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GT198 , located 470 kb downstream of BRCA1 , encodes for the nuclear PSMC3-interacting protein, which functions as co-activator of steroid hormone-mediated gene expression, and is involved in RAD51 and DMC1-mediated homologous recombination during DNA repair of double-strand breaks. Recently, germline variants in GT198 have been identified in hereditary breast and ovarian cancer (HBOC) patients, mainly in cases with early-onset. We screened a cohort of 166 BRCA1/2 mutation-negative HBOC patients, of which 56 developed early-onset breast cancer before the age of 36 years, for GT198 variants. We identified 7 novel or rare GT198 variants in 8 out of 166 index patients: c.-115G>A (rs191843707); c.-70T>A (rs752276800); c.-37A>T (rs199620968); c.-24C>G (rs200359709); c.519G>A p.(Trp173*); c.537+51G>C (rs375509656); c.*24G>A. Three out of 7 identified variants (c.-115G>A, c.519G>A and c.*24G>A) with putative pathogenic impact were found in HBOC patients with breast cancer onset at ≤ 36 years. The nonsense mutation c.519G>A p.(Trp173*) was located within the DNA binding domain of GT198 and is predicted to induce nonsense-mediated mRNA decay. Functional analyses of c.-115G>A, and c.*24A>G indicated an influence of these variants on gene expression. This is the second study that gives evidence for an association between pathogenic GT198 germline variants and early-onset breast cancer in HBOC., Competing Interests: CONFICTS OF INTEREST The authors declared that none of the contributors has commercial, professional or financial affiliations that may be perceived as having biased the presentation. Furthermore all authors have agreed that no conflict of interest with respect to the research, authorship and publication exists. The data presented here have not been submitted, published, and are not being submitted elsewhere for publication.

Familial thrombosis (FT) has been described as a rare autosomal-dominant disorder, mostly caused by activating mutations of the thrombopoietin gene (THPO). Other cases of FT have been linked to one of two different germline mutations in the myeloproliferative leukaemia virus oncogene gene (MPL), which codes for the thrombopoietin receptor MPL. We studied an Arab family with two siblings with severe thrombocytosis by linkage analysis and obtained evidence for linkage to MPL. Sequencing revealed homozygosity for the novel MPL germline mutation p.Pro106Leu (c.317C > T) in the two siblings. Subsequently, homozygosity for p.Pro106Leu was identified in six further FT patients from three other Arab families. Of 18 heterozygous carriers, 14 had normal platelet counts, while four had mild thrombocytosis. Strong support for association of the novel MPL mutation p.Pro106Leu with development of familial thrombocytosis has been obtained. Overall, p.Pro106Leu was absent on 386 alleles of 193 healthy German controls and present on 14 of 426 alleles (3.3%) of 213 unrelated Arabs, which was statistically significantly different (P < 0.001, Fisher's exact test). We assume that p.Pro106Leu is a frequent MPL mutation in the Arab population, leading to severe thrombocytosis in homozygotes and occasionally to mild thrombocytosis in heterozygotes. In the families described the mode of inheritance could be regarded as autosomal-recessive with possible mild heterozygote manifestation rather than autosomal-dominant with high penetrance as usually seen in FT.