Understanding BRCA2 Function as a Tumor Suppressor Based on Domain-Specific Activities in DNA Damage Responses

Abstract

BRCA2 is an essential genome stability gene that has various functions in cells, including roles in homologous recombination, G2 checkpoint control, protection of stalled replication forks, and promotion of cellular resistance to numerous types of DNA damage. Heterozygous mutation of BRCA2 is associated with an increased risk of developing cancers of the breast, ovaries, pancreas, and other sites, thus BRCA2 acts as a classic tumor suppressor gene. However, understanding BRCA2 function as a tumor suppressor is severely limited by the fact that ~70% of the encoded protein has not been tested or assigned a function in the cellular DNA damage response. Remarkably, even the specific role(s) of many known domains in BRCA2 are not well characterized, predominantly because stable expression of the very large BRCA2 protein in cells, for experimental purposes, is challenging. Here, we review what is known about these domains and the assay systems that are available to study the cellular roles of BRCA2 domains in DNA damage responses. We also list criteria for better testing systems because, ultimately, functional assays for assessing the impact of germline and acquired mutations identified in genetic screens are important for guiding cancer prevention measures and for tailored cancer treatments.

Keywords: BRCA2; DNA binding; DNA damage responses; DNA repair; G2 checkpoint; homologous recombination; tumor suppressor; variants of uncertain significance.

Figure 1

Diagram of the positions of identified domains in human BRCA2 and of the 27 exons that encode this protein. (a) At the extreme N-terminus of BRCA2 is a PALB2-binding domain (yellow bar) from a.a. 21 to 39 [38]. The central region of BRCA2 contains a RAD51-binding domain from a.a. 1002 to 2085, including 8 BRC repeats (red bars, BRC1: a.a. 1002–1036, BRC2: a.a. 1212–1246, BRC3: a.a. 1421–1455, BRC4: a.a. 1517–1551, BRC5: a.a. 1664–1698, BRC6: a.a. 1837–1871, BRC7: a.a. 1971–2005, and BRC8: a.a. 2051–2085), and intervening sequences [39,40]. There is also a prominent DNA-binding domain (DBD) at the C-terminus of BRCA2 (C-DBD), spanning a.a. 2482–3184, which includes a helical domain (a.a. 2482–2668, orange bar) and three OB folds (OB1: a.a. 2670–2803, OB2: a.a. 2808–3049, and OB3: a.a. 3055–3184, each shown as a green bar) [41]. There is an additional RAD51-binding domain at the extreme C-terminus of BRCA2 (a.a. 3270–3305, grey bar) [42]. On either side of this motif are three putative nuclear localization signals (NLS) (NLS1: a.a. 3263–3269, NLS2: a.a. 3311–3317 and NLS3: a.a. 3381–3385 [43]) indicated by blue asterisks and bars. A less well characterized N-terminal DBD (N-DBD), present at a.a. 250–500 [44], and a potential transactivation activity revealed via fusion to other factors (present at a.a. 18–105) [45], are shown by a grey line and indicated with grey/italicized text because of uncertainty about their physiological relevance. (b) Exons of BRCA2 are shown relative to the portions of the BRCA2 protein (a) they encode. Exons of BRCA2 by the amino acids they encode: exon 2 (a.a. 1–22), exon 3 (a.a. 23–105), exon 4 (a.a. 106–141), exon 5 (a.a. 142–158), exon 6 (a.a. 159–172), exon 7 (a.a. 173–210), exon 8 (a.a. 211–227), exon 9 (a.a. 228–264), exon 10, (a.a. 265–636), exon 11 (a.a. 637–2280), exon 12 (a.a. 2281–2312), exon 13 (a.a. 2313–2335), exon 14 (a.a. 2336–2478), exon 15 (a.a. 2479–2539), exon 16 (a.a. 2540–2601), exon 17 (a.a. 2602–2658), exon 18 (a.a. 2659–2777), exon 19 (a.a. 2778–2829), exon 20 (a.a. 2830–2877), exon 21 (a.a. 2878–2918), exon 22 (a.a. 2919–2984), exon 23 (a.a. 2985–3039), exon 24 (a.a. 3040–3085), exon 25 (a.a. 3086–3167), exon 26 (a.a. 3168–3216), and exon 27 (a.a. 3217–3418).