A role for BRCA1 in sporadic breast cancer

Abstract

To test the hypothesis that altered expression of BRCA1 protein may play an important role in sporadic breast cancer development, 50 randomly selected primary breast cancers (frozen sections, 5 years' median follow-up) were immunolabelled with two monoclonal BRCA1 antibodies (MS110 and MS13). MS110 labelling was exclusively nuclear showing no relation to outcome or tumour pathology. Western blotting demonstrated crossreactivity, suggesting antibody nonspecificity. MS13 labelling was predominantly cytoplasmic. Intense labelling predicted decreased overall survival (P=0.012), disease-free survival (P=0.029), oestrogen receptor negativity (P=0.0004) and c-erbB-2 overexpression (P=0.006). Western blotting detected a 110 kDa molecule consistent with BRCA1 delta11b splice variant. BRCA1 protein is postulated to function as a tumour suppressor. We demonstrate cytoplasmic localisation in sporadic breast cancer suggesting excess delta11b splice variant production, reduced production of full-length BRCA1 and thus postulate reduced tumour suppressor activity. BRCA1 protein appears to have a significant role in both sporadic and hereditary breast cancers.

Figure 1

(A) Normal breast epithelium labelled with MS110 demonstrating nuclear staining of approximately 1–5% of tumour nuclei; (B) parallel section stained with MS13 demonstrating strong cytoplasmic staining of the ductal and terminal ductal lobular unit; (C) parallel control section; (D) SKBR-3 cell line section labelled with MS110 showing nuclear labelling; (E) a parallel section stained with MS13 demonstrating cytoplasmic staining (histoscore 140); (F) parallel control section; (G) sporadic breast cancer section labelled with MS110 (estimated 30% labelling of the tumour nuclei); (H) parallel section stained with MS13 (mean histoscore 80); (I) parallel control section; (J) sporadic breast cancer section labelled with MS110 (estimated 5% labelling of the tumour nuclei); (K) parallel section stained with MS13 (mean histoscore 270); (L) parallel control section. Control sections were labelled with isotype-matched nonimmune immunoglobulin in place of the primary antibody. Bars=20 μM.

Figure 2

Frequency histograms demonstrating the distribution of (A) MS110 labelling and (B) MS13 labelling in 50 randomly selected sporadic breast cancers.

Figure 3

(A) Survival curve analysis (from all causes) according to low and high immunohistochemical labelling with the MS13 antibody and (B) disease-free survival curve analysis (local and distant recurrence). Arrowheads indicate censored events (Kaplan–Meier estimates and log-rank tests).

Figure 4

SKOV-3 whole, cytoplasmic and nuclear fractions (100 μg aliquots lanes 1 – 3, and 200 μg aliquots lanes 4–6) probed with (A) MS110 demonstrating multiple bands of various molecular weights and (B) MS13 demonstrating one band of 110 kDa in the whole cell and cytoplasmic fractions and faintly in the nuclear fraction.

Figure 5

Sporadic breast cancer tumours (tumour A lanes 1 and 2, tumour B lanes 3 and 4). Whole-cell protein lysate (100 and 200 μg aliquots) probed with MS13 demonstrating the presence of one detected molecule of 110 kDa.