Background/Objectives: The germline polymorphism in the HSD3B1 gene (c.1100 C) results in adrenal-permissive (CC) or adrenal-restrictive (AA) functions of the protein product by regulating the production of high-affinity ligands that activate androgen signaling. Prior studies have indicated that the CC genotype is associated with worse response to hormonal therapies in prostate cancer (PC) patients. Methods: To characterize the impact of germline HSD3B1 variants on somatic tumor features, we examined 6550 primary and metastatic PCs from the Caris Life Sciences database, in which the genomic and transcriptomic landscapes were acquired via paired whole-exome/whole-transcriptome sequencing. Results: The overall prevalence of the HSD3B1 AA genotype (restrictive–homozygous) was 48.8%, AC (permissive–heterozygous) was 32.8%, and CC (permissive–homozygous) was 14.9%. There was enrichment of the CC genotype in these PC patients as compared to prior reports that examined non-cancerous populations. However, the rates of the CC genotype varied between metastatic site and by race. Compared to the AA genotype, tumors harboring the CC genotype did not demonstrate increased AR alterations, nor higher expression of AR, FOXA1, HOXB13, or AR signaling signatures. We instead found significant changes in immune-associated hallmark pathways, immune cell fractions, and biomarkers that inform the use of immune therapies (TMB-high, MSI-high). Further, the CC and AA genotypes exhibited notable differences in the expression of immunoglobulins, MHC class I/II molecules, and cell surface targets. The differences in expression by HSD3B1 genotype were especially notable in lung and liver metastases. Conclusions: Our study indicates that in prostate cancers, HSD3B1 germline c.1100 allele status may not directly influence tumor-intrinsic genomics but is associated with novel functions beyond androgen signaling.

HSD3B1 encodes an enzyme that catalyzes the conversion of adrenal precursors into potent sex steroids. A common germline variant (c.1100C) enhances this effect and is linked to breast cancer (BC) progression. As the HSD3B1 genotypes contribute to differences in local and adrenal steroid production, their transcriptional and phenotypic effects on cancers influenced by hormonal signaling such as BC and endometrial cancer (EC)—particularly in relation to menopausal status—remain unclear. We analyzed BC and EC sequenced from patients that received diagnostic tests in oncology clinics, and we determined the germline HSD3B1 c.1100 genotype (AA, AC, CC) from tumor DNA sequencing by using variant allele frequency, with inferred menopausal status assumed by age at molecular profiling. Whole-transcriptome RNA sequencing and gene set enrichment analysis showed that adrenal-permissive homozygous (CC) tumors in premenopausal ER + BC were enriched for hormone-related pathways, including Estrogen Response Early (NES ≈ +1.8). In premenopausal triple-negative BC, adrenal-restrictive homozygous (AA) tumors exhibited the elevated expression of immune and epithelial genes and the increased prevalence of MED12 alterations (AA 0.25% vs. CC 8%, p < 0.01). In endometrioid EC, CC tumors demonstrated the suppression of immune and proliferative pathways. Postmenopausal cases had higher progesterone receptor IHC positivity (AA 75% vs. CC 83%, p < 0.05) and numerically more frequent ESR1 copy number gains (AA 2.0% vs. CC 4.0%). Results highlight context-specific associations between germline HSD3B1 genotypes and tumor biology in BC and EC.