Mismatch repair protein MLH1 suppresses replicative stress in BRCA2-deficient breast tumors

Area of Science:

• Molecular Biology
• Genetics
• Cancer Research

Background:

• Loss of BRCA2 (breast cancer 2) is typically lethal for normal cells.
• Mechanisms enabling survival and neoplastic transformation in BRCA2-mutant cells are poorly understood.
• BRCA2 deficiency is linked to increased cancer risk, particularly ER-positive breast cancer.

Purpose of the Study:

• To identify genetic interactors of BRCA2 that support cell viability.
• To elucidate the molecular mechanisms by which these interactors promote survival in BRCA2-deficient cells.
• To understand the role of these interactions in BRCA2-mutant tumor development and estrogen signaling.

Main Methods:

• Genetic interaction studies in BRCA2-deficient cells.
• Investigated the interaction between MLH1 (mutL homolog 1) and FEN1 (Flap endonuclease 1).
• Assessed replication fork stability, R-loop formation, and genomic instability.
• Utilized mouse models of Brca2 deficiency and Mlh1 loss.
• Examined estrogen's effect on MLH1 expression via ERα (estrogen receptor α).

Main Results:

• MLH1 overexpression supports the viability of Brca2-null cells.
• MLH1 interacts with FEN1, restraining DNA2 nuclease activity and stabilizing replication forks in BRCA2-deficient cells.
• MLH1 attenuates R-loops, suppressing genomic instability and promoting cell survival.
• Loss of MLH1 is lethal in Brca2-mutant mice and inhibits Brca2-deficient tumor growth.
• Estrogen induces MLH1 expression through ERα, potentially explaining ER-positive breast cancer prevalence in BRCA2 carriers.

Conclusions:

• MLH1 plays a crucial role in relieving replicative stress in BRCA2-deficient cells.
• MLH1 contributes to the establishment and progression of BRCA2-deficient tumors.
• The estrogen-MLH1-ERα pathway may be a key factor in BRCA2-associated breast cancer pathogenesis.