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OBJECTIVE: To evaluate the accuracy of next-generation sequencing-based quantitative cell-free DNA analysis for fetal antigen genotyping in individuals with alloimmunized pregnancies undergoing clinical testing in practices across the United States as early as 10 weeks of gestation, with the objective of identifying individuals with pregnancies at risk for hemolytic disease of the fetus and newborn and guiding management. METHODS: This prospective cohort study included patients with alloimmunized pregnancies undergoing clinical fetal antigen cell-free DNA analysis between 10 0/7 and 37 0/7 weeks of gestation at 120 clinical sites. Both the pregnant person with the alloimmunized pregnancy and the neonates resulting from the pregnancies were included. The laboratory issued the cell-free DNA results prospectively as a part of clinical care. After delivery, neonatal buccal swabs collected between 0 and 270 days of life were sent to an outside independent laboratory for antigen genotyping. The outside laboratory was blinded to the fetal cell-free DNA results, and the results were compared. Concordance was reported for the fetal antigen cell-free DNA analysis for antigens to which the pregnant person was alloimmunized and for all antigens for which the pregnant person was genotype negative. RESULTS: A total of 156 pregnant people who received clinically ordered cell-free DNA fetal antigen testing provided neonatal buccal swabs for genotyping after delivery. Overall, 15.4% of participants were Hispanic, 9.0% were non-Hispanic Black, 65.4% were non-Hispanic White, 4.5% were Asian, 1.3% were more than one race or ethnicity, and 4.5% were unknown. The median gestational age at the time of testing was 16.4 weeks with a median fetal fraction of 11.1%. Concordance between cell-free DNA analysis results and neonatal genotype was determined for 465 antigen calls for the following antigens: K1 (n=143), E (124), C (60), Fya (50), c (47), and D(RhD) (41). These 465 calls included 145 in which the fetus was antigen positive and 320 in which the fetus was antigen negative. We observed complete concordance between prenatal fetal antigen cell-free DNA analysis results and neonatal genotypes for the 465 calls, resulting in 100% sensitivity, specificity, and accuracy. CONCLUSION: In a diverse multicenter cohort, cell-free DNA analysis was highly sensitive and specific for determining fetal antigen genotype as early as 10 weeks of gestation in individuals with alloimmunized pregnancies. Taken together with previously published evidence, this study supports the implementation of cell-free DNA testing to manage individuals with alloimmunized pregnancies in the United States.
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PURPOSE: The ClinGen Actionability Working Group (AWG) developed an evidence-based framework to generate actionability reports and scores of gene-condition pairs in the context of secondary findings from genome sequencing. Here we describe the expansion of the framework to include actionability assertions. METHODS: Initial development of the actionability rubric was based on previously scored adult gene-condition pairs and individual expert evaluation. Rubric refinement was iterative and based on evaluation, feedback, and discussion. The final rubric was pragmatically evaluated via integration into actionability assessments for 27 gene-condition pairs. RESULTS: The resulting rubric has a 4-point scale (limited, moderate, strong, and definitive) and uses the highest-scoring outcome-intervention pair of each gene-condition pair to generate a preliminary assertion. During AWG discussions, predefined criteria and factors guide discussion to produce a consensus assertion for a gene-condition pair, which may differ from the preliminary assertion. The AWG has retrospectively generated assertions for all previously scored gene-condition pairs and are prospectively asserting on gene-condition pairs under assessment, having completed over 170 adult and 188 pediatric gene-condition pairs. CONCLUSION: The AWG expanded its framework to provide actionability assertions to enhance the clinical value of their resources and increase their utility as decision aids regarding return of secondary findings.
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Medicina Baseada em Evidências , Humanos , Medicina Baseada em Evidências/métodos , Testes Genéticos/métodos , Achados Incidentais , Sequenciamento Completo do GenomaRESUMO
The reanalysis of genomic sequencing results has the potential to provide results that are of considerable medical and personal importance to recipients. Employing interviews with forty-seven predominantly medically underserved families and ethnographic observations we argue that there is pressing need to standardize the approach taken to reanalysis. Our findings highlight that study participants were unclear as to the likelihood of reanalysis happening, the process of initiating reanalysis, and whether they would receive revised results. Their reflections mirror the lack a specific focus upon reanalysis within consent and results sessions as observed in clinical settings. Mechanisms need to be put into place that standardize the approach to reanalysis in research and in clinical contexts. This would enable clinicians and genetic counsellors to communicate clearly with research participants with respect to potential for reanalysis of results and the process of reanalysis. We argue that that the role of reanalysis is too important to be referred to in an ad-hoc manner. Furthermore, the ad-hoc nature of the current process may increase health inequities given the likelihood that only those families who have the means to press for reanalysis are likely to receive it.
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The diagnostic yield of exome sequencing (ES) has primarily been evaluated in individuals of European ancestry, with less focus on underrepresented minority (URM) and underserved (US) patients. We evaluated the diagnostic yield of ES in a cohort of predominantly US and URM pediatric and prenatal patients suspected to have a genetic disorder. Eligible pediatric patients had multiple congenital anomalies and/or neurocognitive disabilities and prenatal patients had one or more structural anomalies, disorders of fetal growth, or fetal effusions. URM and US patients were prioritized for enrollment and underwent ES at a single academic center. We identified definitive positive or probable positive results in 201/845 (23.8%) patients, with a significantly higher diagnostic rate in pediatric (26.7%) compared to prenatal patients (19.0%) (P = 0.01). For both pediatric and prenatal patients, the diagnostic yield and frequency of inconclusive findings did not differ significantly between URM and non-URM patients or between patients with US status and those without US status. Our results demonstrate a similar diagnostic yield of ES between prenatal and pediatric URM/US patients and non-URM/US patients for positive and inconclusive results. These data support the use of ES to identify clinically relevant variants in patients from diverse populations.
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PURPOSE: Accurate and understandable information after genetic testing is critical for patients, family members, and professionals alike. METHODS: As part of a cross-site study from the Clinical Sequencing Evidence-Generating Research consortium, we investigated the information-seeking practices among patients and family members at 5 to 7 months after genetic testing results disclosure, assessing the perceived utility of a variety of information sources, such as family and friends, health care providers, support groups, and the internet. RESULTS: We found that individuals placed a high value on information obtained from genetics professionals and health care workers, independent of genetic testing result case classifications as positive, inconclusive, or negative. The internet was also highly utilized and ranked. Study participants rated some information sources as more useful for positive results compared with inconclusive or negative outcomes, emphasizing that it may be difficult to identify helpful information for individuals receiving an uncertain or negative result. There were few data from non-English speakers, highlighting the need to develop strategies to reach this population. CONCLUSION: Our study emphasizes the need for clinicians to provide accurate and comprehensible information to individuals from diverse populations after genetic testing.
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Testes Genéticos , Comportamento de Busca de Informação , Humanos , Grupos Populacionais , Incerteza , FamíliaRESUMO
PURPOSE: Patients undergoing clinical exome sequencing (ES) are routinely offered the option to receive secondary findings (SF). However, little is known about the views of individuals from underrepresented minority pediatric or prenatal populations regarding SF. METHODS: We explored the preferences for receiving hypothetical categories of SF (H-SF) and reasons for accepting or declining actual SF through surveying (n = 149) and/or interviewing (n = 47) 190 families undergoing pediatric or prenatal ES. RESULTS: Underrepresented minorities made up 75% of the probands. In total, 150 families (79%) accepted SF as part of their child/fetus's ES. Most families (63%) wanted all categories of H-SF. Those who declined SF as part of ES were less likely to want H-SF across all categories. Interview findings indicate that some families did not recall their SF decision. Preparing for the future was a major motivator for accepting SF, and concerns about privacy, discrimination, and psychological effect drove decliners. CONCLUSION: A notable subset of families (37%) did not want at least 1 category of H-SF, suggesting more hesitancy about receiving all available results than previously reported. The lack of recollection of SF decisions suggests a need for alternative communication approaches. Results highlight the importance of the inclusion of diverse populations in genomic research.
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Família , Genômica , Criança , Exoma/genética , Feminino , Genoma Humano , Humanos , Gravidez , Sequenciamento do Exoma/métodosRESUMO
The interpretation of genomic variants following whole exome sequencing (WES) can be aided using human phenotype ontology (HPO) terms to standardize clinical features and predict causative genes. We performed WES on 453 patients diagnosed prior to 18 years of age and identified 114 pathogenic (P) or likely pathogenic (LP) variants in 112 patients. We utilized PhenoDB to extract HPO terms from provider notes and then used Phen2Gene to generate a gene score and gene ranking from each list of HPO terms. We assigned Phen2Gene gene rankings to 6 rank classes, with class 1 covering raw gene rankings of 1 to 10 and class 2 covering rankings from 11 to 50 out of a total of 17,126 possible gene rankings. Phen2Gene ranked causative genes into rank class 1 or 2 in 27.7% of cases and the genes in rank class 1 were all associated with well-characterized phenotypes. We found significant associations between the gene score and the number of years, since the gene was first published, the number of HPO terms with an hierarchical depth greater or equal to 11, and the number of Online Mendelian Inheritance in Man terms associated with the phenotype and gene. We conclude that genes associated with recognizable phenotypes and terms deep in the HPO hierarchy have the best chance of producing a high gene score and ranking in class 1 to 2 using Phen2Gene software with HPO terms. Clinicians and laboratory staff should consider these results when HPO terms are employed to prioritize candidate genes.
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Bases de Dados Genéticas , Software , Humanos , Fenótipo , Sequenciamento do ExomaRESUMO
PURPOSE: There is a critical need for genomic medicine research that reflects and benefits socioeconomically and ancestrally diverse populations. However, disparities in research populations persist, highlighting that traditional study designs and materials may be insufficient or inaccessible to all groups. New approaches can be gained through collaborations with patient/community stakeholders. Although some benefits of stakeholder engagement are recognized, routine incorporation into the design and implementation of genomics research has yet to be realized. METHODS: The National Institutes of Health-funded Clinical Sequencing Evidence-Generating Research (CSER) consortium required stakeholder engagement as a dedicated project component. Each CSER project planned and carried out stakeholder engagement activities with differing goals and expected outcomes. Examples were curated from each project to highlight engagement strategies and outcomes throughout the research lifecycle from development through dissemination. RESULTS: Projects tailored strategies to individual study needs, logistical constraints, and other challenges. Lessons learned include starting early with engagement efforts across project stakeholder groups and planned flexibility to enable adaptations throughout the project lifecycle. CONCLUSION: Each CSER project used more than 1 approach to engage with relevant stakeholders, resulting in numerous adaptations and tremendous value added throughout the full research lifecycle. Incorporation of community stakeholder insight improves the outcomes and relevance of genomic medicine research.
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Medicina Genômica , Participação dos Interessados , Genômica , Humanos , Grupos Populacionais , Projetos de PesquisaRESUMO
Chromatin is essentially an array of nucleosomes, each of which consists of the DNA double-stranded fiber wrapped around a histone octamer. This organization supports cellular processes such as DNA replication, DNA transcription, and DNA repair in all eukaryotes. Human histone H4 is encoded by fourteen canonical histone H4 genes, all differing at the nucleotide level but encoding an invariant protein. Here, we present a cohort of 29 subjects with de novo missense variants in six H4 genes (H4C3, H4C4, H4C5, H4C6, H4C9, and H4C11) identified by whole-exome sequencing and matchmaking. All individuals present with neurodevelopmental features of intellectual disability and motor and/or gross developmental delay, while non-neurological features are more variable. Ten amino acids are affected, six recurrently, and are all located within the H4 core or C-terminal tail. These variants cluster to specific regions of the core H4 globular domain, where protein-protein interactions occur with either other histone subunits or histone chaperones. Functional consequences of the identified variants were evaluated in zebrafish embryos, which displayed abnormal general development, defective head organs, and reduced body axis length, providing compelling evidence for the causality of the reported disorder(s). While multiple developmental syndromes have been linked to chromatin-associated factors, missense-bearing histone variants (e.g., H3 oncohistones) are only recently emerging as a major cause of pathogenicity. Our findings establish a broader involvement of H4 variants in developmental syndromes.
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Histonas , Peixe-Zebra , Animais , Cromatina , DNA , Histonas/metabolismo , Humanos , Síndrome , Peixe-Zebra/genética , Peixe-Zebra/metabolismoRESUMO
OBJECTIVE: We aimed to determine the frequency of accepting secondary findings in families undergoing exome sequencing in prenatal and pediatric settings. METHODS: This was a secondary analysis of prospectively enrolled patients undergoing trio exome sequencing for congenital anomalies or developmental disorders in prenatal and pediatric settings, in which families were offered receiving secondary findings (initially assessed in the proband and, if identified, then in the parents). The primary outcome was frequency of accepting secondary findings. Secondary outcomes included frequency of acceptance in prenatal versus pediatric settings, and sociodemographic differences between those who accepted versus declined secondary findings. RESULTS: There were 682 families included in the cohort (289 prenatal and 393 pediatric). Overall, 84% (576/682) of families accepted secondary findings: 86.2% (249/289) of families undergoing prenatal versus 83.2% (327/393) pediatric (p = 0.30) testing. Secondary findings were identified in 2.6% (15/576) of cases, with no difference between prenatal and pediatric settings. There were no differences in sociodemographics between families that accepted versus declined secondary findings. CONCLUSION: The majority of families undergoing exome sequencing accepted secondary findings; this did not differ in prenatal versus pediatric settings. This highlights the need for guidance surrounding the offer of secondary findings in the prenatal setting.
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Exoma , Família , Criança , Estudos de Coortes , Feminino , Humanos , Pais , Gravidez , Diagnóstico Pré-Natal , Sequenciamento do ExomaRESUMO
Nicotinamide adenine dinucleotide (NAD) is an essential coenzyme involved in over 400 cellular reactions. During embryogenesis, mammals synthesize NAD de novo from dietary l -tryptophan via the kynurenine pathway. Biallelic, inactivating variants in three genes encoding enzymes of this biosynthesis pathway (KYNU, HAAO, and NADSYN1) disrupt NAD synthesis and have been identified in patients with multiple malformations of the heart, kidney, vertebrae, and limbs; these patients have Congenital NAD Deficiency Disorder HAAO and four families with biallelic variants in KYNU. These patients present similarly with multiple malformations of the heart, kidney, vertebrae, and limbs, of variable severity. We show that each variant identified in these patients results in loss-of-function, revealed by a significant reduction in NAD levels via yeast genetic complementation assays. For the first time, missense mutations are identified as a cause of malformation and shown to disrupt enzyme function. These missense and frameshift variants cause moderate to severe NAD deficiency in yeast, analogous to insufficient synthesized NAD in patients. We hereby expand the genotypic and corresponding phenotypic spectrum of Congenital NAD Deficiency Disorder.
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NAD , Coluna Vertebral , Animais , Genótipo , Humanos , Mamíferos , Mutação de Sentido Incorreto , Coluna Vertebral/anormalidadesRESUMO
Genomic sequencing results need to be effectively communicated across all populations and practice settings. Projects in the Clinical Sequencing Evidence-Generating Research (CSER) consortium enroll diverse racial/ethnic and medically underserved participants across various clinical contexts. This article explores a set of CSER results disclosure cases to expand the evidence base on experiences returning genomic results. Case details were collected using a structured set of questions. We identified common themes in the case set, and assessed challenges and strategies in achieving six relevant results disclosure objectives. CSER-affiliated patient/community stakeholder impressions of the findings were solicited via video conference calls. Seventeen cases across six CSER projects were included. Case themes sorted into four categories: (1) factors influencing participant understanding, (2) participant emotional response, (3) disease burden, and (4) logistical challenges. Challenges meeting results disclosure objectives included a lack of dialogue, health literacy level, unexpected findings, and complex concepts. Strategies were consistent with traditional genetic counseling practice, but also highlighted approaches being evaluated in CSER projects. Patient/community stakeholders supported the identified themes and provided additional suggestions to improve patient understanding and engagement. These experiences add valuable insights into adapting genomic results disclosure practices to best serve all patient populations.
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The descriptor 'usual care' refers to standard or routine care. Yet, no formal definition exists. The need to define what constitutes usual care arises in clinical research. Often one arm in a trial represents usual care in comparison with a novel intervention. Accordingly, usual care in genetic counseling research appears predominantly in randomized controlled trials. Recent standards for reporting genetic counseling research call for standardization, but do not address usual care. We (1) inventoried all seven studies in the Clinical Sequencing Evidence-Generating Consortium (CSER) about how genetic counseling was conceptualized, conducted, and whether a usual care arm was involved; (2) conducted a review of published randomized control trials in genetic counseling, comparing how researchers describe usual care groups; and (3) reviewed existing professionally endorsed definitions and practice descriptions of genetic counseling. We found wide variation in the content and delivery of usual care. Descriptions frequently detailed the content of usual care, most often noting assessment of genetic risk factors, collecting family histories, and offering testing. A minority included addressing psychological concerns or the risks versus benefits of testing. Descriptions of how care was delivered were vague except for mode and type of clinician, which varied. This significant variation, beyond differences expected among subspecialties, reduces the validity and generalizability of genetic counseling research. Ideally, research reflects clinical practice so that evidence generated can be used to improve clinical outcomes. To address this objective, we propose a definition of usual care in genetic counseling research that merges common elements from the National Society of Genetic Counselors' practice definition, the Reciprocal Engagement Model, and the Accreditation Council for Genetic Counselors' practice-based competencies. Promoting consistent execution of usual care in the design of genetic counseling trials can lead to more consistency in representing clinical care and facilitate the generation of evidence to improve it.
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Aconselhamento , Aconselhamento Genético , Acreditação , HumanosRESUMO
Essential tremor (ET) is the most common adult-onset movement disorder. In the present study, we performed whole exome sequencing of a large ET-affected family (10 affected and 6 un-affected family members) and identified a TUB p.V431I variant (rs75594955) segregating in a manner consistent with autosomal-dominant inheritance. Subsequent targeted re-sequencing of TUB in 820 unrelated individuals with sporadic ET and 630 controls revealed significant enrichment of rare nonsynonymous TUB variants (e.g. rs75594955: p.V431I, rs1241709665: p.Ile20Phe, rs55648406: p.Arg49Gln) in the ET cohort (SKAT-O test p-value = 6.20e-08). TUB encodes a transcription factor predominantly expressed in neuronal cells and has been previously implicated in obesity. ChIP-seq analyses of the TUB transcription factor across different regions of the mouse brain revealed that TUB regulates the pathways responsible for neurotransmitter production as well thyroid hormone signaling. Together, these results support the association of rare variants in TUB with ET.
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Proteínas Adaptadoras de Transdução de Sinal/genética , Tremor Essencial/genética , Proteínas Adaptadoras de Transdução de Sinal/metabolismo , Idoso , Idoso de 80 Anos ou mais , Animais , Sequenciamento de Cromatina por Imunoprecipitação/métodos , Estudos de Coortes , Exoma/genética , Família , Feminino , Humanos , Masculino , Camundongos Endogâmicos C57BL , Pessoa de Meia-Idade , Linhagem , Polimorfismo de Nucleotídeo Único/genética , Fatores de Transcrição/genética , Sequenciamento do Exoma/métodosRESUMO
Recent advances in induced pluripotent stem cell (iPSC) technology and directed differentiation of iPSCs into cardiomyocytes (iPSC-CMs) make it possible to model genetic heart disease in vitro. We apply CRISPR/Cas9 genome editing technology to introduce three RBM20 mutations in iPSCs and differentiate them into iPSC-CMs to establish an in vitro model of RBM20 mutant dilated cardiomyopathy (DCM). In iPSC-CMs harboring a known causal RBM20 variant, the splicing of RBM20 target genes, calcium handling, and contractility are impaired consistent with the disease manifestation in patients. A variant (Pro633Leu) identified by exome sequencing of patient genomes displays the same disease phenotypes, thus establishing this variant as disease causing. We find that all-trans retinoic acid upregulates RBM20 expression and reverts the splicing, calcium handling, and contractility defects in iPSC-CMs with different causal RBM20 mutations. These results suggest that pharmacological upregulation of RBM20 expression is a promising therapeutic strategy for DCM patients with a heterozygous mutation in RBM20.
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Células-Tronco Pluripotentes Induzidas/metabolismo , Splicing de RNA/genética , Proteínas de Ligação a RNA/metabolismo , Humanos , Regulação para CimaRESUMO
NFIB (Nuclear Factor I B) haploinsufficiency has recently been identified as a cause of intellectual disability and macrocephaly. Here we describe two patients with pathogenic variants in NFIB. The first is a 6-year-old Latino male with developmental delays, mild hypotonia, facial anomalies, and brain magnetic resonance imaging findings comprising mild thinning of the corpus callosum, with more marked thinning of the splenium and blunting of the rostrum and cavum septum pellucidum. Exome sequencing identified a previously described de novo variant in NFIB, c.265C>T, predicting p.Arg89Ter. The second is a 5-year-old Latino male with developmental delays, hypotonia, dysmorphic features, a preauricular tag and pit, a small ventricular septal defect, and brain magnetic resonance imaging findings including a dysmorphic corpus callosum and a small posterior fossa. A single nucleotide polymorphism microarray identified a 92 kb interstitial deletion at 9p23 including several exons of NFIB and no other known genes. Our two patients add to the knowledge of this rare condition through our addition of new brain MRI findings and dysmorphic features. Additionally, these are the first known Latino patients to be described with NFIB haploinsufficiency, expanding our understanding of the associated facial features in diverse populations. Further data are needed to determine genotype-phenotype relationships for NFIB.
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Deleção Cromossômica , Deficiências do Desenvolvimento/patologia , Haploinsuficiência , Deficiência Intelectual/patologia , Hipotonia Muscular/patologia , Fatores de Transcrição NFI/genética , Criança , Pré-Escolar , Deficiências do Desenvolvimento/genética , Feminino , Humanos , Deficiência Intelectual/genética , Masculino , Hipotonia Muscular/genética , FenótipoRESUMO
We describe an 11-year old boy with severe global developmental delays, failure to thrive and growth retardation, refractory seizures with recurrent status epilepticus, hypogammaglobulinemia, hypergonadotropic hypogonadism, and duodenal strictures. He had facial and skin findings compatible with trichothiodystrophy, including sparse and brittle hair, thin eyebrows, and dry skin. Exome sequencing showed a hemizygous, truncating variant in RNF113A, c.903_910delGCAGACCA, predicting p.(Gln302fs*12), that was inherited from his mother. Although his clinical features overlap closely with features described in the two previously reported male first cousins with RNF113A loss of function mutations, the duodenal strictures seen in this patient have not been reported. Interestingly, the patient's mother had short stature and 100% skewed X-inactivation as seen in other obligate female carriers. A second male with developmental delays, microcephaly, seizures, ambiguous genitalia, and facial anomalies that included sparse and brittle hair, thin eyebrows and dry skin was recently reported to have c.897_898delTG, predicting p.(Cys299*) in RNF113A and we provide additional clinical details for this patient. This report further supports deleterious variants in RNF113A as a cause of a novel trichothiodystrophy syndrome.
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Proteínas de Ligação a DNA/genética , Doenças Genéticas Ligadas ao Cromossomo X/genética , Predisposição Genética para Doença , Síndromes de Tricotiodistrofia/genética , Criança , Pré-Escolar , Feminino , Estudos de Associação Genética , Doenças Genéticas Ligadas ao Cromossomo X/diagnóstico , Doenças Genéticas Ligadas ao Cromossomo X/patologia , Humanos , Masculino , Mutação/genética , Síndromes de Tricotiodistrofia/diagnóstico , Síndromes de Tricotiodistrofia/patologia , Inativação do Cromossomo X/genéticaRESUMO
Informed consent, the process of gathering autonomous authorization for a medical intervention or medical research participation, is a fundamental component of medical practice. Medical informed consent assumes decision-making capacity, voluntariness, comprehension, and adequate information. The increasing use of genetic testing, particularly genomic sequencing, in clinical and research settings has presented many new challenges for clinicians and researchers when obtaining informed consent. Many of these challenges revolve around the need for patient comprehension of sufficient information. Genomic sequencing is complex-all of the possible results are too numerous to explain, and many of the risks and benefits remain unknown. Thus, historical standards of consent are difficult to apply. Alternative models of consent have been proposed to increase patient understanding, and several have empirically demonstrated effectiveness. However, there is still a striking lack of consensus in the genetics community about what constitutes informed consent in the context of genomic sequencing. Multiple approaches are needed to address this challenge, including consensus building around standards, targeted use of genetic counselors in nongenetics clinics in which genomic testing is ordered, and the development and testing of alternative models for obtaining informed consent.
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Aconselhamento Genético/métodos , Testes Genéticos , Consentimento Livre e Esclarecido/normas , Feminino , Aconselhamento Genético/tendências , Genômica , Sequenciamento de Nucleotídeos em Larga Escala , Humanos , MasculinoRESUMO
Developmental and epileptic encephalopathies are genetic disorders in which both the developmental disability and the frequent epileptic activity are the effect of a specific gene variant. While heterozygous variants in SCN1B have been described in families with generalized epilepsy with febrile seizures plus, Type 1, only three cases of homozygous, missense variants in SCN1B have been reported in association with autosomal recessive inheritance of a severe developmental and epileptic encephalopathy. We present two siblings who are homozygous for a novel, missense variant in SCN1B, c.265C>T, predicting p.Arg89Cys. The proband is an 11-year-old female with infantile-onset, fever-induced, intractable generalized tonic-clonic seizures, myoclonic seizures, and developmental slowing and autism spectrum disorder occurring later in the course of the disease. Her 4-year-old brother had a similar epilepsy phenotype, but still displays normal development. This variant has not been previously reported in the homozygous state in control databases. The variant was predicted to be damaging and occurred in the vicinity of other epileptic encephalopathy-associated missense variants that are biallelic and located in the extracellular immunoglobulin loop domain of the protein, which mediates interaction of the beta-1 subunit with cellular adhesion molecules. Our report is the first set of siblings with homozygosity for the p.Arg89Cys variant in SCN1B and further implicates biallelic mutations in this gene as a cause of epileptic encephalopathy mimicking Dravet syndrome. Interestingly, the phenotype we observed was milder compared to that previously described in patients with recessive SCN1B mutations.