Background: Women of Mexican and African-American descent have a higher mortality and a higher proportion of triple negative or basal-like breast cancer with BRCA like features. Basal-like breast cancer subtype is associated with an elevated Ki67 index and extensive genomic instability. This increase in the Ki67 index indicates cell proliferation and perhaps greater DNA damage; double strand breaks are the most consequential DNA lesions. Knockdown of RING-finger 8 (RNF8), an E3 ubiquitin-protein ligase, by siRNA inhibit focus formation at double strand breaks sites suggesting that RNF8 is an important regulator of the DNA repair system. We are investigating the hypothesis that RNF8 is a modulator of genomic integrity by acting as a regulator of BRCA1 and 53BP1 proteins in human breast cancers.
Methods: We compared RNF8, p53 binding protein 1 (53BP1), cyclin B1 (CB1) and Ki67 mRNA expression in 57 formalin fixed paraffin embedded human breast cancer taken from a population of Mexican women enriched for basal-like tumors. Further we investigated copy number alterations in the 6p21.2 chromosomal region that contains RNF8 in a subset of 971 early stage breast cancers categorized by tumor subtype considering race/ethnicity.
Results: RNF8 and 53BP1 were found to exhibit variable expression at the mRNA level by Ki67 and CB1 expression levels. The combined effect RNF8 high/53BP1 low was associated with high expression of Ki67 (p=0.028). Conversely, RNF8 low expression was associated with low expression of Ki67 (p = 0.017), low CB1 (p = 0.035) and was independent of the 53BP1 expression. In a sample set of 971 stage I/II breast cancers, copy number gain at 6p21.3 (RNF8 locus) was positively associated with triple negative breast cancer (p< 0.0001), high grade tumors (p=0.0017) and large size tumors (p=0.051). Gain in the genomic regions containing the RNF8 locus was significantly associated with greater genomic instability (fdr=0.01). Additionally, though not significant given small sample size, African-Americans were ∼2-fold more likely than Non-Hispanic Whites or Hispanics to be diagnosed with tumors showing gain at the RNF8 locus (p=0.11).
Different groups have established that RNF8 is recruited to DNA damaged sites and subsequently mediates DNA repair by accumulation of BRCA1 and 53BP1 at DNA lesions. Recently, two groups have independently determined, in a BRCA deficient environment, that chromosomal instability is promoted in the presence of 53BP1. Additionally, it was shown that 53BP1 depletion can restore the DNA repair functions. In this report, we demonstrate differential expression of RNF8 and 53BP1 in a series of human breast cancers and an association between RNF8/53BP1 levels, proliferation, and tumor subtype. Our findings suggest that some breast tumors present an altered DNA repair pathway resulting in activation of RNF8 and de-regulation of 53BP1, which we hypothesize, may contribute to genomic instability and more proliferative tumors.
Citation Information: Cancer Epidemiol Biomarkers Prev 2011;20(10 Suppl):A65.