The prevalence of genetic mutations among women with TNBC referred for genetic counseling is high and differs significantly by ethnicity/race and age. This data helps to refine mutation risk estimates among women with TNBC, allowing for more personalized genetic counseling potentially aiding in improved patient decision-making.

Cytochrome b(5) (b5) and NADH cytochrome b(5) reductase (b5R) detoxify reactive hydroxylamine (NHOH) metabolites of known arylamine and heterocyclic amine mammary carcinogens. The aim of this study was to determine whether NHOH reduction for the prototypic arylamine 4-aminobiphenyl (4-ABP) was present in human breast and to determine whether variability in activity was associated with single nucleotide polymorphisms (SNPs) in the coding, promoter, and 3'untranslated region (UTR) regions of the genes encoding b5 (CYB5A) and b5R (CYB5R3). 4-ABP-NHOH reduction was readily detected in pooled human breast microsomes, with a K(m) (280μM) similar to that found with recombinant b5 and b5R, and a V(max) of 1.12 ± 0.19 nmol/min/mg protein 4-ABP-NHOH reduction varied 75-fold across 70 individual breast samples and correlated significantly with both b5 (80-fold variability) and b5R (14-fold) immunoreactive protein. In addition, wide variability in b5 protein expression was significantly associated with variability in CYB5A transcript levels, with a trend toward the same association between b5R and CYB5R3. Although a sample with a novel coding SNP in CYB5A, His22Arg, was found with low reduction and b5 expression, no other SNPs in either gene were associated with outlier activity or protein expression. We conclude that b5 and b5R catalyze the reduction of 4-ABP-NHOH in breast tissue, with very low activity, protein, and messenger RNA expression in some samples, which cannot be attributed to promoter, coding, or 3'UTR SNPs. Further studies are underway to characterize the transcriptional regulation of CYB5A and CYB5R3 and begin to understand the mechanisms of individual variability in this detoxification pathway.

4‐aminobiphenyl (ABP), found in tobacco smoke, is oxidized to a hydroxylamine (ABP‐NHOH) and further bioactivated into a nitrenium ion that binds DNA. While these DNA adducts are thought to play a role in initiating breast cancer, epidemiological studies looking at the relationship between ABP exposure (via smoking) and breast cancer have been inconclusive. We have previously shown that hydroxylamines can be reduced back to their parent compounds by hepatic cytochrome b5 (cyt b5) and NADH cytochrome b5 reductase (b5R), but no studies have investigated these two enzymes in human breast. The aims of this study are to characterize variability in hydroxylamine reduction in 100 human breast samples, and to determine cDNA genotypes for cyt b5 and b5R in these same samples. Our overall goal is to determine how polymorphisms in these two enzymes may modulate local breast detoxification of ABP‐NHOH. Results to date indicate that the kinetic parameters for ABP‐NHOH reduction in breast tissue was similar to that previously found for human liver (Km =280 µM, Vmax =0.66 nmol/mg/min). cDNA sequencing has been complete for 95 samples. Two individuals were heterozygous for novel non‐synonymous single‐nucleotide polymorphisms in either cyt b5 (H22R) or b5R (A158T). Work is ongoing to determine ABP‐NHOH reduction activities in these and other individual breast samples and to determine the relationship between these non‐synonymous cSNPs and altered reduction activities.Supported by NIH grant GM61753, NIEHS grant T32 ES007015