Beyond Tumors: Reduced survival linked to pathogenic PIK3CA and TP53 post-zygotic variants in the uninvolved breast tissue of recurrent cancer patients

Histologically normal mammary tissue from breast cancer patients can harbor significant genetic alterations. We analyzed the spectrum of DNA variants in 408 matched histologically normal tissue and tumors from 77 poor-prognosis patients, 49 patients recruited without prognosis bias, and mammary gland samples from 15 non-cancerous individuals. Whole exome sequencing revealed a higher prevalence of pathogenic post-zygotic variants in cancer-associated genes: AKT1, PIK3CA, PTEN, TBX3, and TP53, affecting poor-prognosis patients (29%), compared to 12.5% in those without prognosis criteria (p=0.0008578). PIK3CA variants were recurrent across patients, while TP53 variants were restricted to those with adverse prognoses. Duplex sequencing detected low-frequency pathogenic PIK3CA and TP53 variants in distant normal tissues of poor-prognosis patients. Disease recurrence significantly reduced survival rates, with poor prognosis patients experiencing higher mortality within 24 months (p=0.0088), further worsened by the presence of pathogenic post-zygotic variants. These findings highlight the importance of genetic monitoring even in microscopically normal mammary tissue.