Pathogenic germline variants in the ATM gene are associated with a 20-30% lifetime risk of breast cancer. Crucially, a relevant fraction of loss-of-function variants in breast cancer susceptibility genes disrupts pre-mRNA splicing. We aimed to perform splicing analysis of ATM splice-site variants identified in the large-scale sequencing project BRIDGES (Breast Cancer After Diagnostic Gene Sequencing). To this end, we bioinformatically selected 47 splice-site variants across 17 exons that were genetically engineered into three minigenes and assayed in MCF-7 cells. Aberrant splicing was observed in 38 variants. Of these, 30 variants, including 7 missense, yielded no or negligible expression of the minigene full-length (mgFL) transcript. A total of 69 different transcripts were characterized, 48 of which harboured a premature termination codon. Some variants, such as c.2922-1G>A, generated complex patterns with up to 10 different transcripts. Alternative 3' or 5' splice-site usage was the predominant event. Integration of ATM minigene read-outs into the ACMG/AMP (American College of Medical Genetics and Genomics/Association for Molecular Pathology)-based specifications for the ATM gene enabled the classification of 30 ATM variants as pathogenic or likely pathogenic and 9 as likely benign. Overall, splicing assays provide key information for variant interpretation and the clinical management of patients.

Background: Polygenic risk scores (PRS) are increasingly recognised as a promising tool for stratifying breast cancer risk, thus contributing to personalised screening strategies. However, their clinical utility, cost-effectiveness, and potential for equitable implementation across European health systems remain uncertain. This study aimed to develop evidence-based, context-sensitive recommendations for the integration of PRS into breast cancer screening using the Grading of Recommendations Assessment, Development and Evaluation Evidence to Decision (GRADE EtD) framework. Methods: A multidisciplinary panel of nine experts was convened under the EU4Health-funded CAN.HEAL project. We synthesised findings from a systematic review of decision-analytic models evaluating the clinical utility of PRS-enhanced screening for breast cancer compared to standard age-based protocols, alongside evidence collected on feasibility, acceptability, cost-effectiveness, and equity. Panellists rated the evidence and issued recommendations via a structured two-round consensus process. Results: Four decision-analytic models demonstrated potential benefits of PRS-enhanced screening in terms of life-years gained and reduced mortality. However, these benefits were partially offset by increases in false positives and overdiagnosis. Evidence on implementation revealed mixed acceptability, particularly regarding psychological impact and data privacy concerns, whilst feasibility was constrained by limited genomic infrastructure and workforce readiness. Cost-effectiveness varied by setting and model assumptions. Equity concerns were substantial, with existing PRS models primarily calibrated to populations of European ancestry. Conclusions: In light of the very low certainty of evidence and context-dependent trade-offs, the panel conditionally recommends against the routine adoption of PRS in organised breast cancer screening at this stage. Instead, national and regional systems should support implementation studies to evaluate clinical utility, encompassing equity and system readiness issues. Responsible implementation will require iterative learning, stakeholder co-design, and built-in evaluation to ensure that future scale-up is evidence-informed, ethically sound, and system-adapted.

BACKGROUND:Breast cancer is etiologically heterogeneous, but which risk factors differ in their associations across tumor subtypes remains unclear. We conducted a large, pooled analysis to evaluate independent, dose-response associations between breast cancer risk factors and quantitative tumor features. METHODS:Analyses of 15,731 invasive breast cancers from 24 studies evaluated associations (p-trend) between reproductive and hormonal factors, body mass index (BMI), alcohol, smoking, and family history in relation to quantitative immunohistochemistry measures on tissue microarrays (ER, PR, HER2, KI67, TP53) and tumor grade. Analyses in a subset of 10 population-based studies estimated subtype-specific odds ratios (ORs) comparing cases to controls. A Bayesian False Discovery Probability (BFDP) <0.2 was used to identify associations with strong statistical evidence. RESULTS:Nulliparity and later age at menopause were associated with higher ER-positivity (p-trend=0.021 and 0.001, respectively), with corresponding OR[ER+] (95% CI) = 1.49 (1.16-1.90) for nulliparous vs. parous and 1.07 (1.03-1.11) per 5 years. Current combined menopausal hormone therapy (MHT) use was associated with lower grade (p-trend<0.001), with OR [grade1] = 3.37 (2.69-4.21) for current vs. never users. Higher BMI was associated with lower ER-positivity and higher grade in premenopausal women (p-trend<0.001 and <0.001), with OR[ER+] = 0.80 (0.74-0.87) and OR[grade1] = 0.75 (0.63-0.88) per 5 units, and with higher PR-positivity and higher grade in postmenopausal women (p-trend<0.001 and <0.001), with OR[PR+] = 1.08 (1.03-1.14) and OR[grade 3] = 1.10 (1.04-1.17) per 5 units. CONCLUSION:This pooled analysis of 15,731 cases showed that nulliparity, age at menopause, MHT, and BMI have independent, dose-response associations with ER, PR, and grade, clarifying patterns of etiologic heterogeneity. Associations with HER2, KI67 and TP53, or other risk factors did not meet our threshold for strong evidence.

Succinate dehydrogenase (SDH) gene variants are the most common cause of the neuroendocrine tumour hereditary paraganglioma, which is associated with an over 20% metastasis risk as well as significant morbidity. There are currently no relevant human tumour cell lines or mouse models, and molecular understanding of downstream tumourigenic pathways is still rudimentary despite over two decades of concerted effort worldwide. These tumours generally show extremely slow in vivo doubling times (4-12 years), presumably existing in a primarily semi-quiescent state with little cell cycling or DNA replication. This characteristic makes deriving a useful tumour cell line impractical. A better alternative would be a cell line in which cell proliferation can be turned on and off at will, allowing expansion to generate sufficient cell numbers and experimentation once tumour cells have returned to their natural semi-quiescent state. The closest models currently available, highly-proliferating rat and mouse adrenal paraganglioma cell lines, are molecularly unrelated to SDH tumours. In this pilot study, we investigated whether primary SDH-derived paraganglioma tumour cells can be made to proliferate in vitro. We successfully transduced primary paraganglioma tumour cells with a lentiviral construct, using the proven strategy of c-MYC¬T58A (c-MYC) controlled by a Tet-On doxycycline-inducible expression system. We present the first evidence that primary paraganglioma chromaffin cells can be induced to proliferate in vitro, even in later passage cultures. Without any prior selection for chromaffin tumour cells, passaged cultures were obtained with over 80% synaptophysin-expressing chromaffin tumour cells, suggesting that this highly promising strategy deserves further exploration.

Polygenic risk scores (PRSs), which quantify inherited susceptibility to complex traits and diseases, have emerged as valuable tools for risk stratification and precision medicine. Despite their promise, PRS developed on European cohorts often demonstrate substantially reduced predictive accuracy in non-European populations, due to differences in genetic architecture. The disproportionate representation of European ancestry cohorts in genome-wide association studies (GWAS) leads to inequitable deployment of PRS technologies across diverse populations. Here, we introduce PRANA (Polygenic Risk Adaptation via Neural-network Architecture), a deep learning framework that adapts an existing PRS developed on one population to other ancestries. Unlike methods that require large-scale GWAS in the target population, PRANA leverages pre-trained PRS models derived from European cohorts and adapts them using modestly sized cohorts from the target population. We evaluated PRANA on seven complex traits in South Asian, East Asian and Ashkenazi Jewish populations, as well as in selected smaller East Asian subpopulations where the scarcity of training data poses a particular challenge. PRANA mostly improved predictive performance of the baseline PRS models by 5%-20% in terms of effect size (β) and Nagelkerke's R2, and, in most cases, outperformed existing cross-ancestry multi-PRS approaches. These results highlight PRANA as a scalable and practical strategy to reduce disparities in genomic risk prediction and advance the equitable application of PRS in diverse populations.

Many breast cancer predisposition genes are involved in DNA damage repair, leading to genome instability that can impact immunosurveillance, neoantigen formation, and the composition of the tumor immune microenvironment. Here, we explored associations between germline protein truncating variants (PTVs) in 34 (putative) breast cancer predisposition genes, of which 26 involved in DNA damage repair, with the abundance of four immune cell markers, i.e., CD8 + , FOXP3 + , CD20 + and CD163 + , across 7,969 invasive breast tumors of women of European ancestry. The most apparent associations were those of CD163, a marker of M2-like tumor-associated macrophages, with genes involved in double- and single-strand break DNA repair, and with the 12 known breast cancer predisposition genes combined. Specifically, DNA damage repair genes, BRCA1, BRCA2, PALB2, RAD51D, and MSH6 were associated with a 1.3 to twofold abundance of CD163-positive cells. Estrogen receptor status was found to mediate associations to a limited extent. Our findings support a role of rare pathogenic germline variants involved in DNA damage repair, and particularly those predisposing to breast cancer, in the immune landscape of breast tumors. These insights may help guide the development of immunomodulatory strategies for breast cancer prevention and treatment.

Breast cancer immune response is important to patient outcome, but the prognostic interaction between tissue-infiltrating immune cell (TIIC) types is not well-characterized. We evaluated the associations between CD8 +, FOXP3+, CD20 +, and CD163+ TIICs and breast cancer-specific survival (BCSS). We developed an AI in Halo to score TIIC percentage by compartment (overall, stromal, or intra-tumoral) in 99,051 microarray images from 12,285 female breast cancers. The associations between log-transformed TIIC scores and BCSS were assessed using Cox regression. CD8+ and FOXP3+ TIICs were associated with better BCSS in ER-negative disease; CD8+ and CD20+ TIICs were associated with a better prognosis in ER-positive disease; and CD163+ TIICs were associated with a poorer prognosis in ER-positive disease in multi-marker models. These results may have implications for breast cancer immunotherapy.

Genes encoding subunits of the mitochondrial tricarboxylic acid cycle enzyme complex succinate dehydrogenase (SDH) are a leading cause of the neuroendocrine tumour syndrome hereditary paraganglioma-pheochromocytoma. Pathogenic variants of SDHD and SDHAF2 confer a remarkable parent-of-origin tumour risk, in which paternally inherited variants cause tumours but maternally inherited variants do not. Formulated to explain this observation, the Hensen hypothesis proposes that loss of an (unknown) imprinted gene(s), together with the remaining wildtype SDH gene, is a prerequisite for tumour formation; in effect a three-hit hypothesis. This study had three objectives, first, as a test of the Hensen model, second, as a potential model for a disease for which no mouse or cell model currently exists, and finally, as a test of chromosome (Ch.) configuration to interrogate large genomic regions carrying an unknown phenotypic modifier. We crossed a gene knockout line (Sdhc, mouse Ch.1) to a Robertsonian chromosome line, Rb(1:7), harbouring the homologous gene imprinting centre (human Ch.11p15, mouse Ch.7) implicated in human tumourigenesis, to create a metacentric chromosome with characteristics of human chromosome 11. We developed 7 cohorts combining Sdhc (mouse Ch.1) wildtype or knockout with distinct configurations of Rb(1:7), confirming both paternal and maternal inheritance of Sdhc. We noted significant weight gain, and in heterozygote Sdhc KO-Rb/wt mice high levels of immune activation. Thyroid abnormalities, including lesions with papillary thyroid carcinoma-like features, were common (30-50%) in Sdhc knockout mice with both heterozygous and homozygous Rb chromosomes, regardless of mode of inheritance. We also observed a single case of bilateral pheochromocytoma in which loss of Sdhc was not the driver. While our findings did not recapitulate features of the Hensen Model, this study does suggest that chromosomal structure, even in the form of a seemingly innocuous single Robertsonian configuration, can dramatically impact clinical phenotype.

The breast cancer risk conferred by germline protein truncating variants (PTVs) in known and putative breast cancer genes has been extensively investigated. However, the effect of FANCM PTVs on breast cancer risk remains unclear. Our previous clinical, genetic and functional results on the N-terminal p.Arg658∗ and the two C-terminal p.Gln1701∗ and p.Gly1906Alafs∗12 variants suggested that FANCM PTVs may confer different risks for ER-negative (ER-neg) and triple-negative (TN) breast cancer subtypes. Here, we performed meta-analyses of seven studies totaling 144 681 breast cancer cases and 123 632 controls. FANCM PTVs were tested for association with breast cancer risk overall and the disease clinical subtypes by single variant and burden analyses. Two CRISPR-Cas9-based functional assays were also conducted to test the fitness of cells after knock-in of the p.Arg658∗, p.Gln1701∗ and p.Gly1906Alafs∗12 PTVs and the sensitivity of different FANCM regions to genome editing. Our results suggest that the N-terminal FANCM region upstream of p.Tyr725 harbors essential functions, whereas downstream regions appear dispensable. This is supported by our genetic data which indicate that all FANCM PTVs, excluding the two C-terminal p.Gln1701∗ and p.Gly1906Alafs∗12, are associated with an increased risk of ER-neg (OR = 1.41, P = 0.023) and TN (OR = 1.64, P = 0.0023). Notably, PTVs upstream of AA position 670 are associated with a moderate risk of developing TN breast cancer, and that even when the p.Arg658∗ carriers were excluded from the analysis. Importantly, our results confirm previous data indicating that p.Arg658∗ carriers are at moderate risk of developing ER-neg (OR = 2.08, P = 0.030) and TN (OR = 3.26; P = 0.0034), whereas carriers of p.Gln1701∗ and p.Gly1906Alafs∗12 should not be considered at increased risk. Our data are useful for counseling carriers of FANCM PTVs, but further analyses are warranted to obtain more precise risk estimates.

Clinical genetic testing identifies variants causal for hereditary cancer, information that is used for risk assessment and clinical management. Unfortunately, some variants identified are of uncertain clinical significance (VUS), complicating patient management. Case-control data is one evidence type used to classify VUS. As an initiative of the Evidence-based Network for the Interpretation of Germline Mutant Alleles (ENIGMA) Analytical Working Group we analyze germline sequencing data of BRCA1 and BRCA2 from 96,691 female breast cancer cases and 302,116 controls from three studies: the BRIDGES study of the Breast Cancer Association Consortium, the Cancer Risk Estimates Related to Susceptibility consortium, and the UK Biobank. We observe 11,207 BRCA1 and BRCA2 variants, with 6909 being coding, covering 23.4% of BRCA1 and BRCA2 VUS in ClinVar and 19.2% of ClinVar curated (likely) benign or pathogenic variants. Case-control likelihood ratio (ccLR) evidence is highly consistent with ClinVar assertions for (likely) benign or pathogenic variants; exhibiting 99.1% sensitivity and 95.3% specificity for BRCA1 and 93.3% sensitivity and 86.6% specificity for BRCA2. This approach provides case-control evidence for 787 unclassified variants; these include 579 with strong or moderate benign evidence and 10 with strong pathogenic evidence for which ccLR evidence is sufficient to alter clinical classification.

SDHD and SDHAF2 pathogenic variants confer a remarkable parent-of-origin tumour risk for the neuroendocrine tumours paraganglioma and pheochromocytoma. Paternally transmitted variants cause tumours but maternally transmitted variants do not. The Hensen hypothesis asserts that loss of an (unknown) imprinted gene(s), together the remaining wildtype SDH gene, is a prerequisite for tumour formation. This study had three objectives, first, as a test of the Hensen model, second, as a potential paraganglioma model, and finally, as a test of chromosomal configuration to interrogate large genomic regions carrying an unknown phenotypic modifier. We crossed an SDH gene knockout line to a Robertsonian (Rb) chromosome line harbouring the gene imprinting centre implicated in human tumourigenesis, to create a metacentric chromosome with characteristics of human chromosome 11. Distinct phenotypes were noted in various cohorts. In heterozygote Rb mice we noted both weight gain and frequent immune activation. In Sdhc knockout mice with both heterozygous and homozygous Rb chromosomes, thyroid abnormalities, including papillary thyroid carcinoma-like tumours, were common due to apparent synergy between the Sdhc KO and the Rb chromosome. We also found a single case of bilateral pheochromocytoma in which loss of Sdhc was not the driver. Although few studies of Robertsonian chromosomes in the mouse have addressed pathology or phenotype, this study suggests that chromosomal structure can dramatically impact clinical phenotype. ### Competing Interest Statement The authors have declared no competing interest.

The 313-variant polygenic risk score (PRS313) provides a promising tool for clinical breast cancer risk prediction. However, evaluation of the PRS313 across different European populations which could influence risk estimation has not been performed. We explored the distribution of PRS313 across European populations using genotype data from 94,072 females without breast cancer diagnosis, of European-ancestry from 21 countries participating in the Breast Cancer Association Consortium (BCAC) and 223,316 females without breast cancer diagnosis from the UK Biobank. The mean PRS was calculated by country in the BCAC dataset and by country of birth in the UK Biobank. We explored different approaches to reduce the observed heterogeneity in the mean PRS across the countries, and investigated the implications of the distribution variability in risk prediction. The mean PRS313 differed markedly across European countries, being highest in individuals from Greece and Italy and lowest in individuals from Ireland. Using the overall European PRS313 distribution to define risk categories, leads to overestimation and underestimation of risk in some individuals from these countries. Adjustment for principal components explained most of the observed heterogeneity in the mean PRS. The mean estimates derived when using an empirical Bayes approach were similar to the predicted means after principal component adjustment. Our results demonstrate that PRS distribution differs even within European ancestry populations leading to underestimation or overestimation of risk in specific European countries, which could potentially influence clinical management of some individuals if is not appropriately accounted for. Population-specific PRS distributions may be used in breast cancer risk estimation to ensure predicted risks are correctly calibrated across risk categories.

Recent exome-wide association studies have explored the role of coding variants in breast cancer risk, highlighting the role of rare variants in multiple genes including BRCA1, BRCA2, CHEK2, ATM and PALB2, as well as new susceptibility genes e.g., MAP3K1. These genes, however, explain a small proportion of the missing heritability of the disease. Much of the missing heritability likely lies in the non-coding genome. We evaluated the role of rare variants in the 5' and 3' untranslated regions (UTRs) of 18,676 genes, and 35,201 putative promoter regions, using whole-genome sequencing data from UK Biobank on 8,001 women with breast cancer and 92,534 women without breast cancer. Burden tests and SKAT-O tests were performed in UTR and promoter regions. For UTR regions of 35 putative breast cancer susceptibility genes, we additionally performed a meta-analysis with a large breast cancer case-control dataset. Associations for 8 regions at P<0.0001 were identified, including several with known roles in tumorigenesis. The strongest evidence of association was for variants in the 5' UTR of CDK5R1 (P=8.5x10-7). These results highlight the potential role of non-coding regulatory regions in breast cancer susceptibility. ### Competing Interest Statement JRBP and EJG are employees of Insmed Innovation UK and holds stock/stock options in Insmed Inc. JRBP also receives research funding from GSK and engages in paid consultancy for WW International Inc. ### Funding Statement The research has been conducted using the UK Biobank Resource under Application Number 28126. N.W. was supported by the International Alliance for Cancer Early Detection, an alliance between Cancer Research UK (C14478/A29329), Canary Center at Stanford University, the University of Cambridge, OHSU Knight Cancer Institute, University College London, and the University of Manchester. Quality control of the UK Biobank sequencing data has been funded by the Medical Research Council (unit programs: MC\_UU\_12015/2, MC\_UU\_00006/2). The BRIDGES project was supported by the European Union Horizon 2020 research and innovation programs BRIDGES (grant number, 634935 to P.D A.G.-N., A,M.D. and D.F.E) and B-CAST (633784 to M.K.S. M.K.S., P.D.P.P. and D.F.E.), the Wellcome Trust (v203477/Z/16/Z to S.H.T ad D.F.E), and Cancer Research UK (C1287/A16563). Details regarding funding of specific BRIDGES studies are provided in the Supplementary Material. ### 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: The organisations providing ethical approval for the contributing studies are summarised by Dorling et al, NEJM, https://www.nejm.org/doi/full/10.1056/NEJMoa1913948 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 Data from UK Biobank are available through application to UK Biobank. Data from the Breast Cancer Association Consortium (BCAC) used in the present study are available upon reasonable request through the BCAC Data Access Coordinating Committee. All data produced in the present study are available upon reasonable request to the authors

Co-observation of a gene variant with a pathogenic variant in another gene that explains the disease presentation has been designated as evidence against pathogenicity for commonly used variant classification guidelines. Multiple variant curation expert panels have specified, from consensus opinion, that this evidence type is not applicable for the classification of breast cancer predisposition gene variants. Statistical analysis of sequence data for 55,815 individuals diagnosed with breast cancer from the BRIDGES sequencing project was undertaken to formally assess the utility of co-observation data for germline variant classification. Our analysis included expected loss-of-function variants in 11 breast cancer predisposition genes and pathogenic missense variants in BRCA1, BRCA2, and TP53. We assessed whether co-observation of pathogenic variants in two different genes occurred more or less often than expected under the assumption of independence. Co-observation of pathogenic variants in each of BRCA1, BRCA2, and PALB2 with the remaining genes was less frequent than expected. This evidence for depletion remained after adjustment for age at diagnosis, study design (familial versus population-based), and country. Co-observation of a variant of uncertain significance in BRCA1, BRCA2, or PALB2 with a pathogenic variant in another breast cancer gene equated to supporting evidence against pathogenicity following criterion strength assignment based on the likelihood ratio and showed utility in reclassification of missense BRCA1 and BRCA2 variants identified in BRIDGES. Our approach has applicability for assessing the value of co-observation as a predictor of variant pathogenicity in other clinical contexts, including for gene-specific guidelines developed by ClinGen Variant Curation Expert Panels.

BACKGROUND Disrupted pre-mRNA splicing is a frequent deleterious mechanism in hereditary cancer. We aimed to functionally analyze candidate spliceogenic variants of the breast cancer susceptibility gene CHEK2 by splicing reporter minigenes. METHODS A total of 128 CHEK2 splice-site variants identified in the Breast Cancer After Diagnostic Gene Sequencing (BRIDGES) project (https://cordis.europa.eu/project/id/634935) were analyzed with MaxEntScan and subsetted to 52 variants predicted to impact splicing. Three CHEK2 minigenes, which span all 15 exons, were constructed and validated. The 52 selected variants were then genetically engineered into the minigenes and assayed in MCF-7 (human breast adenocarcinoma) cells. RESULTS Of 52 variants, 46 (88.5%) impaired splicing. Some of them led to complex splicing patterns with up to 11 different transcripts. Thirty-four variants induced splicing anomalies without any trace or negligible amounts of the full-length transcript. A total of 89 different transcripts were annotated, which derived from different events: single- or multi-exon skipping, alternative site-usage, mutually exclusive exon inclusion, intron retention or combinations of the abovementioned events. Fifty-nine transcripts were predicted to introduce premature termination codons, 7 kept the original open-reading frame, 5 removed the translation start codon, 6 affected the 5'UTR (Untranslated Region), and 2 included missense variations. Analysis of variant c.684-2A > G revealed the activation of a non-canonical TG-acceptor site and exon 6 sequences critical for its recognition. CONCLUSIONS Incorporation of minigene read-outs into an ACMG/AMP (American College of Medical Genetics and Genomics/Association for Molecular Pathology)-based classification scheme allowed us to classify 32 CHEK2 variants (27 pathogenic/likely pathogenic and 5 likely benign). However, 20 variants (38%) remained of uncertain significance, reflecting in part the complex splicing patterns of this gene.

Background: Breast cancer (BC) risk prediction models consider cancer family history (FH) and germline pathogenic variants (PVs) in risk genes. It remains elusive to what extent complementation with polygenic risk score (PRS) and non-genetic risk factor (NGRFs) data affects individual intensified breast surveillance (IBS) recommendations according to European guidelines. Methods: For 425 cancer-free women with cancer FH (mean age 40·6 years, range 21–74), recruited in France, Germany and the Netherlands, germline PV status, NGRFs, and a 306 variant-based PRS (PRS306) were assessed to calculate estimated lifetime risks (eLTR) and estimated 10-year risks (e10YR) using CanRisk. The proportions of women changing country-specific European risk categories for IBS recommendations, i.e. ≥20 % and ≥30 % eLTR, or ≥5 % e10YR were determined. Findings: Of the women with non-informative PV status, including PRS306 and NGRFs changed clinical recommendations for 31·0 %, (57/184, 20 % eLTR), 15·8 % (29/184, 30 % eLTR) and 22·4 % (41/183, 5 % e10YR), respectively whereas of the women tested negative for a PV observed in their family, clinical recommendations changed for 16·7 % (25/150), 1·3 % (2/150) and 9·5 % (14/147). No change was observed for 82 women with PVs in high-risk genes (BRCA1/2, PALB2). Combined consideration of eLTRs and e10YRs identified BRCA1/2 PV carriers benefitting from IBS <30 years, and women tested non-informative/negative for whom IBS may be postponed. Interpretation: For women who tested non-informative/negative, PRS and NGRFs have a considerable impact on IBS recommendations. Combined consideration of eLTRs and e10YRs allows personalizing IBS starting age. Funding: Horizon 2020, German Cancer Aid, Federal Ministry of Education and Research, Köln Fortune.

Background The immune response in breast tumors has an important role in prognosis, but the role of spatial localization of immune cells and of interaction between subtypes is not well characterized. We evaluated the association between spatially resolved tissue infiltrating immune cells (TIICs) and breast cancer specific survival (BCSS) in a large multicenter study. Patients and methods Tissue microarrays with tumor cores from 17,265 breast cancer patients of European descent were stained for CD8, FOXP3, CD20, and CD163. We developed a machine learning based tissue segmentation and immune cell detection algorithm using Halo to score each image for the percentage of marker positive cells by compartment (overall, stroma, or tumor). We assessed the association between log transformed TIIC scores and BCSS using Cox regression. Results Total CD8+ and CD20+ TIICs (stromal and intra-tumoral) were associated with better BCSS in women with ER-negative (HR per standard deviation = 0.91 [95% CI 0.85 - 0.98] and 0.89 [0.84 - 0.94] respectively) and ER-positive disease (HR = 0.92 [95% CI 0.87 - 0.98] and 0.93 [0.86 - 0.99] respectively) in multi-marker models. In contrast, CD163+ macrophages were associated with better BCSS in ER-negative disease (0.94 [0.87 - 1.00]) and a poorer BCSS in ER-positive disease 1.04 [0.99 - 1.10]. There was no association between FOXP3 and BCSS. The observed associations tended to be stronger for intra-tumoral than stromal compartments for all markers. However, the TIIC markers account for only 7.6 percent of the variation in BCSS explained by the multi-marker fully-adjusted model for ER-negative cases and 3.0 percent for ER-positive cases. Conclusions The presence of intra-tumoral and stromal TIICs is associated with better BCSS in both ER-negative and ER-positive breast cancer. This may have implications for the use of immunotherapy. However, the addition of TIICs to existing prognostic models would only result in a small improvement in model performance. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement BCAC was supported by Cancer Research UK grant: PPRPGM-Nov20\100002 and by core funding from the NIHR Cambridge Biomedical Research Centre (NIHR203312). The views expressed are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care. The B-CAST project was supported by the Horizon 2020 Research and Innovation Programs of the European Union B (grant number: 633784) and the NIHR Cambridge Biomedical Research Centre. AJB was supported by the NIH/Oxcam doctoral programme. The funding of the contributing studies is listed in Supplementary Table 8. ### 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: All participants provided written informed consent. The ethics committees or institutional review boards responsible for oversight of the individual studies are listed in Supplementary Table 9. 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 The tissue segmentation and TIIC scores generated by the Halo algorithm together with the phenotype data, the imputed datasets and the analysis code will be available at the European Genome Phenome Archive on publication (https://ega-archive.org/).

PDF file, 107K, Summary characteristics of the participating BCAC case-control studies.