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BRCA1 has been implicated in numerous DNA repair pathways that maintain genome integrity, however the function responsible for its tumor suppressor activity in breast cancer remains obscure. To identify the most highly conserved of the many BRCA1 functions, we screened the evolutionarily distant eukaryote Saccharomyces cerevisiae for mutants that suppressed the G1 checkpoint arrest and lethality induced following heterologous BRCA1 expression. A genome-wide screen in the diploid deletion collection combined with a screen of ionizing radiation sensitive gene deletions identified mutants that permit growth in the presence of BRCA1. These genes delineate a metabolic mRNA pathway that temporally links transcription elongation (SPT4, SPT5, CTK1, DEF1) to nucleopore-mediated mRNA export (ASM4, MLP1, MLP2, NUP2, NUP53, NUP120, NUP133, NUP170, NUP188, POM34) and cytoplasmic mRNA decay at P-bodies (CCR4, DHH1). Strikingly, BRCA1 interacted with the phosphorylated RNA polymerase II (RNAPII) carboxy terminal domain (P-CTD), phosphorylated in the pattern specified by the CTDK-I kinase, to induce DEF1-dependent cleavage and accumulation of a RNAPII fragment containing the P-CTD. Significantly, breast cancer associated BRCT domain defects in BRCA1 that suppressed P-CTD cleavage and lethality in yeast also suppressed the physical interaction of BRCA1 with human SPT5 in breast epithelial cells, thus confirming SPT5 as a relevant target of BRCA1 interaction. Furthermore, enhanced P-CTD cleavage was observed in both yeast and human breast cells following UV-irradiation indicating a conserved eukaryotic damage response. Moreover, P-CTD cleavage in breast epithelial cells was BRCA1-dependent since damage-induced P-CTD cleavage was only observed in the mutant BRCA1 cell line HCC1937 following ectopic expression of wild type BRCA1. Finally, BRCA1, SPT5 and hyperphosphorylated RPB1 form a complex that was rapidly degraded following MMS treatment in wild type but not BRCA1 mutant breast cells. These results extend the mechanistic links between BRCA1 and transcriptional consequences in response to DNA damage and suggest an important role for RNAPII P-CTD cleavage in BRCA1-mediated cancer suppression.

Purpose: This phase II neoadjuvant study investigated whether nab paclitaxel, carboplatin and bevacizumab given before neoadjuvant doxorubicin/cyclophosphamide (AC) produced higher pathologic complete response (pCR) rates in triple- negative breast cancer (TNBC) compared with historical results achieved with standard anthracycline/taxane regimens. Patients and Methods: Eligible patients with operable TNBC ≥2 cm received four cycles of carboplatin (area under the curve 6, day 1) plus nab-paclitaxel (100 mg/m 2 , days 1, 8 and 15) every 28 days, followed by four 14-day cycles of AC neoadjuvantly, with bevacizumab 10 mg/kg every 14 days for the rst 6 cycles of neoadjuvant chemotherapy, resuming postoperatively to complete 1 year of antibody treatment. In breast pCR and pCR (breast + nodes) were primary and secondary endpoints, respectively. Results: Due to slow accrual, the study was closed after enrollment of 42 of 60 planned patients. Of the 38 patients who underwent surgery (ef cacy population), 22 (58%) achieved an in-breast pCR and 19 (50%) achieved a pCR (breast + nodes). Neutropenia was the most common Grade 3/4 adverse event (57% Grade 3 and 31% Grade 4), but only 1 patient required hospitalisation and IV antibiotics for neutropenic fever. Other Grade 3/4 events included anaemia (24%), thrombocytopenia (29%) and peripheral neuropathy (Grade 3, 5%). Conclusion: Our results demonstrate a substantially higher pCR rate, both in-breast and breast + nodes, with the combination of nab paclitaxel plus carboplatin followed by AC, with concurrent bevacizumab, versus historic pCR rates with anthracycline-taxane regimens alone, supporting further investigation of this regimen, preferably in molecularly driven subsets, for the neoadjuvant treatment of patients with TNBC. Key words: Bevacizumab, breast cancer, carboplatin, nab-paclitaxel, neoadjuvant, triple-negative