pathogenic variants are associated with spondyloepiphyseal dysplasia, Kondo-Fu type (MIM:618392; cataract, alopecia, oral mucosal disorder, and psoriasis-like syndrome, and Silver-Russell-like syndrome). In this report, we describe a 14-year-old female with a complex medical history including white matter volume loss, early-onset cataracts, retrognathia, laryngomalacia, inguinal hernia, joint hypermobility, feeding dysfunction, and speech delay. Additionally, features of ectodermal dysplasia that she has include decreased sweating, heat intolerance, dysplastic nails, chronically dry skin, and abnormal hair growth issues. Exome sequencing analysis identified compound heterozygous variants in the MBTPS1 gene: c.2255G > T p.(Gly752Val) predicted to affect important function of the protein, which was inherited from the mother, and a splice site variant c.2831 + 5G > T, which was inherited from the father. The RNA-seq analysis of the splice variant showed skipping of exon 21, predicted to result in frameshifting p.(Ser901fs28*) leading to non-sense mediated decay. To our knowledge, only eight studies have been published that described the MBPTS1-related disorders. Interestingly, we observed the features of ectodermal dysplasia in our patient that further expands the phenotypic spectrum of MBTPS1 gene-related disorders.

Systematic evaluation of genome sequencing for the diagnostic assessment of autism spectrum disorder and fetal structural anomalies.

Short-read genome sequencing (GS) holds the promise of becoming the primary diagnostic approach for the assessment of autism spectrum disorder (ASD) and fetal structural anomalies (FSAs). However, few studies have comprehensively evaluated its performance against current standard-of-care diagnostic tests: karyotype, chromosomal microarray (CMA), and exome sequencing (ES). To assess the clinical utility of GS, we compared its diagnostic yield against these three tests in 1,612 quartet families including an individual with ASD and in 295 prenatal families. Our GS analytic framework identified a diagnostic variant in 7.8% of ASD probands, almost 2-fold more than CMA (4.3%) and 3-fold more than ES (2.7%). However, when we systematically captured copy-number variants (CNVs) from the exome data, the diagnostic yield of ES (7.4%) was brought much closer to, but did not surpass, GS. Similarly, we estimated that GS could achieve an overall diagnostic yield of 46.1% in unselected FSAs, representing a 17.2% increased yield over karyotype, 14.1% over CMA, and 4.1% over ES with CNV calling or 36.1% increase without CNV discovery. Overall, GS provided an added diagnostic yield of 0.4% and 0.8% beyond the combination of all three standard-of-care tests in ASD and FSAs, respectively. This corresponded to nine GS unique diagnostic variants, including sequence variants in exons not captured by ES, structural variants (SVs) inaccessible to existing standard-of-care tests, and SVs where the resolution of GS changed variant classification. Overall, this large-scale evaluation demonstrated that GS significantly outperforms each individual standard-of-care test while also outperforming the combination of all three tests, thus warranting consideration as the first-tier diagnostic approach for the assessment of ASD and FSAs.

HNRNPC haploinsufficiency affects alternative splicing of intellectual disability-associated genes and causes a neurodevelopmental disorder.

Heterogeneous nuclear ribonucleoprotein C (HNRNPC) is an essential, ubiquitously abundant protein involved in mRNA processing. Genetic variants in other members of the HNRNP family have been associated with neurodevelopmental disorders. Here, we describe 13 individuals with global developmental delay, intellectual disability, behavioral abnormalities, and subtle facial dysmorphology with heterozygous HNRNPC germline variants. Five of them bear an identical in-frame deletion of nine amino acids in the extreme C terminus. To study the effect of this recurrent variant as well as HNRNPC haploinsufficiency, we used induced pluripotent stem cells (iPSCs) and fibroblasts obtained from affected individuals. While protein localization and oligomerization were unaffected by the recurrent C-terminal deletion variant, total HNRNPC levels were decreased. Previously, reduced HNRNPC levels have been associated with changes in alternative splicing. Therefore, we performed a meta-analysis on published RNA-seq datasets of three different cell lines to identify a ubiquitous HNRNPC-dependent signature of alternative spliced exons. The identified signature was not only confirmed in fibroblasts obtained from an affected individual but also showed a significant enrichment for genes associated with intellectual disability. Hence, we assessed the effect of decreased and increased levels of HNRNPC on neuronal arborization and neuronal migration and found that either condition affects neuronal function. Taken together, our data indicate that HNRNPC haploinsufficiency affects alternative splicing of multiple intellectual disability-associated genes and that the developing brain is sensitive to aberrant levels of HNRNPC. Hence, our data strongly support the inclusion of HNRNPC to the family of HNRNP-related neurodevelopmental disorders.

Macrocephaly and developmental delay caused by missense variants in RAB5C.

Rab GTPases are important regulators of intracellular vesicular trafficking. RAB5C is a member of the Rab GTPase family that plays an important role in the endocytic pathway, membrane protein recycling and signaling. Here we report on 12 individuals with nine different heterozygous de novo variants in RAB5C. All but one patient with missense variants (n = 9) exhibited macrocephaly, combined with mild-to-moderate developmental delay. Patients with loss of function variants (n = 2) had an apparently more severe clinical phenotype with refractory epilepsy and intellectual disability but a normal head circumference. Four missense variants were investigated experimentally. In vitro biochemical studies revealed that all four variants were damaging, resulting in increased nucleotide exchange rate, attenuated responsivity to guanine exchange factors and heterogeneous effects on interactions with effector proteins. Studies in C. elegans confirmed that all four variants were damaging in vivo and showed defects in endocytic pathway function. The variant heterozygotes displayed phenotypes that were not observed in null heterozygotes, with two shown to be through a dominant negative mechanism. Expression of the human RAB5C variants in zebrafish embryos resulted in defective development, further underscoring the damaging effects of the RAB5C variants. Our combined bioinformatic, in vitro and in vivo experimental studies and clinical data support the association of RAB5C missense variants with a neurodevelopmental disorder characterized by macrocephaly and mild-to-moderate developmental delay through disruption of the endocytic pathway.

Characterization of a novel deep-intronic variant in DYNC2H1 identified by whole-exome sequencing in a patient with a lethal form of a short-rib thoracic dysplasia type III.

Biallelic pathogenic variants in DYNC2H1 are the cause of short-rib thoracic dysplasia type III with or without polydactyly (OMIM #613091), a skeletal ciliopathy characterized by thoracic hypoplasia due to short ribs. In this report, we review the case of a patient who was admitted to the Neonatal Intensive Care Unit (NICU) of Indiana University Health (IUH) for respiratory support after experiencing respiratory distress secondary to a small, narrow chest causing restrictive lung disease. Additional phenotypic features include postaxial polydactyly, short proximal long bones, and ambiguous genitalia were noted. Exome sequencing (ES) revealed a maternally inherited likely pathogenic variant c.10322C > T p.(Leu3448Pro) in the DYNC2H1 gene. However, there was no variant found on the paternal allele. Microarray analysis to detect deletion or duplication in DYNC2H1 was normal. Therefore, there was insufficient evidence to establish a molecular diagnosis. To further explore the data and perform additional investigations, the patient was subsequently enrolled in the Undiagnosed Rare Disease Clinic (URDC) at Indiana University School of Medicine (IUSM). The investigators at the URDC performed a reanalysis of the ES raw data, which revealed a paternally inherited DYNC2H1 deep-intronic variant c.10606-14A > G predicted to create a strong cryptic acceptor splice site. Additionally, the RNA sequencing of fibroblasts demonstrated partial intron retention predicted to cause a premature stop codon and nonsense-mediated mRNA decay (NMD). Droplet digital RT-PCR (RT-ddPCR) showed a drastic reduction by 74% of DYNCH2H1 mRNA levels. As a result, the intronic variant was subsequently reclassified as likely pathogenic resulting in a definitive clinical and genetic diagnosis for this patient. Reanalysis of ES and fibroblast mRNA experiments confirmed the pathogenicity of the splicing variants to supplement critical information not revealed in original ES or CMA reports. The NICU and URDC collaboration ended the diagnostic odyssey for this family; furthermore, its importance is emphasized by the possibility of prenatally diagnosing the mother's current pregnancy.

Exome sequencing identified a novel HIST1H1E heterozygous protein-truncating variant in a 6-month-old male patient with Rahman syndrome: A case report.

Rahman syndrome is a rare congenital anomaly syndrome recently described, which results from pathogenic variants in the HIST1H1E gene. The condition is characterized by variable somatic overgrowth, macrocephaly, distinctive facial features, intellectual disability, and behavioral problems. This report extends the genotype and clinical phenotype of HIST1H1E-associated Rahman syndrome.

Reanalysis of a novel variant in the IGF1R gene in a family with variable prenatal and postnatal growth retardation and dysmorphic features: benefits and feasibility of IUSM-URDC (Undiagnosed Rare Disease Clinic) program.

IGF1R-related disorders are associated with intrauterine growth restriction (IUGR), postnatal growth failure, short stature, microcephaly, developmental delay, and dysmorphic facial features. We report a patient who presented to medical genetics at 7 mo of age with a history of IUGR, poor feeding, mild developmental delays, microcephaly, and dysmorphic facial features. Whole-exome sequencing revealed a novel c.1464T > G p.(Cys488Trp) variant in the IGF1R gene, initially classified as a variation of uncertain significance (VUS). We enrolled the patient in the URDC (Undiagnosed Rare Disease Clinic) and performed additional studies including deep phenotyping and familial segregation analysis, which demonstrated that the patient's IGF1R VUS was present in phenotypically similar family members. Furthermore, biochemical testing revealed an elevated serum IGF-1 level consistent with abnormal IGF-1 receptor function. Workup resulted in the patient's variant being upgraded from a VUS to likely pathogenic. Our report expands the variant and phenotypic spectrum of IGF1R-related disorders and illustrates benefits and feasibility of reassessing a VUS beyond the initial molecular diagnosis by deep phenotyping, 3D modeling, additional biochemical testing, and familial segregation studies through the URDC, a multidisciplinary clinical program whose major goal is to end the diagnostic odyssey in patients with rare diseases.

Familial Autonomic Ganglionopathy Caused by Rare CHRNA3 Genetic Variants.

OBJECTIVE: To determine the molecular basis of a new monogenetic recessive disorder that results in familial autonomic ganglionopathy with diffuse autonomic failure. METHODS: Two adult siblings from one family (I-4 and I-5) and another participant from a second family (II-3) presented with severe neurogenic orthostatic hypotension (nOH), small nonreactive pupils, and constipation. All 3 affected members had low norepinephrine levels and diffuse panautonomic failure. RESULTS: Whole exome sequencing of DNA from I-4 and I-5 showed compound heterozygosity for c.907_908delCT (p.L303Dfs*115)/c.688 G>A (p.D230N) pathologic variants in the acetylcholine receptor, neuronal nicotinic, α3 subunit gene (CHRNA3). II-3 from the second family was homozygous for the same frameshift (fs) variant (p.L303Dfs*115//p.L303Dfs*115). CHRNA3 encodes a critical subunit of the nicotinic acetylcholine receptors (nAChRs) responsible for fast synaptic transmission in the autonomic ganglia. The fs variant is clearly pathogenic and the p.D230N variant is predicted to be damaging (SIFT)/probably damaging (PolyPhen2). The p.D230N variant lies on the interface between CHRNA3 and other nAChR subunits based on structural modeling and is predicted to destabilize the nAChR pentameric complex. CONCLUSIONS: We report a novel genetic disease that affected 3 individuals from 2 unrelated families who presented with severe nOH, miosis, and constipation. These patients had rare pathologic variants in the CHRNA3 gene that cosegregate with and are predicted to be the likely cause of their diffuse panautonomic failure.

Characterization of Reference Materials with an Association for Molecular Pathology Pharmacogenetics Working Group Tier 2 Status: CYP2C9, CYP2C19, VKORC1, CYP2C Cluster Variant, and GGCX: A GeT-RM Collaborative Project.

Pharmacogenetic testing is increasingly available from clinical and research laboratories. However, only a limited number of quality control and other reference materials are currently available for many of the variants that are tested. The Association for Molecular Pathology Pharmacogenetic Work Group has published a series of papers recommending alleles for inclusion in clinical testing. Several of the alleles were not considered for tier 1 because of a lack of reference materials. To address this need, the Division of Laboratory Systems, Centers for Disease Control and Prevention-based Genetic Testing Reference Material (GeT-RM) program, in collaboration with members of the pharmacogenetic testing and research communities and the Coriell Institute for Medical Research, has characterized 18 DNA samples derived from Coriell cell lines. DNA samples were distributed to five volunteer testing laboratories for genotyping using three commercially available and laboratory developed tests. Several tier 2 variants, including CYP2C9∗13, CYP2C19∗35, the CYP2C cluster variant (rs12777823), two variants in VKORC1 (rs61742245 and rs72547529) related to warfarin resistance, and two variants in GGCX (rs12714145 and rs11676382) related to clotting factor activation, were identified among these samples. These publicly available materials complement the pharmacogenetic reference materials previously characterized by the GeT-RM program and will support the quality assurance and quality control programs of clinical laboratories that perform pharmacogenetic testing.

Characterization of Reference Materials for Spinal Muscular Atrophy Genetic Testing: A Genetic Testing Reference Materials Coordination Program Collaborative Project.

Spinal muscular atrophy (SMA) is an autosomal recessive disorder predominately caused by bi-allelic loss of the SMN1 gene. Increased copies of SMN2, a low functioning nearly identical paralog, are associated with a less severe phenotype. SMA was recently recommended for inclusion in newborn screening. Clinical laboratories must accurately measure SMN1 and SMN2 copy number to identify SMA patients and carriers, and to identify individuals likely to benefit from therapeutic interventions. Having publicly available and appropriately characterized reference materials with various combinations of SMN1 and SMN2 copy number variants is critical to assure accurate SMA clinical testing. To address this need, the CDC-based Genetic Testing Reference Materials Coordination Program, in collaboration with members of the genetic testing community and the Coriell Institute for Medical Research, has characterized 15 SMA reference materials derived from publicly available cell lines. DNA samples were distributed to four volunteer testing laboratories for genotyping using three different methods. The characterized samples had zero to four copies of SMN1 and zero to five copies SMN2. The samples also contained clinically important allele combinations (eg, zero copies SMN1, three copies SMN2), and several had markers indicative of an SMA carrier. These and other reference materials characterized by the Genetic Testing Reference Materials Coordination Program are available from the Coriell Institute and are proposed to support the quality of clinical laboratory testing.

BICRA, a SWI/SNF Complex Member, Is Associated with BAF-Disorder Related Phenotypes in Humans and Model Organisms.

SWI/SNF-related intellectual disability disorders (SSRIDDs) are rare neurodevelopmental disorders characterized by developmental disability, coarse facial features, and fifth digit/nail hypoplasia that are caused by pathogenic variants in genes that encode for members of the SWI/SNF (or BAF) family of chromatin remodeling complexes. We have identified 12 individuals with rare variants (10 loss-of-function, 2 missense) in the BICRA (BRD4 interacting chromatin remodeling complex-associated protein) gene, also known as GLTSCR1, which encodes a subunit of the non-canonical BAF (ncBAF) complex. These individuals exhibited neurodevelopmental phenotypes that include developmental delay, intellectual disability, autism spectrum disorder, and behavioral abnormalities as well as dysmorphic features. Notably, the majority of individuals lack the fifth digit/nail hypoplasia phenotype, a hallmark of most SSRIDDs. To confirm the role of BICRA in the development of these phenotypes, we performed functional characterization of the zebrafish and Drosophila orthologs of BICRA. In zebrafish, a mutation of bicra that mimics one of the loss-of-function variants leads to craniofacial defects possibly akin to the dysmorphic facial features seen in individuals harboring putatively pathogenic BICRA variants. We further show that Bicra physically binds to other non-canonical ncBAF complex members, including the BRD9/7 ortholog, CG7154, and is the defining member of the ncBAF complex in flies. Like other SWI/SNF complex members, loss of Bicra function in flies acts as a dominant enhancer of position effect variegation but in a more context-specific manner. We conclude that haploinsufficiency of BICRA leads to a unique SSRIDD in humans whose phenotypes overlap with those previously reported.

EVEN-PLUS syndrome: A case report with novel variants in HSPA9 and evidence of HSPA9 gene dysfunction.

EVEN-PLUS syndrome is a rare condition characterized by its involvement of the Epiphyses, Vertebrae, Ears, and Nose, PLUS other associated findings. We report here the fifth case of EVEN-PLUS syndrome with novel variants c.818 T > G (p.L273X) and c.955C > T (p.L319F) in the HSPA9 gene identified through whole-exome sequencing. The patient is the first male known to be affected and presented with additional features not previously described with EVEN-PLUS syndrome. These features include agenesis of the septum pellucidum, a short chest and sternum, 13 pairs of ribs, a single hemivertebra, laterally displaced nipples, hydronephrosis, unilateral cryptorchidism, unilateral single palmar crease, bilateral clubfoot, and hypotonia. qPCR analysis provides supporting evidence for a nonsense-mediated decay mechanism for the HSPA9 truncating variant. In silico 3D modeling supports the pathogenicity of the c.955C > T (p.L319F) missense variant. The study presented here further describes the syndrome and broadens its mutational and phenotypic spectrum. Our study also lends support to HSPA9 variants as the underlying etiology of EVEN-PLUS syndrome and ultimately provides a better understanding of the molecular basis of the condition.

CNVs cause autosomal recessive genetic diseases with or without involvement of SNV/indels.

PURPOSE: Improved resolution of molecular diagnostic technologies enabled detection of smaller sized exonic level copy-number variants (CNVs). The contribution of CNVs to autosomal recessive (AR) conditions may be better recognized using a large clinical cohort. METHODS: We retrospectively investigated the CNVs' contribution to AR conditions in cases subjected to chromosomal microarray analysis (CMA, N = ~70,000) and/or clinical exome sequencing (ES, N = ~12,000) at Baylor Genetics; most had pediatric onset neurodevelopmental disorders. RESULTS: CNVs contributed to biallelic variations in 87 cases, including 81 singletons and three affected sibling pairs. Seventy cases had CNVs affecting both alleles, and 17 had a CNV and a single-nucleotide variant (SNV)/indel in trans. In total, 94.3% of AR-CNVs affected one gene; among these 41.4% were single-exon and 35.0% were multiexon partial-gene events. Sixty-nine percent of homozygous AR-CNVs were embedded in homozygous genomic intervals. Five cases had large deletions unmasking an SNV/indel on the intact allele for a recessive condition, resulting in multiple molecular diagnoses. CONCLUSIONS: AR-CNVs are often smaller in size, transmitted through generations, and underrecognized due to limitations in clinical CNV detection methods. Our findings from a large clinical cohort emphasized integrated CNV and SNV/indel analyses for precise clinical and molecular diagnosis especially in the context of genomic disorders.

An unusual cause for Coffin-Lowry syndrome: Three brothers with a novel microduplication in RPS6KA3.

Coffin-Lowry syndrome (CLS) is a rare X-linked disorder characterized by moderate to severe intellectual disability, hypotonia, craniofacial features, tapering digits, short stature, and skeletal deformities. Using whole exome sequencing and high-resolution targeted comparative genomic hybridization array analysis, we identified a novel microduplication encompassing exons five through nine of RPS6KA3 in three full brothers. Each brother presented with intellectual disability and clinical and radiographic features consistent with CLS. qRT-PCR analyses performed on mRNA from the peripheral blood of the three siblings revealed a marked reduction of RPS6KA3 levels suggesting a loss-of-function mechanism. PCR analysis of the patients' cDNA detected a band greater than expected for an exon 4-10 amplicon, suggesting this was likely a direct duplication that lies between exons 4 through 10, which was later confirmed by Sanger sequencing. This microduplication is only the third intragenic duplication of RPS6KA3, and the second and smallest reported to date thought to cause CLS. Our study further supports the clinical utility of methods such as next-generation sequencing and high-resolution genomic arrays to detect small intragenic duplications. These methods, coupled with expression studies and cDNA structural analysis have the capacity to confirm the diagnosis of CLS in these rare cases.

Copy number variant and runs of homozygosity detection by microarrays enabled more precise molecular diagnoses in 11,020 clinical exome cases.

BACKGROUND: Exome sequencing (ES) has been successfully applied in clinical detection of single nucleotide variants (SNVs) and small indels. However, identification of copy number variants (CNVs) using ES data remains challenging. The purpose of this study is to understand the contribution of CNVs and copy neutral runs of homozygosity (ROH) in molecular diagnosis of patients referred for ES. METHODS: In a cohort of 11,020 consecutive ES patients, an Illumina SNP array analysis interrogating mostly coding SNPs was performed as a quality control (QC) measurement and for CNV/ROH detection. Among these patients, clinical chromosomal microarray analysis (CMA) was performed at Baylor Genetics (BG) on 3229 patients, either before, concurrently, or after ES. We retrospectively analyzed the findings from CMA and the QC array. RESULTS: The QC array can detect ~ 70% of pathogenic/likely pathogenic CNVs (PCNVs) detectable by CMA. Out of the 11,020 ES cases, the QC array identified PCNVs in 327 patients and uniparental disomy (UPD) disorder-related ROH in 10 patients. The overall PCNV/UPD detection rate was 5.9% in the 3229 ES patients who also had CMA at BG; PCNV/UPD detection rate was higher in concurrent ES and CMA than in ES with prior CMA (7.2% vs 4.6%). The PCNVs/UPD contributed to the molecular diagnoses in 17.4% (189/1089) of molecularly diagnosed ES cases with CMA and were estimated to contribute in 10.6% of all molecularly diagnosed ES cases. Dual diagnoses with both PCNVs and SNVs were detected in 38 patients. PCNVs affecting single recessive disorder genes in a compound heterozygous state with SNVs were detected in 4 patients, and homozygous deletions (mostly exonic deletions) were detected in 17 patients. A higher PCNV detection rate was observed for patients with syndromic phenotypes and/or cardiovascular abnormalities. CONCLUSIONS: Our clinical genomics study demonstrates that detection of PCNV/UPD through the QC array or CMA increases ES diagnostic rate, provides more precise molecular diagnosis for dominant as well as recessive traits, and enables more complete genetic diagnoses in patients with dual or multiple molecular diagnoses. Concurrent ES and CMA using an array with exonic coverage for disease genes enables most effective detection of both CNVs and SNVs and therefore is recommended especially in time-sensitive clinical situations.

De novo and inherited TCF20 pathogenic variants are associated with intellectual disability, dysmorphic features, hypotonia, and neurological impairments with similarities to Smith-Magenis syndrome.

BACKGROUND: Neurodevelopmental disorders are genetically and phenotypically heterogeneous encompassing developmental delay (DD), intellectual disability (ID), autism spectrum disorders (ASDs), structural brain abnormalities, and neurological manifestations with variants in a large number of genes (hundreds) associated. To date, a few de novo mutations potentially disrupting TCF20 function in patients with ID, ASD, and hypotonia have been reported. TCF20 encodes a transcriptional co-regulator structurally related to RAI1, the dosage-sensitive gene responsible for Smith-Magenis syndrome (deletion/haploinsufficiency) and Potocki-Lupski syndrome (duplication/triplosensitivity). METHODS: Genome-wide analyses by exome sequencing (ES) and chromosomal microarray analysis (CMA) identified individuals with heterozygous, likely damaging, loss-of-function alleles in TCF20. We implemented further molecular and clinical analyses to determine the inheritance of the pathogenic variant alleles and studied the spectrum of phenotypes. RESULTS: We report 25 unique inactivating single nucleotide variants/indels (1 missense, 1 canonical splice-site variant, 18 frameshift, and 5 nonsense) and 4 deletions of TCF20. The pathogenic variants were detected in 32 patients and 4 affected parents from 31 unrelated families. Among cases with available parental samples, the variants were de novo in 20 instances and inherited from 4 symptomatic parents in 5, including in one set of monozygotic twins. Two pathogenic loss-of-function variants were recurrent in unrelated families. Patients presented with a phenotype characterized by developmental delay, intellectual disability, hypotonia, variable dysmorphic features, movement disorders, and sleep disturbances. CONCLUSIONS: TCF20 pathogenic variants are associated with a novel syndrome manifesting clinical characteristics similar to those observed in Smith-Magenis syndrome. Together with previously described cases, the clinical entity of TCF20-associated neurodevelopmental disorders (TAND) emerges from a genotype-driven perspective.