Fig. 1: Mammary progenitor cells from BRCA1 mutation carriers express low BRCA1 and enhanced radiosensitivity.

a Schematic overview of the experimental strategy. Non-cancer breast tissue samples from three premenopausal non-carrier samples (N1, N2, and N3) and three premenopausal BRCA1 mutation carriers (B1, B2, and B3) were preserved as mammary epithelial organoids and dissociated. Luminal progenitor (LPs) and basal cell (BC) subpopulations were FACS-purified based on surface marker expression, and cultured to measure proliferation and cell division orientation. b Immunofluorescence analysis of BRCA1 and phospho-Histone H2AX (Ser139) (γH2Ax) in Cyclin B1-negative (not shown) LPs isolated from a premenopausal non-carrier donor (N2) or a BRCA1 mutation (mut) carrier (B1). LPs were sham-treated, or irradiated with 1 Gy X-radiation, and fixed 30 min or 24 h later. Scale bars = 4 μm. c BRCA1-positive foci per µm² nucleus area measured in LPs or BCs isolated from premenopausal non-carrier donors or BRCA1 mutation carriers. BRCA1-positive foci are reduced 3-fold (30 min post-IR) to >20-fold (sham) in BRCA1 mutation carriers (mean ± SD, triplicate experiment values shown, n = 20 cells per experiment). ****P = 2.48E−07, ****P = 4.81E−06, **P = 0.0069, ****P = 6.06E−07, ****P = 2.76E−07, ****P = 5.72E−05; two-tailed unpaired t-test. d γH2Ax-positive foci per µm² nucleus area measured in LPs and BCs isolated from premenopausal non-carrier donors and BRCA1 mutation carriers (mean ± SD, triplicate experiment values shown, n = 20 cells per experiment). ns P = 0.1214, ns P = 0.595, *P = 0.0273, *P = 0.0216, ns P = 0.6898, *P = 0.0114; two-tailed unpaired t-test. e Percentage of abnormal nuclei measured in BCs and LPs isolated from premenopausal non-carrier donors or BRCA1 mutation carriers (mean ± SD, field of view values shown, triplicate experiments, 3 fields of view per experiment). ns P = 0.4906, ns P = 0.2888, ****P < 1E−15, ns P = 0.9006, ns P = 0.7111, ****P = 1E−15; two-tailed Mann–Whitney test. Representative images of micronucleus and nuclear budding are provided in Supplementary Figure S2a. Abnormal nuclei are subdivided as micronucleus or nuclear budding in Supplementary Figure S2b.