Fig. 2: Mammary progenitor cells from BRCA1 mutation carriers display an elevated mitotic rate, but do not orient cell divisions.

a Colonies formed after 8 days per 500 cells seeded for BCs and LPs isolated from premenopausal non-carrier donors (N1, N2, and N3) or BRCA1 mutation carriers (B1, B2, and B3). Prior to seeding, cells were sham-treated or X-radiated with 1 Gy (mean ± SD, triplicate experiment values shown). ns P = 0.9851, *P = 0.0225, ns P = 0.6867, ****P = 1.73E−05; two-tailed unpaired t-test. b Individual LPs or BCs from a premenopausal non-carrier donor (N1) or a BRCA1 mutation carrier (B1) seeded on fibronectin-coated, L-shaped micropatterns and imaged during cell division to determine the orientation of cell division at anaphase indicated by a line connecting the spindle poles. Scale bars = 20 μm. c Percentage of adherent cells that underwent mitosis in a 24-h period following seeding on L-shaped micropatterns (mean ± SD, field of view values shown, three fields per experiment, triplicate experiments). Data is presented for BCs and LPs isolated from premenopausal non-carrier donors (N1, N2, and N3) or BRCA1 mutation carriers (B1, B2, and B3). Representative images are provided in Supplementary Figure 3a. ****P < 1E−15, ***P = 0.0006; two-tailed unpaired t-test. d Distribution of cell division angles measured at anaphase in 10°-wide sectors for BCs and LPs isolated from premenopausal non-carrier donors and BRCA1 mutation carriers. Data is pooled for three patient samples per genotype (each patient has n = 50 cells per experiment, duplicate experiments). Distribution of cell division angles for individual samples is provided in Supplementary Figure S3c. Gray percentages indicate the percent of total mitotic cells examined.