Genotype first: Clinical genomics research through a reverse phenotyping approach.

Although genomic research has predominantly relied on phenotypic ascertainment of individuals affected with heritable disease, the falling costs of sequencing allow consideration of genomic ascertainment and reverse phenotyping (the ascertainment of individuals with specific genomic variants and subsequent evaluation of physical characteristics). In this research modality, the scientific question is inverted: investigators gather individuals with a genomic variant and test the hypothesis that there is an associated phenotype via targeted phenotypic evaluations. Genomic ascertainment research is thus a model of predictive genomic medicine and genomic screening. Here, we provide our experience implementing this research method. We describe the infrastructure we developed to perform reverse phenotyping studies, including aggregating a super-cohort of sequenced individuals who consented to recontact for genomic ascertainment research. We assessed 13 studies completed at the National Institutes of Health (NIH) that piloted our reverse phenotyping approach. The studies can be broadly categorized as (1) facilitating novel genotype-disease associations, (2) expanding the phenotypic spectra, or (3) demonstrating ex vivo functional mechanisms of disease. We highlight three examples of reverse phenotyping studies in detail and describe how using a targeted reverse phenotyping approach (as opposed to phenotypic ascertainment or clinical informatics approaches) was crucial to the conclusions reached. Finally, we propose a framework and address challenges to building collaborative genomic ascertainment research programs at other institutions. Our goal is for more researchers to take advantage of this approach, which will expand our understanding of the predictive capability of genomic medicine and increase the opportunity to mitigate genomic disease.

Reappraisal of Idiopathic CD4 Lymphocytopenia at 30 Years.

BACKGROUND: Idiopathic CD4 lymphocytopenia (ICL) is a clinical syndrome that is defined by CD4 lymphopenia of less than 300 cells per cubic millimeter in the absence of any primary or acquired cause of immunodeficiency. Some 30 years after its original identification, ICL has remained a disease of obscure cause, with limited evidence with respect to its prognosis or management, despite diagnostic and therapeutic innovations. METHODS: We evaluated the clinical, genetic, immunologic, and prognostic characteristics of 108 patients who were enrolled during an 11-year period. We performed whole-exome and targeted gene sequencing to identify genetic causes of lymphopenia. We also performed longitudinal linear mixed-model analyses of T-cell count trajectories and evaluated predictors of clinical events, the response to immunization against coronavirus disease 2019 (Covid-19), and mortality. RESULTS: After the exclusion of patients with genetic and acquired causes of CD4 lymphopenia, the study population included 91 patients with ICL during 374 person-years of follow-up. The median CD4+ T-cell count among the patients was 80 cells per cubic millimeter. The most prevalent opportunistic infections were diseases related to human papillomavirus (in 29%), cryptococcosis (in 24%), molluscum contagiosum (in 9%), and nontuberculous mycobacterial diseases (in 5%). A reduced CD4 count (<100 cells per cubic millimeter), as compared with a CD4 count of 101 to 300 cells, was associated with a higher risk of opportunistic infection (odds ratio, 5.3; 95% confidence interval [CI], 2.8 to 10.7) and invasive cancer (odds ratio, 2.1; 95% CI, 1.1 to 4.3) and a lower risk of autoimmunity (odds ratio, 0.5; 95% CI, 0.2 to 0.9). The risk of death was similar to that in the age- and sex-adjusted general population, but the prevalence of cancer was higher. CONCLUSIONS: Among the study patients, ICL continued to be associated with increased susceptibility to viral, encapsulated fungal, and mycobacterial diseases, as well as with a reduced response to novel antigens and an increased risk of cancer. (Funded by the National Institute of Allergy and Infectious Diseases and the National Cancer Institute; ClinicalTrials.gov number, NCT00867269.).

Psychological state at the time of psychiatric genetic counseling impacts patient empowerment: A pre-post analysis.

Psychiatric genetic counseling (GC) has been associated with patient-reported increases in empowerment (perceived control, emotional regulation, and hope). We sought to evaluate the extent to which patients' psychological state at the time of GC is related to changes in empowerment. Participants with a history of major depressive disorder and/or bipolar disorder that had been refractory to treatment underwent psychiatric GC remotely from 2022 to 2023. GC was performed by four genetic counselors and included discussion of perceived causes of illness, multifactorial inheritance, and protective factors. Empowerment, depression, and anxiety were measured immediately prior to GC via online survey by the GCOS-16, PHQ-9, and GAD-7, respectively. Empowerment was re-assessed 2 weeks later. In total, 66/161 (41.0%) invited individuals completed both the baseline and follow-up surveys. Participants completing both surveys were 54.6% female, 84.8% white, and ranged in age from 22 to 78 years (mean = 54.8 years). Overall, a significant change in mean empowerment was not observed (p = 0.38); however, there were moderating effects by baseline psychological state. A multiple linear regression model incorporating PHQ-9, GAD-7 and baseline GCOS-16 score predicted change in empowerment with a large effect (F = 5.49, R2 = 0.21, p < 0.01). A higher score on the PHQ-9 was associated with decreases in empowerment from pre to post GC. Higher scores on the GAD-7 and lower baseline GCOS-16 scores were associated with increases in empowerment. Further, two-way ANOVA was conducted to assess change in empowerment between subgroups based on the level of anxiety and depression. Those with low depression and high anxiety reported significant increases in empowerment (F = 6.64, p = 0.01). These findings suggest that psychiatric GC may be especially helpful to individuals experiencing anxiety and low baseline empowerment. Alternative approaches may be needed to best meet the needs of those experiencing significant depression.

Immunodeficiency and bone marrow failure with mosaic and germline TLR8 gain of function.

Inborn errors of immunity (IEI) are a genetically heterogeneous group of disorders with a broad clinical spectrum. Identification of molecular and functional bases of these disorders is important for diagnosis, treatment, and an understanding of the human immune response. We identified 6 unrelated males with neutropenia, infections, lymphoproliferation, humoral immune defects, and in some cases bone marrow failure associated with 3 different variants in the X-linked gene TLR8, encoding the endosomal Toll-like receptor 8 (TLR8). Interestingly, 5 patients had somatic variants in TLR8 with <30% mosaicism, suggesting a dominant mechanism responsible for the clinical phenotype. Mosaicism was also detected in skin-derived fibroblasts in 3 patients, demonstrating that mutations were not limited to the hematopoietic compartment. All patients had refractory chronic neutropenia, and 3 patients underwent allogeneic hematopoietic cell transplantation. All variants conferred gain of function to TLR8 protein, and immune phenotyping demonstrated a proinflammatory phenotype with activated T cells and elevated serum cytokines associated with impaired B-cell maturation. Differentiation of myeloid cells from patient-derived induced pluripotent stem cells demonstrated increased responsiveness to TLR8. Together, these findings demonstrate that gain-of-function variants in TLR8 lead to a novel childhood-onset IEI with lymphoproliferation, neutropenia, infectious susceptibility, B- and T-cell defects, and in some cases, bone marrow failure. Somatic mosaicism is a prominent molecular mechanism of this new disease.

SASH3 variants cause a novel form of X-linked combined immunodeficiency with immune dysregulation.

Sterile alpha motif (SAM) and Src homology-3 (SH3) domain-containing 3 (SASH3), also called SH3-containing lymphocyte protein (SLY1), is a putative adaptor protein that is postulated to play an important role in the organization of signaling complexes and propagation of signal transduction cascades in lymphocytes. The SASH3 gene is located on the X-chromosome. Here, we identified 3 novel SASH3 deleterious variants in 4 unrelated male patients with a history of combined immunodeficiency and immune dysregulation that manifested as recurrent sinopulmonary, cutaneous, and mucosal infections and refractory autoimmune cytopenias. Patients exhibited CD4+ T-cell lymphopenia, decreased T-cell proliferation, cell cycle progression, and increased T-cell apoptosis in response to mitogens. In vitro T-cell differentiation of CD34+ cells and molecular signatures of rearrangements at the T-cell receptor α (TRA) locus were indicative of impaired thymocyte survival. These patients also manifested neutropenia and B-cell and natural killer (NK)-cell lymphopenia. Lentivirus-mediated transfer of the SASH3 complementary DNA-corrected protein expression, in vitro proliferation, and signaling in SASH3-deficient Jurkat and patient-derived T cells. These findings define a new type of X-linked combined immunodeficiency in humans that recapitulates many of the abnormalities reported in mice with Sly1-/- and Sly1Δ/Δ mutations, highlighting an important role of SASH3 in human lymphocyte function and survival.

Research participants' perspectives about the return of uninformative genomic test results in a clinical research setting.

The majority of genomic sequencing and microarray results are clinically uninformative, meaning that they do not suggest a need for any behavioral action or medical intervention. Prior studies have shown that recipients of uninformative genomic testing results ("uninformative results" hereafter) may incorrectly interpret them to imply a lowered risk of disease or false reassurance about future health risks. Few studies have examined how patients understand uninformative results when they are returned in a research setting, where there is wide variation in analytical specifications of testing, interpretation and reporting practices, and resources to support the return of results. We conducted cross-sectional interviews (N = 17) to explore how a subset of research participants in one genomics study at the National Institutes of Health reacted to and understood their uninformative test results, which were returned to them via a patient portal without genetic counseling. We found that most participants did not remember the details of the informed consent process, including the distinction between "primary" and "secondary" findings. Participants had questions about what genes were tested for and, in most cases, requested a list of the genes covered. Several participants incorrectly assumed that autosomal recessive carrier results would have been reported to them if detected. Some participants interpreted their uninformative results to mean that they could forgo prenatal testing, and participants had mixed expectations about whether their results might be reinterpreted in the future. These themes suggest that there are specific challenges to returning uninformative results in research settings. Educational supplements to uninformative test reports may be most useful if they contextualize results in relation to other types of clinical genetic or genomic testing that may be made available to research participants in their lifetimes.

FOXI3 haploinsufficiency contributes to low T-cell receptor excision circles and T-cell lymphopenia.

BACKGROUND: Newborn screening can identify neonatal T-cell lymphopenia through detection of a low number of copies of T-cell receptor excision circles in dried blood spots collected at birth. After a positive screening result, further diagnostic testing is required to determine whether the subject has severe combined immunodeficiency or other causes of T-cell lymphopenia. Even after thorough evaluation, approximately 15% of children with a positive result of newborn screening for T-cell receptor excision circles remain genetically undiagnosed. Identifying the underlying genetic etiology is necessary to guide subsequent clinical management and family planning. OBJECTIVE: We sought to elucidate the genetic basis of patients with T-cell lymphopenia without an apparent genetic diagnosis. METHODS: We used clinical genomic testing as well as functional and immunologic assays to identify and elucidate the genetic and mechanistic basis of T-cell lymphopenia. RESULTS: We report 2 unrelated individuals with nonsevere T-cell lymphopenia and abnormal T-cell receptor excision circles who harbor heterozygous loss-of-function variants in forkhead box I3 transcription factor (FOXI3). CONCLUSION: Our findings support the notion that haploinsufficiency of FOXI3 results in T-cell lymphopenia with variable expressivity and that FOXI3 may be a key modulator of thymus development.

Clinical exome sequencing of 1000 families with complex immune phenotypes: Toward comprehensive genomic evaluations.

BACKGROUND: Prospective genetic evaluation of patients at this referral research hospital presents clinical research challenges. OBJECTIVES: This study sought not only a single-gene explanation for participants' immune-related presentations, but viewed each participant holistically, with the potential to have multiple genetic contributions to their immune phenotype and other heritable comorbidities relevant to their presentation and health. METHODS: This study developed a program integrating exome sequencing, chromosomal microarray, phenotyping, results return with genetic counseling, and reanalysis in 1505 individuals from 1000 families with suspected or known inborn errors of immunity. RESULTS: Probands were 50.8% female, 71.5% were ≥18 years, and had diverse immune presentations. Overall, 327 of 1000 probands (32.7%) received 361 molecular diagnoses. These included 17 probands with diagnostic copy number variants, 32 probands with secondary findings, and 31 probands with multiple molecular diagnoses. Reanalysis added 22 molecular diagnoses, predominantly due to new disease-gene associations (9 of 22, 40.9%). One-quarter of the molecular diagnoses (92 of 361) did not involve immune-associated genes. Molecular diagnosis was correlated with younger age, male sex, and a higher number of organ systems involved. This program also facilitated the discovery of new gene-disease associations such as SASH3-related immunodeficiency. A review of treatment options and ClinGen actionability curations suggest that at least 251 of 361 of these molecular diagnoses (69.5%) could translate into ≥1 management option. CONCLUSIONS: This program contributes to our understanding of the diagnostic and clinical utility whole exome analysis on a large scale.

A Double-Blind, Placebo-Controlled, Crossover Study of Magnesium Supplementation in Patients with XMEN Disease.

X-linked MAGT1 deficiency with increased susceptibility to Epstein-Barr virus (EBV) infection and N-linked glycosylation defect (XMEN) disease is an inborn error of immunity caused by loss-of-function mutations in the magnesium transporter 1 (MAGT1) gene. The original studies of XMEN patients focused on impaired magnesium regulation, leading to decreased EBV-cytotoxicity and the loss of surface expression of the activating receptor "natural killer group 2D" (NKG2D) on CD8+ T cells and NK cells. In vitro studies showed that supraphysiological supplementation of magnesium rescued these defects. Observational studies in 2 patients suggested oral magnesium supplementation could decrease EBV viremia. Hence, we performed a randomized, double-blind, placebo-controlled, crossover study in 2 parts. In part 1, patients received either oral magnesium L-threonate (MLT) or placebo for 12 weeks followed by 12 weeks of the other treatment. Part 2 began with 3 days of high-dose intravenous (IV) magnesium sulfate (MgSO4) followed by open-label MLT for 24 weeks. One EBV-infected and 3 EBV-naïve patients completed part 1. One EBV-naïve patient was removed from part 2 of the study due to asymptomatic elevation of liver enzymes during IV MgSO4. No change in EBV or NKG2D status was observed. In vitro magnesium supplementation experiments in cells from 14 XMEN patients failed to significantly rescue NKG2D expression and the clinical trial was stopped. Although small, this study indicates magnesium supplementation is unlikely to be an effective therapeutic option in XMEN disease.

PhenoTagger: a hybrid method for phenotype concept recognition using human phenotype ontology.

MOTIVATION: Automatic phenotype concept recognition from unstructured text remains a challenging task in biomedical text mining research. Previous works that address the task typically use dictionary-based matching methods, which can achieve high precision but suffer from lower recall. Recently, machine learning-based methods have been proposed to identify biomedical concepts, which can recognize more unseen concept synonyms by automatic feature learning. However, most methods require large corpora of manually annotated data for model training, which is difficult to obtain due to the high cost of human annotation. RESULTS: In this article, we propose PhenoTagger, a hybrid method that combines both dictionary and machine learning-based methods to recognize Human Phenotype Ontology (HPO) concepts in unstructured biomedical text. We first use all concepts and synonyms in HPO to construct a dictionary, which is then used to automatically build a distantly supervised training dataset for machine learning. Next, a cutting-edge deep learning model is trained to classify each candidate phrase (n-gram from input sentence) into a corresponding concept label. Finally, the dictionary and machine learning-based prediction results are combined for improved performance. Our method is validated with two HPO corpora, and the results show that PhenoTagger compares favorably to previous methods. In addition, to demonstrate the generalizability of our method, we retrained PhenoTagger using the disease ontology MEDIC for disease concept recognition to investigate the effect of training on different ontologies. Experimental results on the NCBI disease corpus show that PhenoTagger without requiring manually annotated training data achieves competitive performance as compared with state-of-the-art supervised methods. AVAILABILITYAND IMPLEMENTATION: The source code, API information and data for PhenoTagger are freely available at https://github.com/ncbi-nlp/PhenoTagger. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Genomic tools for health: Secondary findings as findings to be shared.

PURPOSE: Whether and how to disclose secondary finding (SF) information to children is ethically debated. Some argue that genetic testing of minors should be limited to preserve the child's future autonomy. Others suggest that disclosure of SFs can occur if it is in the best interests of the child. However, the ways that parents conceptualize and weigh their child's future autonomy against the interests of their child and other family members are unknown. METHODS: To explore how parents understand SF disclosure in the context of their child and other family members' lives, we conducted semistructured interviews with 30 families (40 parents in total). All parents had children who were enrolled in a genetic sequencing protocol that returned results by default. RESULTS: We found that parents did not routinely conceptualize SFs as distinctive health information. Rather parents saw this information as part of their child's overall health. To make decisions about disclosure, parents weighed their child's ability to understand the SF information and their other family member's need to know. CONCLUSION: Because most families desired SF information, we argue that disclosure of SF be reconceptualized to reflect the lived experiences of those who may receive this information.

PhenoRerank: A re-ranking model for phenotypic concept recognition pre-trained on human phenotype ontology.

The study aims at developing a neural network model to improve the performance of Human Phenotype Ontology (HPO) concept recognition tools. We used the terms, definitions, and comments about the phenotypic concepts in the HPO database to train our model. The document to be analyzed is first split into sentences and annotated with a base method to generate candidate concepts. The sentences, along with the candidate concepts, are then fed into the pre-trained model for re-ranking. Our model comprises the pre-trained BlueBERT and a feature selection module, followed by a contrastive loss. We re-ranked the results generated by three robust HPO annotation tools and compared the performance against most of the existing approaches. The experimental results show that our model can improve the performance of the existing methods. Significantly, it boosted 3.0% and 5.6% in F1 score on the two evaluated datasets compared with the base methods. It removed more than 80% of the false positives predicted by the base methods, resulting in up to 18% improvement in precision. Our model utilizes the descriptive data in the ontology and the contextual information in the sentences for re-ranking. The results indicate that the additional information and the re-ranking model can significantly enhance the precision of HPO concept recognition compared with the base method.

The Monarch Initiative in 2019: an integrative data and analytic platform connecting phenotypes to genotypes across species.

In biology and biomedicine, relating phenotypic outcomes with genetic variation and environmental factors remains a challenge: patient phenotypes may not match known diseases, candidate variants may be in genes that haven't been characterized, research organisms may not recapitulate human or veterinary diseases, environmental factors affecting disease outcomes are unknown or undocumented, and many resources must be queried to find potentially significant phenotypic associations. The Monarch Initiative (https://monarchinitiative.org) integrates information on genes, variants, genotypes, phenotypes and diseases in a variety of species, and allows powerful ontology-based search. We develop many widely adopted ontologies that together enable sophisticated computational analysis, mechanistic discovery and diagnostics of Mendelian diseases. Our algorithms and tools are widely used to identify animal models of human disease through phenotypic similarity, for differential diagnostics and to facilitate translational research. Launched in 2015, Monarch has grown with regards to data (new organisms, more sources, better modeling); new API and standards; ontologies (new Mondo unified disease ontology, improvements to ontologies such as HPO and uPheno); user interface (a redesigned website); and community development. Monarch data, algorithms and tools are being used and extended by resources such as GA4GH and NCATS Translator, among others, to aid mechanistic discovery and diagnostics.

Expansion of the Human Phenotype Ontology (HPO) knowledge base and resources.

The Human Phenotype Ontology (HPO)-a standardized vocabulary of phenotypic abnormalities associated with 7000+ diseases-is used by thousands of researchers, clinicians, informaticians and electronic health record systems around the world. Its detailed descriptions of clinical abnormalities and computable disease definitions have made HPO the de facto standard for deep phenotyping in the field of rare disease. The HPO's interoperability with other ontologies has enabled it to be used to improve diagnostic accuracy by incorporating model organism data. It also plays a key role in the popular Exomiser tool, which identifies potential disease-causing variants from whole-exome or whole-genome sequencing data. Since the HPO was first introduced in 2008, its users have become both more numerous and more diverse. To meet these emerging needs, the project has added new content, language translations, mappings and computational tooling, as well as integrations with external community data. The HPO continues to collaborate with clinical adopters to improve specific areas of the ontology and extend standardized disease descriptions. The newly redesigned HPO website (www.human-phenotype-ontology.org) simplifies browsing terms and exploring clinical features, diseases, and human genes.

Exome sequencing study in a clinical research setting finds general acceptance of study returning secondary genomic findings with little decisional conflict.

The most appropriate strategies for managing secondary genomic findings (SF) in clinical research are being developed and evaluated. We surveyed patients at the National Institute of Allergy and Infectious Diseases (NIAID) to evaluate decisional conflict regarding enrolling in a study that returns SF. Responses were collected using a cross-sectional survey after informed consent but before return of SF. Sixty-six adults of 116 eligible participants responded. No participant explicitly declined because they did not want to possibly receive a SF. Sixty-five of 66 (98%) participants thought it was appropriate to return SFs in research; one participant was unsure. Decisional conflict regarding enrolling in a study returning SF was low overall with 68% of participants reporting a score of less than 10 on a 100-point decisional conflict scale, implying that they felt informed, clear on what they wanted, and supported. Lower genetic literacy was weakly associated with higher decisional conflict (Spearman's rho = -0.297, p = .015). Six participants reported confusion related to the choices about SFs. Our data suggest that participants in our study feel it is appropriate to receive SF and have little decisional conflict about potentially receiving such information; however, some participants may need further education and counseling.

Life with a Primary Immune Deficiency: a Systematic Synthesis of the Literature and Proposed Research Agenda.

PURPOSE: The clinical immunology literature is punctuated with research on psychosocial dimensions of illness. Studies investigating the lived experiences and stated needs of patients with primary immune deficiencies and their families are essential to improving clinical management and determining the research questions that matter to patients and other stakeholders. Yet, to move the field forward, a systematic review of literature and proposed agenda is needed. METHODS: A systematic review was conducted via PubMed and Scopus to include original research on psychological, social, or behavioral aspects of primary immune deficiencies published between 1999 and 2015. A Title/Abstract keyword search was conducted, 317 candidate article abstracts were manually reviewed, and forward/backward reference searches were completed. RESULTS: Twenty-nine studies met inclusion criteria. These illuminate the complex psychological, social, and emotional experiences of primary immune deficiency. Themes included the potential for negative psychosocial impact from disease; adaptation over time; the multi-dimensional assessments of quality of life; familial impact; the important roles of hope, developing a sense of control, social support; and addressing anxiety/depression in our patients and their families. Methodological considerations and areas for improvement are discussed. CONCLUSION: We propose the research agenda focus on study creativity and rigor, with improved engagement with existing literature and critical study design (e.g., methodology with adequate statistical power, careful variable selection, etc.). This review highlights opportunities to advance psychosocial research and bring a brighter future to clinicians, researchers, and families affected by primary immune deficiency.

Predispositions to Lymphoma: A Practical Review for Genetic Counselors.

This review provides a synopsis for genetic counselors of the major concepts of lymphoma predisposition: genomic instability, immune deficiency, inappropriate lymphoproliferation, and chronic antigen stimulation. We discuss syndromes typifying each of these mechanisms. Importantly, our review of the genetic counseling literature reveals sparse discussion of genetically-based immune-mediated lymphoma predisposition, which we address in depth here. We aim to increase awareness among genetic counselors and colleagues in oncology about familial susceptibility and facilitate critical thinking about lymphoma risk assessment. Clinical application of this knowledge is aided by recommendations for collection of personal and family history to guide risk assessment and testing. Lastly, we include a special discussion of genetic counseling issues including perceptions of the context, nature, and magnitude of lymphoma risk, as well as coping with awareness of susceptibility to lymphoma.

Patient Perceptions of Telephone vs. In-Person BRCA1/BRCA2 Genetic Counseling.

Telephone genetic counseling (TC) for hereditary breast/ovarian cancer risk has been associated with positive outcomes in high risk women. However, little is known about how patients perceive TC. As part of a randomized trial of TC versus usual care (UC; in-person genetic counseling), we compared high risk women's perceptions of: (1) overall satisfaction with genetic counseling; (2) convenience; (3) attentiveness during the session; (4) counselor effectiveness in providing support; and (5) counselor ability to recognize emotional responses during the session. Among the 554 participants (TC, N = 272; UC, N = 282), delivery mode was not associated with self-reported satisfaction. However, TC participants found counseling significantly more convenient than UC participants (OR = 4.78, 95 % CI = 3.32, 6.89) while also perceiving lower levels of support (OR = 0.56, 95 % CI = 0.40-0.80) and emotional recognition (OR = 0.53, 95 % CI = 0.37-0.76). In exploratory analyses, we found that non-Hispanic white participants reported higher counselor support in UC than in TC (69.4 % vs. 52.8 %; OR = 3.06, 95 % CI = 1.39-6.74), while minority women perceived less support in UC vs. TC (58.3 % vs. 38.7 %; OR = 0.80, 95 % CI = 0.39-1.65). We discuss potential research and practice implications of these findings which may further improve the effectiveness and utilization of TC.

Genetic Risk Factors for Early-Onset Merkel Cell Carcinoma.

Importance: Merkel cell carcinoma (MCC) is a rare, aggressive neuroendocrine skin cancer. Of the patients who develop MCC annually, only 4% are younger than 50 years. Objective: To identify genetic risk factors for early-onset MCC via genomic sequencing. Design, Setting, and Participants: The study represents a multicenter collaboration between the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), the National Institute of Allergy and Infectious Diseases (NIAID), and the University of Washington. Participants with early-onset and later-onset MCC were prospectively enrolled in an institutional review board-approved study at the University of Washington between January 2003 and May 2019. Unrelated controls were enrolled in the NIAID Centralized Sequencing Program (CSP) between September 2017 and September 2021. Analysis was performed from September 2021 and March 2023. Early-onset MCC was defined as disease occurrence in individuals younger than 50 years. Later-onset MCC was defined as disease occurrence at age 50 years or older. Unrelated controls were evaluated by the NIAID CSP for reasons other than familial cancer syndromes, including immunological, neurological, and psychiatric disorders. Results: This case-control analysis included 1012 participants: 37 with early-onset MCC, 45 with later-onset MCC, and 930 unrelated controls. Among 37 patients with early-onset MCC, 7 (19%) had well-described variants in genes associated with cancer predisposition. Six patients had variants associated with hereditary cancer syndromes (ATM = 2, BRCA1 = 2, BRCA2 = 1, and TP53 = 1) and 1 patient had a variant associated with immunodeficiency and lymphoma (MAGT1). Compared with 930 unrelated controls, the early-onset MCC cohort was significantly enriched for cancer-predisposing pathogenic or likely pathogenic variants in these 5 genes (odds ratio, 30.35; 95% CI, 8.89-106.30; P < .001). No germline disease variants in these genes were identified in 45 patients with later-onset MCC. Additional variants in DNA repair genes were also identified among patients with MCC. Conclusions and Relevance: Because variants in certain DNA repair and cancer predisposition genes are associated with early-onset MCC, genetic counseling and testing should be considered for patients presenting at younger than 50 years.