Circulating prostate-specific antigen (PSA) discriminates poorly in the diagnostic gray zone (3.0-9.99 ng/mL), where ~75% of biopsies yield no clinically significant prostate cancer (PCa). We evaluated whether urinary creatine riboside (CR), a tumor-derived metabolite excreted through the prostatic urethra, complements PSA for gray-zone detection and independently predicts prostate-cancer-specific mortality (PCSM). In the NCI-Maryland PCa Case-Control Study (951 cases, 962 controls; 47.6% African American men; median follow-up 11.5 years), urinary CR was quantified by UPLC-MS/MS. Within the PSA gray zone (n = 668), urinary CR was complementary to PSA, with markedly higher single-marker discrimination than PSA (AUC 0.93, 95% CI 0.88-0.98 vs 0.77, 0.66-0.89) and additive when combined (ΔAUC +0.17, p < 0.001; 91.4% sensitivity at 80% specificity). After adjustment for 11 clinical and sociodemographic covariates, urinary CR independently predicted PCSM complementary to PSA (Fine-Gray SHR 1.72, 1.35-2.19 for CR; 1.35, 1.08-1.68 for PSA; Harrell's C 0.85 for CR + PSA vs 0.77 for PSA alone), with strongest signal in African American men (SHR 2.43, 1.57-3.75 for CR). We conclude that urinary CR is a candidate non-invasive biomarker complementary to PSA - improving gray-zone triage and predicting PCSM; prospective validation in biopsy-referred cohorts is warranted. ### Competing Interest Statement The authors have declared no competing interest. ### 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: Institutional Review Board of the National Cancer Institute, National Institutes of Health 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I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the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. Individual-level data from the NCI-Maryland Prostate Cancer Case-Control Study are not publicly deposited due to participant privacy protections governing this NIH intramural research cohort. Intramural Research Program of the U.S. National Institutes of Health (NIH), National Cancer Institute, Center for Cancer Research, grant ZIA BC 011492 National Center for Advancing Translational Sciences, ZIC TR000547

Background Identification and characterization of prostate tumor markers differentially expressed in European American (EA) men vs African American (AA) men has been one of the focus areas positioned to understand and address prostate cancer disparity in the US. Whereas some genes are differentially expressed, validation studies are limited, and the impact of these expression differences on recurrence risk remains unclear.Methods This study focused on phospholipase A2 group 7 (PLA2G7) in prostate cancer surgical specimens from EA and AA patients. A nested case-control study was conducted to evaluate the prognostic value of the PLA2G7 protein while controlling for age, stage, and Gleason, followed by re-analysis of a published dataset of 556 EA and 596 AA prostate cancer patients. Expression signatures correlated with PLA2G7 were investigated in both patient populations.Results PLA2G7 was more frequently overexpressed in EA tumors than AA tumors, and higher tumor-specific PLA2G7 expression was associated with divergent outcomes: lower biochemical recurrence risk in EA men but elevated risk in AA men. Expression in noncancer tissues showed no prognostic value. Whereas androgen response signatures were consistently enriched in high-PLA2G7 tumors across both groups, immune and inflammatory response gene sets showed opposite enrichment trends between AA and EA men.Conclusion This study represents the first identification and validation of a tumor-specific marker that is both differentially expressed between prostate cancers in AA and EA men and demonstrates opposite prognostic effects based on self-reported race/ethnicity. The findings emphasize the importance of evaluating prostate tumor markers across diverse geographic ancestries.

e17149 Background: Distinguishing clinically significant prostate cancer from indolent disease remains a major challenge in prostate cancer diagnosis and contributes to unnecessary biopsy and overtreatment. Prostate-specific antigen (PSA) testing, while widely used, has limited specificity for clinically significant disease and provides limited information for individualized risk assessment. Creatine riboside is a tumor-associated urinary metabolite previously linked to aggressive prostate cancer biology. We evaluated whether urinary creatine riboside could provide clinically interpretable risk stratification for clinically significant prostate cancer using a likelihood-ratio–based framework designed to complement existing diagnostic approaches and inform decision-making beyond PSA alone. Methods: Urinary creatine riboside concentrations were quantified using targeted liquid chromatography–mass spectrometry in men undergoing evaluation for prostate cancer. Clinically significant prostate cancer was defined according to standard pathological criteria. Risk strata were prespecified using percentile thresholds derived from the empirical distribution of urinary creatine riboside concentrations. Diagnostic performance across strata was evaluated using sensitivity, specificity, and positive likelihood ratios (LR+), interpreted according to established clinical benchmarks to define low-risk, intermediate-risk, and high-risk categories. Results: Urinary creatine riboside concentrations were higher among men with clinically significant prostate cancer than among those without. Diagnostic discrimination increased progressively across ascending percentile thresholds. Lower-risk strata were associated with minimal changes in post-test probability (LR+ approximately 1–2), consistent with low-risk classification. Intermediate-risk strata demonstrated moderate enrichment of risk (LR+ approximately 2–5). At higher percentile thresholds, urinary creatine riboside demonstrated strong rule-in performance, with LR+ values exceeding 10 and reaching greater than 30 at the highest thresholds. This likelihood-ratio–based framework enabled clear separation of patients into clinically actionable low-risk, intermediate-risk, and high-risk categories using a single noninvasive biomarker. Conclusions: Urinary creatine riboside enables robust, likelihood-ratio–based risk stratification for clinically significant prostate cancer, providing clinically interpretable decision support beyond PSA. This approach has the potential to reduce unnecessary biopsies while improving identification of aggressive disease and advancing precision prostate cancer diagnostics.

Tumor NOS2 and COX2 expression induced by IFNg and TNFa

PURPOSE:As oncologic precision medicine develops, there is concern that tumor genomic profiling among Black patients and other underrepresented demographics will lag and amplify health disparities. We evaluate whether a precision medicine navigator (PMN), a clinical navigator specialized in obtaining tumor genomic testing for patients, is associated with increased genomic testing rates and whether this is the case among Black patients and other disadvantaged groups. METHODS:We retrospectively reviewed prostate cancer consults within one health care system from November 2, 2021, to January 2, 2023. We compared the proportion of patients who received genomic testing in the 7 months before the PMN start (pre-PMN) to the 7 months afterward (post-PMN). Binary logistic regression was used to calculate the odds of receiving testing. RESULTS:The sample included 693 patients, 311 pre-PMN and 382 post-PMN. Racial distribution and clinical stage were well-balanced between the pre- and post-PMN groups. From pre- to post-PMN, the proportion of patients receiving testing increased from 18% to 70% (P = .0002). Significant increases in testing post-PMN occurred among Black patients, lower median income patients, patients with Medicare/Medicaid, and community hospital patients. However, utilization post-PMN was significantly higher in non-Black compared with Black patients, 75.4% and 58.5%, respectively (P = .0006). CONCLUSION:For patients with prostate cancer, the presence of a PMN is associated with increased rates of genomic testing overall, including among Black patients and other groups that experience health inequities. Utilization post-PMN was significantly lower in Black patients compared with non-Black patients. More research is needed into why Black patients show lower rates of genomic testing.

Abstract Background: The tumor immune microenvironment (TIME) reflects both tumor-intrinsic biology and host-modifiable factors. Characterizing how tumor features, body mass index (BMI), and reproductive history relate to immune activity may help explain heterogeneity in breast cancer and inform precision prevention and survivorship strategies. Methods: We analyzed 461 invasive breast tumors from Kenyan patients using a custom NanoString nCounter® immune-focused gene panel. Intrinsic subtypes were assigned with PAM50, and immune cell composition (relative proportions for 22 immune cells) was estimated by CIBERSORTx. Composite immune activity scores were derived to represent functional modules: z_hot (CD8+, M1 macrophages, NK activated, T follicular helper) and z_suppression (Treg + M2 macrophages) from CIBERSORTx; z_cytotoxic (GZMB, PRF1), z_exhaustion (PDCD1, LAG3, CTLA4), and z_checkpoint (PDCD1, PDCD1LG2, CTLA4, LAG3) from GSVA signatures. Associations between immune scores and tumor features (PAM50 subtypes, risk of recurrence (ROR), RNA-based TP53 status, tumor grade) or host factors (BMI, menopausal status, parity, breastfeeding) were evaluated in age-adjusted linear regression models. Independent effects of host factors were further examined in multivariable regression models adjusted for tumor characteristics and lifestyle covariates. Multiple testing was accommodated using a false discovery rate adjusted p-values. Results: Participants had a mean age of 50.3 years; 43.8% of tumors were luminal A and 21.5% basal-like. Most women were overweight/obese (BMI ≥ 25, 73.1%) and 65.7% had ≥ 3 children. High-grade, basal-like, high-ROR, and TP53 mutant-like tumors showed strong evidence of elevated z_hot, z_cytotoxic, z_checkpoint, and z_exhaustion scores (BH-adjusted p < 0.001), along with modestly lower z_suppression in basal-like and TP53 mutant-like tumors (p < 0.05), indicating an active TIME characteristic of aggressive tumors. In contrast, established BC risk factors showed weaker influence on TIME. Overweight/obese patients were more likely to have cold TIME (lower CD8 (p = 0.09), T follicular helper (p < 0.05), and z_hot score (p < 0.05)), while patients with longer duration of breastfeeding had more active TIME (higher T follicular helper and z_cytotoxicity; lower M2 macrophages and z_suppression (all p < 0.05)). Conclusions: Tumor-intrinsic subtype classification remains the dominant determinant of immune heterogeneity, while established BC risk factors (BMI and reproductive risk factors) may modulate immune responsiveness. Integrating tumor, reproductive, and lifestyle data can help clarify immune variation across diverse populations. Citation Format: Li Feng, Amber N. Hurson, Shahin Sayed, Hela Koka, Viviane Oluoch, Veronica Ngundo, Alfred Mburu Githuka, Zaitun Ajuoga, Shaoqi Fan, Kristine Jones, Belynda Hicks, Amy Hutchinson, Maria Brown, Petra Lenz, Aaron M. Rozeboom, Difei Wang, Francis Makokha, Stefan Ambs, Jonine D. Figueroa, Ruth M. Pfeiffer, Xiaohong Rose Yang. Tumor and Host Determinants of the Breast Tumor Immune Microenvironment in Kenyan Breast Cancer Women [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7606.

Nitric oxide synthase 2 (NOS2) and cyclooxygenase 2 (COX2) tumor expression present significant obstacles for effective treatment of aggressive tumors including ER-negative breast cancer. Spatial analysis of NOS2, COX2 and CD8 expression in patient tumors has identified mechanisms of treatment inhibition that were further elucidated using the 4T1 mouse model of triple negative breast cancer and live cell culture studies. NOS2 and COX2 activate each other via a feed-forward paracrine mechanism. While NOS2 promotes cancer stemness and the formation of metastatic niches, COX2 mediates CD8+ T cell suppression. Quantitative spatial analysis has revealed NOS2/COX2 roles during the temporal progression from immune hot in low COX2 expressing tumors to three immune cold stages in the tumor microenvironment of high COX2 expressing tumors. Type 1: immune cold with stroma-restricted CD8+ T cell secretion of interferon gamma, which activates COX2 expression at the tumor margin as well as NOS2 expression at the tumor periphery. Type 2: developing immune desserts lack stroma-restricted CD8+ T cells with tumor NOS2 and COX2 restricted to the tumor periphery. Type 3: mature immune desserts exhibit abated NOS2, COX2 and CD8+ T cells, and induction of B7H4 and cancer-associated fibroblasts driven by significant tumor hypoxia and necrosis. These three types of immune desserts can coexist with each other and with immune hot regions in the same tumor. The coordinated interplay of NOS2 and COX2 indicates that targeting both these enzymes provides an effective treatment strategy that is supported by ongoing clinical trials demonstrating improved clinical outcomes in patients who have otherwise exhausted treatment options.

Alterations in peripheral immunity may contribute to fatal prostate cancer and the excessive disease burden among African American (AA) men. Thus, we investigated peripheral immunity in prostate cancer patients in association with disease outcome and AA descent. We performed single-cell RNA and T cell receptor (TCR) sequencing using peripheral blood mononuclear cells (PBMC) from a cohort of 43 prostate cancer patients with survival follow-up and 16 healthy controls. Analyzing 273,229 high-quality PBMC, we observed that men who developed fatal prostate cancer (n=20) distinctively expressed the LAG3 and TIGIT exhaustion markers in their CD8⁺ T effector memory cells (CD8 TEM). These men also showed a loss of type-2 conventional dendritic cells (cDC2). Further analyses revealed additional transcriptome-inferred functional differences in cDC2, CD16+ monocytes, and CD8 TEM comparing men with and without fatal disease. Those describe cDC2 as immunosuppressed, CD16+ monocytes as pro-inflammatory but less cytotoxic, and CD8 TEM as chronically activated in patients with fatal disease. Comparing AA with European American patients, we observed upregulated LAG3 and TIGIT in AA PBMC. For cDC2 and CD16+ monocytes, fatal disease and AA patients showed shared marker expression. Furthermore, TCR profiling discovered over-representation of large, hyperexpanded clonotypes and a lower TCR diversity among fatal disease and AA patients. Lastly, the CD8 TEM functional status and LAG3 expression in PBMC emerged as a candidate predictor of disease fatality. We conclude that peripheral immunity may distinctly vary in association with disease outcome and AA descent of prostate cancer patients.

Polygenic risk score (PRS) models effectively predict breast cancer (BC) risk in European-ancestry women but have limited accuracy for African-ancestry women, particularly for aggressive subtypes. We developed PRS models for overall BC, estrogen receptor (ER)-positive, ER-negative and triple-negative BC (TNBC) in African-ancestry women using data from the African Ancestry Breast Cancer Genetics consortium (17,391 cases and 18,800 controls). We applied several PRS methods and integrated information across ancestries and BC subtypes. The best models for overall, ER-positive, ER-negative and TNBC showed an area under the receiving operating curve of 0.612, 0.621, 0.611 and 0.639, respectively, and maintained predictive accuracy in external validation studies with area under the receiving operating curves of 0.612, 0.640, 0.605 and 0.652. We further introduce a parsimonious 162-variant PRS for TNBC with comparable accuracy (0.626). These findings demonstrate markedly improved PRS accuracy for BC risk prediction in African-ancestry women. Using these PRS models for screening will help promote more equitable cancer prevention efforts.

Supplementary Table I summarizes pathological features of tumor immune microenvironment where NOS2+ inflamed regions are significantly higher in tumors from Deceased patients.

Despite growing evidence on the impact of intra-tumoral heterogeneity (ITH) in breast cancer (BC), its biological drivers and clinically relevant mitigation strategies remain poorly characterized, particularly in low-resource settings where spatial profiling is rarely applied. To address this gap, we conducted a comprehensive, multiplatform spatial profiling study of BC in a Kenyan population. Using the NanoString GeoMx™ Digital Spatial Profiler, we quantified 44 protein markers across 707 spatially defined, PanCK-segmented Areas of Illumination (AOIs) from 31 tumors. All tumors exhibited measurable ITH, with one-third showing marked heterogeneity. Sixteen patients exhibited two or more intrinsic subtypes within their tumors, three of whom had three subtypes. Immune-related ITH was particularly pronounced for CD8 and CD68. AOI phenotype (epithelium- vs. stroma-rich) emerged as the primary driver of marker variability. Linear mixed-effects (LME) modeling further revealed that intrinsic subtype influenced not only classical subtyping markers (ER, PR, HER2) but also immune and signaling markers including IDO1, S100B, PTEN, and BCL-2. Tissue morphology and spatial neighborhood contributed additional variance, particularly in stroma-rich AOIs, while incorporating AOI spatial coordinates improved model fit and revealed further spatially structured heterogeneity. Nonetheless, over 25

Genome-wide association studies (GWAS) have identified over 200 genetic risk loci for breast cancer, yet the target genes in these loci remain largely unknown. To address this knowledge gap, we conducted a series of multi-ancestry transcriptome-wide association studies (TWAS) to discover potential breast cancer susceptibility genes. We developed and validated ancestry-specific genetic models to predict levels of gene expression, alternative splicing, and 3' UTR alternative polyadenylation, using genomic and transcriptomic data from normal breast tissue samples of 652 females of African, Asian, or European ancestry. These models were then applied to GWAS data of 178,534 breast cancer cases and 248,300 controls from these ancestry groups for association analyses. We identified 290 genes associated with breast cancer risk, including 103 previously unreported in TWAS and 46 located at least 500Kb away from any previously identified risk variants. Among them, 39 genes exhibited distinct associations with breast cancer risk by estrogen receptor status. The identified genes were enriched in pathways related to homologous recombination, apoptosis, p53, PI3K/AKT/mTOR, estrogen, and IL-2/STAT5 signaling. Single-cell RNA sequencing and in vitro experiment data provided additional functional evidence for 169 genes. Our study uncovered large numbers of candidate breast cancer susceptibility genes and contributed valuable insights into the genetics and biology of this common cancer.

The tumor suppressor p53 is a transcription factor that controls the expression of hundreds of genes. Emerging evidence indicates that the p53-induced RNA-binding protein ZMAT3 acts as a key splicing regulator that contributes to p53-dependent tumor suppression in vitro and in vivo. However, the mechanism by which ZMAT3 functions within the p53 pathway remains largely unclear. Here, we discovered a function of ZMAT3 in inhibiting transcription of HKDC1, a hexokinase that regulates glucose metabolism and mitochondrial respiration in human cancer cells. Quantitative proteomics revealed HKDC1 as the most significantly upregulated protein in ZMAT3-depleted colorectal cancer cells. ZMAT3 depletion resulted in increased mitochondrial respiration, which was rescued by simultaneous depletion of HKDC1, suggesting that HKDC1 is a critical downstream effector of ZMAT3. Unexpectedly, ZMAT3 did not bind to HKDC1 RNA or DNA; however, proteomic analysis of the ZMAT3 interactome identified its interaction with the oncogenic transcription factor JUN. ZMAT3 depletion enhanced JUN binding to the HKDC1 locus, leading to increased HKDC1 transcription that was rescued upon JUN depletion, suggesting that JUN activates HKDC1 transcription in ZMAT3-depleted cells. Collectively, these findings uncover a mechanism by which ZMAT3 regulates transcription through JUN and demonstrate that HKDC1 is a key component of the ZMAT3-regulated transcriptome in the context of mitochondrial respiration regulation.

Prostate Cancer (PCa) remains the second leading cause of male cancer-related deaths in the U.S., with men of African ancestry (AA) exhibiting disproportionately higher mortality rates compared to European American (EA) men. Emerging evidence suggests that race-related immunobiological variations in PCa patients may contribute to disparities in tumor aggressiveness and clinical outcomes. This study explored the autoantibody response to the glycolytic enzymes Enolase-1 (ENO1) and -2 (ENO2) in AA and EA PCa patients. Based on prior research, we hypothesized that ENO1 and ENO2 are targets of autoantibodies in PCa patients and that these immune responses differ in frequency, levels, and clinical associations between AA and EA patients. Gene Expression Profiling Interactive Analysis (GEPIA) revealed elevated ENO1 expression in primary PCa, while ENO2 expression remained unchanged. Given that increased ENO1 expression may elicit an anti-tumor autoantibody response, we used ELISA to evaluate the prevalence of serum autoantibodies against purified ENO1 and ENO2 in PCa patients (n=307) and non-PCa controls (n=200). Our findings revealed significantly higher frequency and levels of ENO1 autoantibodies in PCa patients compared to controls (p<0.05), whereas ENO2 autoantibody frequency and levels were not significantly different between the two groups. However, stratifying PCa patients by self-reported racial groups uncovered significantly higher levels of ENO1 (p<0.001) and ENO2 (p<0.001) autoantibodies in AA patients compared to EA patients. ELISA results were confirmed by multiplex Western blotting. Kaplan-Meier survival and logistic regression analyses indicated that elevated ENO1 autoantibody levels, but not ENO2, were associated with poorer overall and cancer-specific patient survival, as well as advanced tumor stage and AA race. These results align with the growing recognition of race-related differences in PCa immunobiology. The heightened anti-ENO1 response observed in AA PCa patients may also reflect the presence of more aggressive, metabolically active tumors. Future studies will focus on the functional implications of patient-derived ENO1 and ENO2 autoantibodies, using proliferation, migration, invasion, clonogenic, tumorsphere, and metabolic assays, to determine their anti-cancer properties in racially distinct pre-clinical PCa models. Kai Wen Cheng, Pedro Ochoa, Alfonso Duran, Zainab Alramadhan, Michael Reeves, Diana Lozano, Evelyn Sanchez-Hernandez, Saied Mirshahidi, Susan Montgomery, Brain Hu, Frankis Almaguel, Stefan Ambs, Carlos A. Casiano. Elevated autoantibody response to enolase-1 is associated with poor overall survival in African American patients with prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB146.

Tumor NOS2 and COX2 expression induced by IFNg and TNFa

Breast cancer biology is influenced by both genetic ancestry and the environment with implications for disease outcomes. However, we lack the knowledge how non-cancerous cells within tumors are functionally altered by these factors at single-cell resolution. Here, we created a single-nucleus (sn) dataset from 33 African American, 25 Kenyan, and 24 European American women with breast cancer using combined RNA and ATAC sequencing. We successfully isolated intact, high-quality single nuclei from archival frozen breast tumor tissue using an optimized combination of enzymatic digestion and automated tissue homogenization. SnMultiome sequencing of 82 tumors was performed using the 10x Genomics platform. Following filtering, normalization, peak calling, and integration, our dataset includes a total of 292,458 nuclei. Cancerous (163,419 nuclei) and non-cancerous (129,039 nuclei) cells were distinguished based on DNA copy number. Within the microenvironment, 11 major immune, epithelial, and stromal cell types were identified. CD45+, EPCAM- cells were further analytically extracted, clustered, and manually annotated based on known marker genes, detecting 26 distinct immune subpopulations across our samples. After adjustment for potential confounders (age, body mass index, and tumor molecular subtype) in all analyses, we found that women of African descent possessed distinct, activated mesenchymal and endothelial cell subpopulations within the tumor vasculature that promote inflammation, angiogenesis, and cancer cell invasion. Women of African descent exhibited a more immune-suppressive tumor microenvironment, with elevated expression of immune checkpoint markers (PD-1, TIGIT, CTLA-4, and LAG-3), functional impairment of dendritic cells, and enrichment of regulatory and tolerogenic lymphocytic subpopulations. Further, malignant cells from African descent patients expressed a discrete set of CAR-T targets with potential implications for precision immunotherapy. Lastly, we developed three gene signatures within the microenvironment that are predictive of survival with external validation in both tumor-adjacent normal and cancerous tissues from the TCGA. Together, we uncovered molecular breast tumor characteristics of clinical significance in women of African ancestry. Alexandra R. Harris, Huaitian Liu, Brittany D. Jenkins, Sanna Madan, Tiangen Chang, Tiffany H. Dorsey, Eytan Ruppin, Shahin Sayed, Francis Makokha, Gretchen L. Gierach, Stefan Ambs. Single-nucleus sequencing reveals a functionally distinct breast tumor microenvironment by ancestral group with implications for disease progression and therapy [abstract]. In: Proceedings of the 18th AACR Conference on the Science of Cancer Health Disparities; 2025 Sep 18-21; Baltimore, MD. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2025;34(9 Suppl):Abstract nr C109.

Genome‐wide association studies (GWAS) have identified more than 200 risk loci for breast cancer. However, target genes and their encoded proteins in these loci remain largely unknown. In this study, we utilized genetic prediction models for 1349 circulating proteins derived from individuals of African ( n = 1871) and European ( n = 7213) ancestry to investigate genetically predicted protein levels in association with breast cancer risk among females of African ( n = 40,138), Asian ( n = 137,677), and European ( n = 247,173) ancestry. We identified 51 blood protein biomarkers associated with breast cancer risk, overall or by subtypes, at a false discovery rate (FDR) < 0.05, including 27 proteins encoded by genes located at least 1 Mb away from any of the known risk loci identified in GWAS. Of them, 32 proteins showed significant associations with breast cancer risk at the Bonferroni‐corrected significance level ( p < 2.45 × 10 −4 ). Of the 24 proteins located at GWAS‐identified risk loci, associations for 14 proteins were significantly attenuated after adjustment for the index risk variant of each respective locus, suggesting that these proteins may be target proteins for the risk loci. Encoding gene expression levels in normal breast tissue could be genetically predicted for 23 of the 51 identified proteins, and 13 encoding genes were associated with breast cancer risk in the same direction ( p < .05). Our study identified potential protein targets of GWAS risk loci and biomarkers for breast cancer risk and provided additional insights into breast cancer genetics and etiology.