Affiliations: Institut Curie, France
Journal reference: https://advances.sciencemag.org/content/6/5/eaay2611
Summary: This excellent paper explains how mutations in a protein called BRCA2 confer predisposition to hereditary breast cancer. Using these hallmarks of cancer predisposition could improve risk assessment and prevention strategies in at risk patients.
BRCA2 is a tumor suppressor protein
BRCA2 is a tumor suppressor protein that plays a key role in DNA repair. It is essential for the maintenance of the genome integrity of the cell, that is, the preservation of all genetic elements in a cell for its proper functioning.
BRCA2 deficiency causes breast cancer in humans
Women with BRCA2 mutations have a 50 to 80% risk of developing breast cancer. The BRCA2 mutation is mono-allelic, meaning it only needs to happen on one of the two alleles of a gene to confer predisposition to hereditary breast cancer. Although the deleterious effect of BRCA2 deficiency is well-established, what drives tumor formation in BRCA2 mutation carriers has not been completely elucidated yet.
According to the “two-hit” model, the essential step for carcinogenesis (i.e. the formation of a cancer) in BRCA2 mutated cells seems to be the inactivation of the BRCA2 wild-type (non-mutated) allele. This phenomenon is also referred to as loss of heterozygosity (LOH).
However, the two-hit model has been revisited as a fraction of breast cancers arising in BRCA2 mutation carriers may retain an intact wild type allele. This means that a normal phenotype (with no cancer) would require two wild-type copies. Or, in other words, the wild type allele is incapable of producing a normal phenotype, a phenomenon called haploinsufficiency. However, to date, no direct evidence for BRCA2 haploinsufficiency has been identified in noncancerous human breast tissue.
To tackle this controversial subject, the authors of this paper decided to analyze noncancerous breast tissue from BRCA2 mutation carriers compared to control breast tissue with no mutation. The goal was to see whether BRCA2 deficiency preceded histologic abnormalities, thus providing material for early diagnosis.
The analysis focused on a specific subset of breast cells, the Luminal Progenitor (LP), which are considered to be the potential origin of BRCA2-associated carcinogenesis.
Once isolated, BRCA2 mutated and control cells were then subjected to a method called Whole Genome Sequencing that detects the presence of genetic copy number variations (CNVs), one of the earliest events in cancer progression.
The analysis revealed that breast LP cells from BRCA2 mutation carriers exhibited numerous CNVs whereas almost no CNVs were detected in controls. Results also confirmed the integrity of the wild-type BRCA2 allele, thus providing evidence for haploinsufficiency.
The authors further sought to assess DNA damage levels caused by CNVs in BRCA2 mutated cells. Results revealed that breast LP cells from BRCA2 mutation carriers displayed increased DNA damage compared to the controls. The difference further increased upon treatment with a compound called hydroxyurea that induces replicative stress (which broadly impairs DNA replication). Surprisingly, despite the ongoing genomic stress, BRCA2 mutated cells failed to activate the checkpoint protein that coordinates and initiates the DNA damage response. In other words, mutated cells accumulated DNA damage but lost their ability to respond to it and repair themselves. Moreover, due to decreased apoptotic response (the cell death process that is necessary for the normal development of cells), damaged cells accumulated in breast tissue in an age-associated manner.
Interestingly, while DNA damage failed to activate the checkpoint protein that responds to this damage, it did activate TP53, a tumor-suppressor protein that prevents malignant transformation. Loss of TP53 typically is another hallmark event that cooperates with BRCA2 deficiency in cancer pathogenesis. In this setting, TP53 enhanced activation could act as an early barrier to malignant progression.
Indeed, it was further associated with other downstream consequences such as the suppression of a tumorigenesis driver called NF-KB signaling that classically induces the production of many cytokines and inflammatory factors in most tumor cells. These results suggest that DNA damage and TP53 activation in BRCA2 mutated cells could be the earliest abnormalities that underlie malignancies.
Altogether, the authors demonstrated that breast cells from BRCA2 mutation carriers display increasing DNA damage but fail to activate responses to DNA damage and replication stress. Since the deregulated phenotype observed in BRCA2 mutated cells occur without losing the BRCA2 wild-type allele, collectively these findings seem to suggest that an haploinsufficient phenotype for BRCA2 may also occur in vivo.
Tracking the prevalence of DNA-damaged cells could thus potentially improve risk prediction for women patients faced with the difficult choice of whether to undergo mastectomy before cancer development.