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Purpose: Germline variants in known breast cancer (BC) predisposing genes explain less than half of hereditary BC cases. This study aimed to identify missing genetic determinants of BC., Methods: Whole exome sequencing (WES) of lymphocyte DNA was performed for 49 Russian patients with clinical signs of genetic BC predisposition, who lacked Slavic founder mutations in BRCA1, BRCA2, CHEK2, and NBS1 genes., Results: Bioinformatic analysis of WES data was allowed to compile a list of 229 candidate mutations. 79 of these mutations were subjected to a three-stage case-control analysis. The initial two stages, which involved up to 797 high-risk BC patients, 1504 consecutive BC cases, and 1081 healthy women, indicated a potentially BC-predisposing role for 6 candidates, i.e., USP39 c.*208G > C, PZP p.Arg680Ter, LEPREL1 p.Pro636Ser, SLIT3 p.Arg154Cys, CREB3 p.Lys157Glu, and ING1 p.Pro319Leu. USP39 c.*208G > C was strongly associated with triple-negative breast tumors (p = 0.0001). In the third replication stage, we genotyped the truncating variant of PZP (rs145240281) and the potential splice variant of USP39 (rs112653307) in three independent cohorts of Russian, Byelorussian, and German ancestry, comprising a total of 3216 cases and 2525 controls. The data obtained for USP39 rs112653307 supported the association identified in the initial stages (the combined OR 1.72, p = 0.035)., Conclusions: This study suggests the role of a rare splicing variant in BC susceptibility. USP39 encodes an ubiquitin-specific peptidase that regulates cancer-relevant tumor suppressors including CHEK2. Further epidemiological and functional studies involving these gene variants are warranted.

Inherited mutations in PALB2 are known to be associated with increased breast cancer risk. We aimed to investigate the prevalence and risk association of a recurrent PALB2 mutation, c.509_510delGA, among 3,924 unselected breast cancer patients from Belarus, Russia or Germany. High-resolution melting analyses and direct sequencing identified the c.509_510delGA allele in 3/1,008 (0.3 %) German breast cancer patients, 2/994 (0.2 %) Russian breast cancer patients and 5/1,922 (0.3 %) Byelorussian breast cancer patients. Breast tumours were mainly estrogen receptor positive and included both ductal and lobular histology. Only one of the ten patients had a first-degree family history of breast cancer. The mutation was not detected in 2,827 healthy females from the same populations, confirming the association of PALB2*c.509_510delGA with breast cancer risk (p = 0.007). These data indicate that the PALB2*c.509_510delGA mutation is prevalent in about 1 in 400 breast cancer patients from Central and Eastern Europe, and the low occurrence of familial clustering is consistent with a moderate penetrance of this mutation.

17 double heterozygous (DH) breast cancer (BC) patients were identified upon the analysis of 5,391 affected women for recurrent Slavic mutations in BRCA1, CHEK2, NBN/NBS1, ATM, and BLM genes. Double heterozygosity was found for BRCA1 and BLM (4 patients), BRCA1 and CHEK2 (4 patients), CHEK2 and NBS1 (3 patients), BRCA1 and ATM (2 patients), CHEK2 and BLM (2 patients), CHEK2 and ATM (1 patient), and NBS1 and BLM (1 patient). DH BC patients were on average not younger than single mutation carriers and did not have an excess of bilateral BC; an additional non-breast tumor was documented in two BRCA1/BLM DH patients (ovarian cancer and lymphoplasmacytic lymphoma). Loss-of-heterozygosity (LOH) analysis of involved genes was performed in 5 tumors, and revealed a single instance of somatic loss of the wild-type allele (LOH at CHEK2 locus in BRCA1/CHEK2 double heterozygote). Distribution of mutations in patients and controls favors the hypothesis on multiplicative interaction between at least some of the analyzed genes. Other studies on double heterozygosity for BC-predisposing germ-line mutations are reviewed.

Blood relatives of patients with ataxia-telangiectasia (A-T) have an increased risk to develop breast cancer. Allelic heterogeneity has made it difficult to confirm the role of ATM, the gene mutated in A-T, for breast cancer susceptibility in the general population. We now report that a nonsense mutation, p.E1978X (c.5932G>T), is both a classical A-T mutation and a breast cancer susceptibility allele in Eastern European populations. In a case-control study from Belarus, the E1978X mutation was identified in 10/1,891 Byelorussian breast cancer cases (0.5%) compared with 1/1,019 population controls [odds ratio (OR): 5.4; 95% confidence interval (95% CI), 0.7-42.4, P = 0.1]. A second case-control study from Russia identified the E1978X mutation in two Russian and one Ukrainian cases out of 611 breast cancer patients but not in any Russian or Ukrainian controls (P = 0.1). In a third case-control study from Poland, E1978X was observed in 7/3,910 Polish breast cancer cases (0.2%) compared with 1/2,010 cancer-free population controls (OR: 3.6; 95% CI: 0.4-29.3, P = 0.4). In the combined analysis, E1978X was significantly associated with breast cancer (Mantel-Haenszel OR: 5.6, 95% CI: 1.3-21.4, P = 0.01). Taken together, this study provides first evidence for the association of a common A-T causing mutation with breast cancer in Eastern European founder populations.