Intratumor Heterogeneity of Homologous Recombination Deficiency in Primary Breast Cancer.

The 3-biomarker homologous recombination deficiency (HRD) assay measures the number of telomeric allelic imbalances, loss of heterozygosity, and large-scale state transitions in tumor DNA and combines these metrics into a single score that reflects DNA repair deficiency. The goal of this study is to assess the consistency of these HRD measures in different biopsies from distinct areas of the same cancer. HRD scores, BRCA mutation status, and promoter methylation were assessed in 99 samples from 33 surgically resected, stage I-III breast cancers; each cancer was biopsied in three distinct areas. Homologous recombination repair (HR) deficiency was defined as either high HRD score (≥42) or tumor BRCA mutation. Eighty-one biopsies from 32 cancers were analyzed. Tumor BRCA status was available for all samples, HRD scores for 70, and methylation values for 76 samples. The mutation and promoter methylation status and HR category showed perfect concordance across all biopsies from the same cancer. All tumors with mutations or promoter methylation had high HRD scores, as did 17% (4/24) of the wild-type and nonmethylated tumors. The HRD scores were also highly consistent between different biopsies from the same tumor with an intraclass correlation coefficient of 0.977, indicating that only 2.3% of the variance is attributed to within-tumor biopsy-to-biopsy variation. These results indicate that within-tumor spatial heterogeneity for HRD metrics and the technical noise in the assay are small and do not influence HRD scores and HR status. .