Nonsense-mediated decay (NMD) is a conserved RNA quality-control pathway that degrades transcripts containing premature termination codons. Because roughly a third of pathogenic variants in ClinVar can lead to truncated protein synthesis, predicting whether such transcripts undergo NMD is central to interpreting variant effects, yet the canonical 50-55 nucleotide rule explains only about half of observed outcome variability. Using paired whole-genome and RNA-sequencing from 10,306 individual samples in the Trans-Omics for Precision Medicine (TOPMed) program, we quantified NMD efficiency for 5,749 germline truncating variants via allele-specific expression and trained a gradient-boosting classifier, TrunCat, that distinguished NMD-sensitive from NMD-escape transcripts with ∼78% ROC-AUC (Receiver Operating Characteristic - Area Under the Curve). A reduced model using the ten features with the highest mean SHAP (SHapley Additive exPlanations) value as a measure of each feature's average contribution to predictions nearly matched this performance. Applied across large variant databases and a rare-disease cohort, the model produced NMD outcome predictions, with variants of uncertain significance showing higher predicted escape than pathogenic ones. This framework confirms the canonical rule, identifies non-canonical determinants, and offers a scalable resource for interpreting protein-truncating variants.

Background: A large fraction of clinically relevant pathogenic genomic variation consists of premature termination codons (PTC). While PTCs can trigger nonsense mediated mRNA decay (NMDtrig) causing loss of function (LOF), those near the end of transcripts can escape NMD (NMDesc), allowing expression of truncated proteins. As NMDesc PTCs are not well studied,and can lead to possible gain of function (GOF) effects, we investigated the impact of NMDtrig/NMDesc PTCs on human congenital heart disease (CHD). Results: Whole exome sequencing data from ~3,000 CHD patients and ~5,000 control subjects were analyzed for PTCs using known rules for classifying variants as NMDtrig/NMDesc. CHD patients had an increased burden of PTCs in gene-sets related to both heart and brain developmental processes. NMDesc PTCs were enriched in the MAPK pathway, known to harbor GOF variants that cause Noonan syndrome. Also identified were Hedgehog and ERBB signaling, pathways with prominent roles in heart development but not previously reported in nonsyndromic CHD. NMD analysis of PTCs at the transcript level identified 11 genes (9 novel) associated with CHD. Enrichment of digenic combinations between these 11 genes and digenic and higher order combinations of PTCs within heart developmental genes suggest a role for oligogenicity. Conclusions: We observed NMDesc PTCs play a role in CHD pathogenesis, and this role may be disproportionately specific to heart development. Our analysis uncovered new genes and pathways, identifying MAPK, Hedgehog, and ERBB signaling as pathways contributing to nonsyndromic CHD. The additional observation of digenic and higher order combinations of PTCs suggests an oligogenic framework to the genetic architecture of human CHD. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study was funded by the PCGC-CDDRC Fellowship and Predoctoral Individual National Research Service Award (F31) from NHLBI ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: IRB of University of Pittsburgh, School of Medicine gave ethical approval for this work I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors or available online from dbGaP

Nonsense-mediated mRNA decay (NMD) degrades transcripts containing premature termination codons (PTCs), critically shaping the disease outcomes of protein-truncating variants. While existing NMD rules categorize PTCs as NMD-triggering or NMD-evading, they cannot quantitatively predict the degree of NMD activity for a given endogenous PTC variant. To provide quantitative insight into NMD, we used saturation genome editing (SGE) to systematically introduce all possible PTCs (TAA, TAG, TGA) at every codon position spanning the first, penultimate, and three internal exons of the Lamin A/C (LMNA) gene. Combining targeted sequencing with NMD inhibition, we measured mRNA expression and NMD activity for 722 PTCs and 211 single nucleotide variants (SNVs). Our data validate known positional trends in NMD activity but reveal unexpected complexity. In the penultimate exon, the PTC position effect extends beyond the binary 50-55 nucleotides (nt) rule, revealing a quantitative relationship between the PTC-EJC distance and NMD activity. At the 5' end, NMD is completely absent in the first 21 codons of LMNA, followed by sharp activation over the next 4 codons. Both patterns, at the 5' end and in the penultimate exon, are unexplained by current models. Finally, internal exons show robust NMD with outliers consistently mapping to predicted readthrough-permissive sequence contexts, including the conserved readthrough promoting TGA-CT motif. This comprehensive dataset provides an unprecedented resource for understanding the quantitative impact of PTC position and sequence context on NMD, with direct implications for the clinical interpretation of nonsense variants in the human population.

Nonsense mediated decay (NMD) as an RNA surveillance pathway degrades transcripts with variants introducing premature termination codons (i.e., PTC variants),yet a substantial subset of pathogenic PTC variants downstream of the final exon exon junction is predicted to escape NMD (NMD escape) based on the canonical 50 bp rule. Our systematic analysis of germline pathogenic PTC variants from ClinVar revealed 148 autosomal dominant (AD) disease genes enriched for predicted NMD escape alleles. These genes span nonsense (N=63), minus1 frameshift (N=34), and plus1 frameshift (N=22) variants, with 23 genes enriched for two classes and 6 for all three. Although their loss of function intolerance score distributions did not differ from controls (P = 0.407), these genes exhibited significantly higher protein protein interaction (PPI) network centrality (P < 0.05) with their NMD escape regions enriched for PPI interfaces (P < 0.001 for minus1 and nonsense) and low-complexity sequences (P < 0.03 for minus1 and plus1). P/LP variants also produced significantly longer mutant C terminal tails than controls (P < 0.01), increasing potential for functional disruption. Structural modeling of altered C terminal tails revealed recurrent gains of glycine/proline (P < 0.03) and changes in aromatic residue content consistent with altered intrinsic disorder. Integration with neurodevelopmental disorder gene sets identified 25 dosage sensitive genes with predicted NMD escape P/LP variants, seven (28%) encoding condensate-forming proteins. Variant-level modeling in representative genes (e.g., KAT6B) showed altered phase separation propensity driven by truncated and/or altered intrinsically disordered regions. Overall, this study implicates condensate dysregulation as a potential downstream biophysical consequence of NMD escape disease alleles, providing a protein-feature viewer for variant interpretation (https://github.com/schmidtjacob46/NMDesc-protein-viewer). ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was supported in part by the US National Human Genome Research Institute/National Heart Blood Lung Institute jointly funded Baylor-Hopkins Center for Mendelian Genomics (UM1HG006542), by the National Institutes of Health (NIH) (5R01 HD039056, 5R01 HL091771), by the Genomic Research Elucidates the Genetics of Rare Disease (GREGoR) Program (U01 HG011758) to J.E.P., J.R.L., and R.A.G., and by the National Institute of Neurological Disorders and Stroke (NINDS R35 NS105078) to J.R.L. J.X. was supported by Simons Foundation pilot award (AGT011737). J.S. was supported by the Genomic Research Elucidates the Genetics of Rare Disease (GREGoR) Program (U01 HG011758). S.J was supported by the University of Colorado School of Medicine Translational Research Scholars Program, Simons Foundation pilot award (AGT011737), and the National Institutes of Health grant R35GM133433. Z.C.-A. was supported by the TOPMed NHLBI Fellowship, Simons Foundation pilot award (AGT011737) and the Genomic Research Elucidates the Genetics of Rare Disease (GREGoR) Program (U01 HG011758). S.S.Y. is a Partner Faculty Member of the GREGoR Consortium. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: We used the human genetic variation data available from ClinVar and gnomAD control databases. They are all publicly available. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present work are contained in the manuscript.

Inversions are known contributors to the pathogenesis of genetic diseases. Identifying inversions poses significant challenges, making it one of the most demanding structural variants (SVs) to detect and interpret. Recent advancements in sequencing technologies and the development of publicly available SV datasets have substantially enhanced our capability to explore inversions. However, a cross-comparison in those datasets remains unexplored. In this study, we reported a proband with familial hemophagocytic lymphohistiocytosis type-3 carrying a splicing variant (c.1389+1G>A) in trans with an inversion present in 0.006345% of individuals in gnomAD (v4.0) that disrupts UNC13D. Based on this result, we investigate the features of potentially pathogenic inversions in gnomAD which revealed 98.9% of them are rare and disrupt 5% of protein-coding genes associated with a phenotype in OMIM. We then conducted a comparative analysis of additional public datasets, including DGV, 1KGP, and two recent studies from the Human Genome Structural Variation Consortium which revealed common and dataset-specific inversion characteristics suggesting methodology detection biases. Next, we investigated the genetic features of inversions disrupting the protein-coding genes. Notably, we found that the majority of protein-coding genes in OMIM disrupted by inversions are associated with autosomal recessive phenotypes supporting the hypothesis that inversions in trans with other variants are potential hidden causes of monogenic diseases. This effort aims to fill the gap in our understanding of the molecular characteristics of inversions with low frequency in the population and highlight the importance of identifying them in rare disease studies.

Laterality defects are defined by perturbations in the usual left-right asymmetry of organs. The genetic etiology that underlies congenital heart disease (CHD) is often unknown (less than 40%), so we used a digenic model approach for the identification of contributing variants in known laterality-defect-associated genes (n = 115) in the exome/genome sequencing (ES/GS) data from individuals with clinically diagnosed laterality defects. The unsolved ES/GS data were analyzed from three CHD cohorts: Baylor College of Medicine-Genomics Research to Elucidate the Genetics of Rare Diseases (BCM-GREGoR; n= 251 proband ES), Gabriella Miller Kids First Pediatric Research Program (Kids First; n = 158 trio GS), and Pediatric Cardiac Genomics Consortium (PCGC; n = 163 trio ES). trans-heterozygous digenic variants were identified in 2.8% (inherited digenic variants in 0.4%), 8.2%, and 13.5% of individuals, respectively; this was significantly higher than in 602 control trios provided by the 1000 Genomes Project (p = 0.001, 1.4e-07, and 8.9e-13, respectively). trans-heterozygous digenic variants were also identified in 0.4% and 1.4% of individuals with non-laterality CHD in Kids First and PCGC datasets, respectively, which was not statistically significant as compared to control trios (p = 1 and 0.059, respectively). Altogether, in laterality cohorts, 23% of digenic pairs were in the same structural complex of motile cilia. Out of 39 unique digenic pairs in laterality CHD, 29 are more likely to be potential digenic hits as predicted by the DiGePred tool. These findings provide further evidence that digenic epistatic interactions can contribute to the complex genetics of laterality defects.

Childhood -onset essential hypertension (COEH) is an uncommon form of hypertension that manifests in childhood or adolescence and, in the United States, disproportionately affects children of African ancestry. The etiology of COEH is unknown, but its childhood onset, low prevalence, high heritability, and skewed ancestral demography suggest the potential to identify rare genetic variation segregating in a Mendelian manner among affected individuals and thereby implicate genes important to disease pathogenesis. However, no COEH genes have been reported to date. Here, we identify recessive segregation of rare and putatively damaging missense variation in the spectrin domain of spectrin repeat containing nuclear envelope protein 1 ( SYNE1 ), a cardiovascular candidate gene, in 3 of 16 families with early -onset COEH without an antecedent family history. By leveraging exome sequence data from an additional 48 COEH families, 1,700 in-house trios, and publicly available data sets, we demonstrate that compound heterozygous SYNE1 variation in these COEH individuals occurred more often than expected by chance and that this class of biallelic rare variation was significantly enriched among individuals of African genetic ancestry. Using in vitro shRNA knockdown of SYNE1 , we show that reduced SYNE1 expression resulted in a substantial decrease in the elasticity of smooth muscle vascular cells that could be rescued by pharmacological inhibition of the downstream RhoA/Rho-associated protein kinase pathway. These results provide insights into the molecular genetics and underlying pathophysiology of COEH and suggest a role for precision therapeutics in the future.

Rare diseases affect millions of people worldwide, and most have a genetic etiology. The incorporation of next-generation sequencing into clinical settings, particularly exome and genome sequencing, has resulted in an unprecedented improvement in diagnosis and discovery in the past decade. Nevertheless, these tools are unavailable in many countries, increasing health care gaps between high- and low-and-middle-income countries and prolonging the “diagnostic odyssey” for patients. To advance genomic diagnoses in a setting of limited genomic resources, we developed DECIPHERD, an undiagnosed diseases program in Chile. DECIPHERD was implemented in two phases: training and local development. The training phase relied on international collaboration with Baylor College of Medicine, and the local development was structured as a hybrid model, where clinical and bioinformatics analysis were performed in-house and sequencing outsourced abroad, due to lack of high-throughput equipment in Chile. We describe the implementation process and findings of the first 103 patients. They had heterogeneous phenotypes, including congenital anomalies, intellectual disabilities and/or immune system dysfunction. Patients underwent clinical exome or research exome sequencing, as solo cases or with parents using a trio design. We identified pathogenic, likely pathogenic or variants of unknown significance in genes related to the patients´ phenotypes in 47 (45.6%) of them. Half were de novo informative variants, and half of the identified variants have not been previously reported in public databases. DECIPHERD ended the diagnostic odyssey for many participants. This hybrid strategy may be useful for settings of similarly limited genomic resources and lead to discoveries in understudied populations.

Homozygous duplications contribute to genetic disease by altering gene dosage or disrupting gene regulation and can be more deleterious to organismal biology than heterozygous duplications. Intragenic exonic duplications can result in loss-of-function (LoF) or gain-of-function (GoF) alleles that when homozygosed, i.e. brought to homozygous state at a locus by identity by descent or state, could potentially result in autosomal recessive (AR) rare disease traits. However, the detection and functional interpretation of homozygous duplications from exome sequencing data remains a challenge. We developed a framework algorithm, HMZDupFinder, that is designed to detect exonic homozygous duplications from exome sequencing (ES) data. The HMZDupFinder algorithm can efficiently process large datasets and accurately identifies small intragenic duplications, including those associated with rare disease traits. HMZDupFinder called 965 homozygous duplications with three or less exons from 8,707 ES with a recall rate of 70.9% and a precision of 16.1%. We experimentally confirmed 8/10 rare homozygous duplications. Pathogenicity assessment of these copy number variant alleles allowed clinical genomics contextualization for three homozygous duplications alleles, including two affecting known OMIM disease genes EDAR (MIM# 224900), TNNT1(MIM# 605355), and one variant in a novel candidate disease gene: PAAF1.

ABSTRACT Purpose The variome of the Turkish (TK) population, a population with a considerable history of admixture and consanguinity, has not been deeply investigated deeply for its potential impact on the genomic architecture of disease traits. Methods We generated and analyzed a database of variants derived from exome sequencing (ES) data of 773 TK unrelated, clinically affected individuals with various suspected Mendelian disease traits, and 643 unaffected relatives. Results Using uniform manifold approximation and projection (UMAP), we showed that the TK genomes are more similar to those of Europeans and consist of two main subpopulations: clusters 1 and 2 (N=235 and 1,181) that differ in admixture proportion and variome ( https://turkishvariomedb.shinyapps.io/tvdb/ ). Furthermore, the higher inbreeding coefficient ( F ) values observed in the TK affected compared to unaffected individuals correlated with a larger median span of long-sized (>2.64 Mb) runs of homozygosity (ROH) regions ( p -value=2.09e-18). We show that long-sized ROHs are more likely to be formed on recently configured haplotypes enriched for rare homozygous deleterious variants in the TK-affected compared to TK-unaffected individuals ( p -value= 3.35e-11). Analysis of genotype-phenotype correlations reveals that genes with rare homozygous deleterious variants in long-sized ROHs provide the most comprehensive set of molecular diagnoses for the observed disease traits with a systematic quantitative analysis of HPO (Human Phenotype Ontology) terms. Conclusion Our findings support the notion that novel rare variants on newly configured haplotypes arising within the recent past generations of a family or clan contribute significantly to recessive disease traits in the TK population.

Abstract Background NODAL signaling plays a critical role in embryonic patterning and heart development in vertebrates. Genetic variants resulting in perturbations of the TGF-β/NODAL signaling pathway have reproducibly been shown to cause laterality defects in humans. To further explore this association and improve genetic diagnosis, the study aims to identify and characterize a broader range of NODAL variants in a large number of individuals with laterality defects. Methods We re-analyzed a cohort of 321 proband-only exomes of individuals with clinically diagnosed laterality congenital heart disease (CHD) using family-based, rare variant genomic analyses. To this cohort we added 12 affected subjects with known NODAL variants and CHD from institutional research and clinical cohorts to investigate an allelic series. For those with candidate contributory variants, variant allele confirmation and segregation analysis were studied by Sanger sequencing in available family members. Array comparative genomic hybridization and droplet digital PCR were utilized for copy number variants (CNV) validation and characterization. We performed Human Phenotype Ontology (HPO)-based quantitative phenotypic analyses to dissect allele-specific phenotypic differences. Results Missense, nonsense, splice site, indels, and/or structural variants of NODAL were identified as potential causes of heterotaxy and other laterality defects in 33 CHD cases. We describe a recurrent complex indel variant for which the nucleic acid secondary structure predictions implicate secondary structure mutagenesis as a possible mechanism for formation. We identified two CNV deletion alleles spanning NODAL in two unrelated CHD cases. Furthermore, 17 CHD individuals were found (16/17 with known Hispanic ancestry) to have the c.778G > A:p.G260R NODAL missense variant which we propose reclassification from variant of uncertain significance (VUS) to likely pathogenic. Quantitative HPO-based analyses of the observed clinical phenotype for all cases with p.G260R variation, including heterozygous, homozygous, and compound heterozygous cases, reveal clustering of individuals with biallelic variation. This finding provides evidence for a genotypic-phenotypic correlation and an allele-specific gene dosage model. Conclusion Our data further support a role for rare deleterious variants in NODAL as a cause for sporadic human laterality defects, expand the repertoire of observed anatomical complexity of potential cardiovascular anomalies, and implicate an allele specific gene dosage model.

Background Multilocus pathogenic variants (MPVs) are genetic changes that affect multiple gene loci or regions of the genome, collectively leading to multiple molecular diagnoses. MPVs may also contribute to intrafamilial phenotypic variability between affected individuals within a nuclear family. In this study, we aim to gain further insights into the influence of MPVs on a disease manifestation in individual research subjects and explore the complexities of the human genome within a familial context.Methods We conducted a systematic reanalysis of exome sequencing data and runs of homozygosity (ROH) regions of 47 sibling pairs previously diagnosed with various neurodevelopmental disorders (NDD).Results We found siblings with MPVs driven by long ROH regions in 8.5% of families (4/47). The patients with MPVs exhibited significantly higher FROH values (p-value = 1.4e-2) and larger total ROH length (p-value = 1.8e-2). Long ROH regions mainly contribute to this pattern; the siblings with MPVs have a larger total size of long ROH regions than their siblings in all families (p-value = 6.9e-3). Whereas the short ROH regions in the siblings with MPVs are lower in total size compared to their sibling pairs with single locus pathogenic variants (p-value = 0.029), and there are no statistically significant differences in medium ROH regions between sibling pairs (p-value = 0.52).Conclusion This study sheds light on the significance of considering MPVs in families with affected sibling pairs and the role of ROH as an adjuvant tool in explaining clinical variability within families. Identifying individuals carrying MPVs may have implications for disease management, identification of possible disease risks to different family members, genetic counseling and exploring personalized treatment approaches.

AbstractFLVCR1encodes Feline leukemia virus subgroup C receptor 1 (FLVCR1), a solute carrier (SLC) transporter within the Major Facilitator Superfamily. FLVCR1 is a widely expressed transmembrane protein with plasma membrane and mitochondrial isoforms implicated in heme, choline, and ethanolamine transport. WhileFlvcr1knockout mice diein uterowith skeletal malformations and defective erythropoiesis reminiscent of Diamond-Blackfan anemia, rare biallelic pathogenicFLVCR1variants are linked to childhood or adult-onset neurodegeneration of the retina, spinal cord, and peripheral nervous system.We ascertained from research and clinical exome sequencing 27 individuals from 20 unrelated families with biallelic ultra-rare missense and predicted loss-of-function (pLoF)FLVCR1variant alleles. We characterize an expansiveFLVCR1phenotypic spectrum ranging from adult-onset retinitis pigmentosa to severe developmental disorders with microcephaly, reduced brain volume, epilepsy, spasticity, and premature death. The most severely affected individuals, including three individuals with homozygous pLoF variants, share traits withFlvcr1knockout mice and Diamond-Blackfan anemia including macrocytic anemia and congenital skeletal malformations. PathogenicFLVCR1missense variants primarily lie within transmembrane domains and reduce choline and ethanolamine transport activity compared with wild-typeFLVCR1with minimal impact on FLVCR1 stability or subcellular localization. Several variants disrupt splicing in a mini-gene assay which may contribute to genotype-phenotype correlations. Taken together, these data support an allele-specific gene dosage model in which phenotypic severity reflects residual FLVCR1 activity. This study expands our understanding of Mendelian disorders of choline and ethanolamine transport and demonstrates the importance of choline and ethanolamine in neurodevelopment and neuronal homeostasis.

DExD/H-box RNA helicases (DDX/DHX) are encoded by a large paralogous gene family; in a subset of these human helicase genes, pathogenic variation causes neurodevelopmental disorder (NDD) traits and cancer. DHX9 encodes a BRCA1-interacting nuclear helicase regulating transcription, R-loops, and homologous recombination and exhibits the highest mutational constraint of all DDX/DHX paralogs but remains without disease trait associations. Using exome sequencing and family-based rare variant analysis, we identified 20 individuals with de novo , ultra-rare, heterozygous missense or loss-of-function (LoF) DHX9 variant alleles. Phenotypes ranged from NDDs to the distal symmetric polyneuropathy axonal Charcot-Marie-Tooth disease (CMT2). Quantitative HPO analysis demonstrated genotype-phenotype correlations with LoF variants causing mild NDD phenotypes and nuclear localization signal (NLS) missense variants causing severe NDD. We investigated DHX9 variant-associated cellular phenotypes in human cell lines. Whereas wild-type DHX9 restricted to the nucleus, NLS missense variants abnormally accumulated in the cytoplasm. Fibroblasts from a patient with an NLS variant also showed abnormal cytoplasmic DHX9 accumulation. CMT2-associated missense variants caused aberrant nucleolar DHX9 accumulation, a phenomenon previously associated with cellular stress. Two NDD-associated variants, p.(Gly411Glu) and p.(Arg761Gln), altered DHX9 ATPase activity. The severe NDD-associated variant p.(Arg141Gln) did not impact DHX9 localization but instead increased R-loop levels and double-stranded DNA breaks. Dhx9 -/-mice exhibit hypoactivity in novel environments, tremor, and sensorineural hearing loss. Taken together, these results establish DHX9 as a critical regulator of mammalian neurodevelopment and neuronal homeostasis.

Purpose Pathogenic variants in genes encoding ubiquitin E3 ligases are known to cause neurodevelopmental syndromes. Additional neurodevelopmental disorders associated with the other genes encoding E3 ligases are yet to be identified. Methods Chromosomal analysis and exome sequencing were used to identify the genetic causes in 10 patients from 7 unrelated families with syndromic neurodevelopmental, seizure, and movement disorders and neurobehavioral phenotypes. Results In total, 4 patients were found to have 3 different homozygous loss-of-function (LoF) variants, and 3 patients had 4 compound heterozygous missense variants in the candidate E3 ligase gene, HECTD4, that were rare, absent from controls as homozygous, and predicted to be deleterious in silico. In 3 patients from 2 families with Angelman-like syndrome, paralog-directed candidate gene approach detected 2 LoF variants in the other candidate E3 ligase gene, UBE3C, a paralog of the Angelman syndrome E3 ligase gene, UBE3A. The RNA studies in 4 patients with LoF variants in HECTD4 and UBE3C provided evidence for the LoF effect. Conclusion HECTD4 and UBE3C are novel biallelic rare disease genes, expand the association of the other HECT E3 ligase group with neurodevelopmental syndromes, and could explain some of the missing heritability in patients with a suggestive clinical diagnosis of Angelman syndrome.

Protein phosphatase 1 regulatory subunit 35 (PPP1R35) encodes a centrosomal protein required for recruiting microtubule-binding elongation machinery. Several proteins in this centriole biogenesis pathway correspond to established primary microcephaly (MCPH) genes, and multiple model organism studies hypothesize PPP1R35 as a candidate MCPH gene. Here, using exome sequencing (ES) and family-based rare variant analyses, we report a homozygous, frameshifting indel deleting the canonical stop codon in the last exon of PPP1R35 [Chr7: c.753_*3delGGAAGCGTAGACCinsCG (p.Trp251Cysfs*22)]; the variant allele maps in a 3.7 Mb block of absence of heterozygosity (AOH) in a proband with severe MCPH (-4.3 SD at birth, -6.1 SD by 42 months), pachygyria, and global developmental delay from a consanguineous Turkish kindred. Droplet digital PCR (ddPCR) confirmed mutant mRNA expression in fibroblasts. In silico prediction of the translation of mutant PPP1R35 is expected to be elongated by 18 amino acids before encountering a downstream stop codon. This complex indel allele is absent in public databases (ClinVar, gnomAD, ARIC, 1000 genomes) and our in-house database of 14,000+ exomes including 1800+ Turkish exomes supporting predicted pathogenicity. Comprehensive literature searches for PPP1R35 variants yielded two probands affected with severe microcephaly (-15 SD and -12 SD) with the same homozygous indel from a single, consanguineous, Iranian family from a cohort of 404 predominantly Iranian families. The lack of heterozygous cases in two large cohorts representative of the genetic background of these two families decreased our suspicion of a founder allele and supports the contention of a recurrent mutation. We propose two potential secondary structure mutagenesis models for the origin of this variant allele mediated by hairpin formation between complementary GC rich segments flanking the stop codon via secondary structure mutagenesis.

Traditionally, disease causal mutations were thought to disrupt gene function. However, it becomes more clear that many deleterious mutations could exhibit a "gain-of-function" (GOF) behavior. Systematic investigation of such mutations has been lacking and largely overlooked. Advances in next-generation sequencing have identified thousands of genomic variants that perturb the normal functions of proteins, further contributing to diverse phenotypic consequences in disease. Elucidating the functional pathways rewired by GOF mutations will be crucial for prioritizing disease-causing variants and their resultant therapeutic liabilities. In distinct cell types (with varying genotypes), precise signal transduction controls cell decision, including gene regulation and phenotypic output. When signal transduction goes awry due to GOF mutations, it would give rise to various disease types. Quantitative and molecular understanding of network perturbations by GOF mutations may provide explanations for 'missing heritability" in previous genome-wide association studies. We envision that it will be instrumental to push current paradigm toward a thorough functional and quantitative modeling of all GOF mutations and their mechanistic molecular events involved in disease development and progression. Many fundamental questions pertaining to genotype-phenotype relationships remain unresolved. For example, which GOF mutations are key for gene regulation and cellular decisions? What are the GOF mechanisms at various regulation levels? How do interaction networks undergo rewiring upon GOF mutations? Is it possible to leverage GOF mutations to reprogram signal transduction in cells, aiming to cure disease? To begin to address these questions, we will cover a wide range of topics regarding GOF disease mutations and their characterization by multi-omic networks. We highlight the fundamental function of GOF mutations and discuss the potential mechanistic effects in the context of signaling networks. We also discuss advances in bioinformatic and computational resources, which will dramatically help with studies on the functional and phenotypic consequences of GOF mutations.

Telomere maintenance 2 (TELO2), Tel2 interacting protein 2 (TTI2), and Tel2 interacting protein 1 (TTI1) are the three components of the conserved Triple T (TTT) complex that modulates activity of phosphatidylinositol 3-kinase-related protein kinases (PIKKs), including mTOR, ATM, and ATR, by regulating the assembly of mTOR complex 1 (mTORC1). The TTT complex is essential for the expression, maturation, and stability of ATM and ATR in response to DNA damage. TELO2- and TTI2-related bi-allelic autosomal-recessive (AR) encephalopathies have been described in individuals with moderate to severe intellectual disability (ID), short stature, postnatal microcephaly, and a movement disorder (in the case of variants within TELO2). We present clinical, genomic, and functional data from 11 individuals in 9 unrelated families with bi-allelic variants in TTI1. All present with ID, and most with microcephaly, short stature, and a movement disorder. Functional studies performed in HEK293T cell lines and fibroblasts and lymphoblastoid cells derived from 4 unrelated individuals showed impairment of the TTT complex and of mTOR pathway activity which is improved by treatment with Rapamycin. Our data delineate a TTI1-related neurodevelopmental disorder and expand the group of disorders related to the TTT complex.