A glycolytic metabolite, methylglyoxal (MGO), bypasses the "two-hit" tumor suppression mechanism by BRCA2, leading to genome-wide single-base substitution (SBS) mutations in nonmalignant mammary cells and patient-derived organoids. MGO transiently inactivates BRCA2, causing functional haploinsufficiency and enabling cancer initiation. This mechanism links metabolic reprogramming, metabolic disorders, or diet to early cancer development. Germline monoallelic BRCA2 mutations predispose cells to MGO-induced BRCA2 inactivation, which can occur without biallelic inactivation. MGO accumulation and DNA damage in Kras-driven pancreatic and breast cancers also exhibit similar SBS signatures. MGO triggers BRCA2 proteolysis, temporarily disabling its tumor suppressor functions in DNA repair and replication. Intermittent MGO exposure causes episodic SBS mutations without permanent BRCA2 inactivation. This metabolic bypass connects glycolysis activation by oncogenes, metabolic disorders, or dietary factors to mutational signatures in cancer evolution. The study reveals that MGO-induced BRCA2 haploinsufficiency can temporarily bypass the two-hit requirement, linking metabolic changes to cancer initiation. The findings suggest that metabolic alterations, such as those in glycolysis, can influence cancer development by affecting BRCA2 function. The study also highlights the role of MGO in DNA damage and mutagenesis, particularly in BRCA2-mutant cells. The results indicate that MGO-induced BRCA2 haploinsufficiency can lead to mutational signatures similar to those seen in biallelic BRCA2 inactivation, even in the absence of permanent inactivation. The study underscores the importance of metabolic pathways in cancer development and the potential of MGO as a contributor to cancer initiation through its effects on BRCA2 function. The research provides insights into the mechanisms by which metabolic changes can influence cancer progression and highlights the significance of BRCA2 in tumor suppression. The findings suggest that metabolic factors, such as those involved in glycolysis, can play a critical role in cancer development by affecting BRCA2 function and leading to mutational signatures associated with cancer. The study also emphasizes the potential of MGO as a metabolic factor that can contribute to cancer initiation by inducing BRCA2 haploinsufficiency. The research highlights the importance of understanding the interplay between metabolic processes and tumor suppression mechanisms in cancer development. The results suggest that metabolic changes, such as those involving glycolysis, can influence cancer progression by affecting BRCA2 function and leading to mutational signatures associated with cancer. The study provides evidence that MGO can induce BRCA2 haploinsufficiency, leading to mutational signatures similar to those seen in biallelic BRCA2 inactivation, even in the absence of permanent inactivation. The findings highlight the role of MGO in DNA damage and mutagenA glycolytic metabolite, methylglyoxal (MGO), bypasses the "two-hit" tumor suppression mechanism by BRCA2, leading to genome-wide single-base substitution (SBS) mutations in nonmalignant mammary cells and patient-derived organoids. MGO transiently inactivates BRCA2, causing functional haploinsufficiency and enabling cancer initiation. This mechanism links metabolic reprogramming, metabolic disorders, or diet to early cancer development. Germline monoallelic BRCA2 mutations predispose cells to MGO-induced BRCA2 inactivation, which can occur without biallelic inactivation. MGO accumulation and DNA damage in Kras-driven pancreatic and breast cancers also exhibit similar SBS signatures. MGO triggers BRCA2 proteolysis, temporarily disabling its tumor suppressor functions in DNA repair and replication. Intermittent MGO exposure causes episodic SBS mutations without permanent BRCA2 inactivation. This metabolic bypass connects glycolysis activation by oncogenes, metabolic disorders, or dietary factors to mutational signatures in cancer evolution. The study reveals that MGO-induced BRCA2 haploinsufficiency can temporarily bypass the two-hit requirement, linking metabolic changes to cancer initiation. The findings suggest that metabolic alterations, such as those in glycolysis, can influence cancer development by affecting BRCA2 function. The study also highlights the role of MGO in DNA damage and mutagenesis, particularly in BRCA2-mutant cells. The results indicate that MGO-induced BRCA2 haploinsufficiency can lead to mutational signatures similar to those seen in biallelic BRCA2 inactivation, even in the absence of permanent inactivation. The study underscores the importance of metabolic pathways in cancer development and the potential of MGO as a contributor to cancer initiation through its effects on BRCA2 function. The research provides insights into the mechanisms by which metabolic changes can influence cancer progression and highlights the significance of BRCA2 in tumor suppression. The findings suggest that metabolic factors, such as those involved in glycolysis, can play a critical role in cancer development by affecting BRCA2 function and leading to mutational signatures associated with cancer. The study also emphasizes the potential of MGO as a metabolic factor that can contribute to cancer initiation by inducing BRCA2 haploinsufficiency. The research highlights the importance of understanding the interplay between metabolic processes and tumor suppression mechanisms in cancer development. The results suggest that metabolic changes, such as those involving glycolysis, can influence cancer progression by affecting BRCA2 function and leading to mutational signatures associated with cancer. The study provides evidence that MGO can induce BRCA2 haploinsufficiency, leading to mutational signatures similar to those seen in biallelic BRCA2 inactivation, even in the absence of permanent inactivation. The findings highlight the role of MGO in DNA damage and mutagen