Early triple negative breast cancers in a Singapore cohort exhibit high PIK3CA mutation rates associated with low PD-L1 expression

Abstract Mutations in the PI3K pathway, particularly of PIK3CA, were reported to be intimately associated with triple negative breast cancer (TNBC) progression and development of treatment resistance. We profiled PIK3CA and other genes on 166 early-stage TNBC tumors from Singapore, for comparison to publicly available TNBC cohorts. These tumors were profiled transcriptionally using a Nanostring panel of immune genes and multiplex immunohistochemistry, then manually scored for PD-L1-positivity using two clinically relevant clones, SP142 and 22C3. We discovered a higher rate of PIK3CA mutations in our TNBC cohort as compared to non-Asian cohorts, along with TP53, BRCA1, PTPN11, and MAP3K1 alterations. PIK3CA mutations did not affect overall or recurrence-free survival, and when compared to PIK3CAWT tumors, there were no differences in immune infiltration. Using two clinically approved antibodies, PIK3CAmut tumors were associated with PD-L1 negativity. Analysis of co-mutation frequencies further revealed that PIK3CA mutations tended to be accompanied by MAP kinase pathway mutation. The mechanism and impact of PIK3CA alterations on the TNBC tumor immune microenvironment and PD-L1 positivity warrant further study.