Obesity-associated changes in molecular biology of primary breast cancer

Abstract Worldwide, there is a growing proportion of women who are overweight or obese. While obesity has been associated with an increased risk of developing breast cancer (BC) and worse prognosis in BC patients, yet the impact of adiposity (abnormal or excess body fat) on BC biology remains understudied in humans. This retrospective study aimed to investigate how the biology of primary BC would differ according to patients’ body mass index (BMI). We examined clinicopathological data (including BMI at the time of diagnosis) and molecular data (including genomic, bulk and single-cell transcriptomic data) of treatment-naïve (early stage) BC patients from five cohorts (N = 2071). We identified several genomic alterations considered actionable or of potential clinical relevance which had a different prevalence in overweight or obese patients compared to lean patients, for instances, less PIK3CA gene mutations, and more CCND1, CCNE1 and IGFR1 amplifications. Moreover, we found evidence supporting an ageing accelerating effect of obesity at the genetic level, through its association with an age-associated mutational signature. We showed that BMI-associated differences in transcriptomic profile were subtle at the bulk resolution while single cell profiling allowed detection of more pronounced changes in different cell compartments. Investigation at the single cell resolution revealed an elevated and unresolved inflammation of the BC tumor microenvironment (TME) associated with obesity, which had distinct characteristics contingent on the estrogen receptor status. Collectively, analyses at both genomic and transcriptomic levels implied that obesity is associated with an inflammaging-like phenotype of the TME. Our results indicate that patient adiposity might play a significant role in the heterogeneity of BC and should be considered in the context of precision medicine.