Mechanistic Insights into the Hypermethylation of BRCA1 Evinces a Novel Pathway to Breast Tumorigenesis

Abstract

BackgroundBreast cancer is the most common and dreadful form of cancer globally. Inherited mutations in the breast cancer susceptibility gene, BRCA1, increases the cumulative risk of developing breast cancer by 70‐80%. However, hereditary mutations account for only 5‐10% of the breast cancer burden. At the same time, the rest of this menace is attributed to sporadic cases where patients rarely encounter a BRCA1 mutation. Instead, upto 60% of these patients manifest low BRCA1 expression associated with BRCA1 promoter hypermethylation—moreover, many hereditary cases with a mutated BRCA1 allele exhibit promoter hypermethylation in the second allele. Interestingly, most of these cases also develop into a hyper‐aggressive and drug‐resistant form, Triple Negative Breast Cancer (TNBC), where Estrogen Receptors (ER), Progesterone receptors (PR) and Human epidermal growth factor receptors (Her2) are absent or minimally expressed. But the molecular signaling governing this tumorigenesis has remained elusive with an unresolved question of whether promoter hypermethylation is the master controller of the process of tumorigenesis or is a mere consequence of tumor progression.Objective & MethodologyTo unravel the truth behind this enigma, we induced site‐specific methylations in the CpG island of BRCA1 promoter on wildtype BRCA1 backgrounds using a modified version of CRISPR technology.ResultsInducing site‐specific methylation on the BRCA1 promoter effectuated its downregulation via alteration in the balance between its alternate transcripts ‘β’ and ‘α’. Methylation also attenuated the expression of a long noncoding RNA, NBR2 (Neighbor of BRCA1 gene 2), which is transcribed through the BRCA1 promoter in the reverse direction. It, in turn, activates a feedback loop leading to further downregulation of BRCA1 associated with impaired DNA damage repair and a shift in the balance between apoptosis and autophagy evident under glucose starvation conditions. Further, the β subunit of hCG (human chorionic gonadotrophin) hormone, which is often associated with aggressive forms of BRCA1 mutated breast cancers, was significantly overexpressed. With regards to TNBC progression, methylation also led to the downregulation of the hormone receptors except for ER‐ α, which was found to be overexpressed. This ER‐ α surge helps the cells survive the onslaught following DNA Damage under BRCA1 defective condition.ConclusionFor the first time, this study elucidates the molecular signaling behind breast tumorigenesis involving BRCA1 hypermethylation. Like BRCA1 mutation screening, our findings open up an outstanding opportunity for screening sporadic breast cancers in females using the combination of NBR2, β‐hCG, and BRCA1 hypermethylation as biomarkers from blood. Moreover, as therapeutic intervention, this study also provides scope for the use of BRCA1 hypermethylation reversing drugs like methotrexate, drugs like biguanides that show better action under NBR2 depleted conditions and development of β‐hCG inhibitors for a better prognosis.