Abstract P5-10-09: Correlation between germline and tumor CYP450 2D6 gene polymorphisms
Authors: Dehghani Mehdi , Gonzalez Anneliese O , Hashemi-Sadraei Neda , Zhang Songlin , Rosenblatt Kevin P 
Published: May 2015

Tamoxifen is used for the treatment and prevention of ER (estrogen receptor)-positive BrCa (Breast Cancer). Several studies have shown that genetic variations in the CYP2D6 gene and/or drug-dependent inhibition of CYP2D6 enzyme activity (drug-drug interactions or DDIs) are associated with significant reductions in the circulating levels of endoxifen the active tamoxifen metabolite. These studies demonstrated that changes in CYP2D6 metabolism, as predicted from CYP2D6 genotyping, affect efficacy. Recent negative results with regards to CYP2D6 genotyping from two large studies suggest that testing for CYP2D6 status has no practical clinical value. Tumor DNA was used for genotyping in a large fraction of the BrCa patients in these studies. Genetic bias may result when CYP2D6 genotyping is carried out on the tumor (somatic) genome rather than the host genome (germline DNA) since it is the host genome that determines CYP450 enzyme activity within the liver and GI tract. Also expanded CYP2D6 polymorphism coverage, using a more comprehensive genotyping panel, may improve risk stratification when using CYP2D6 genotyping as a prognosticator in BrCa patients treated with tamoxifen. We hypothesize that BrCa tissues will harbor numerous mutations, due to a mutator phenotype inherent in most cancers, including within the CYP450 family of genes and, specifically, within the CYP2D6 gene. We expect that comparisons between germline DNA isolated from peripheral blood, and mutated DNA isolated from BrCa (somatic DNA) specimens within the same patient will reveal extensive differences in CYP2D6 genotypes.

The aim of this study, is to extensively genotype 70 BrCa patients using a retrospective cohort with matched blood and tumor tissue from an existing biobank at UTHealth. We plan to perform genotype to phenotype conversions on each patient, for both germline and somatic DNA. We will look for discrepancies in genotypes and phenotypes and determine the magnitude of genetic bias possible in such cohorts.

Method: DNA samples were extracted from matched archived tumor cells, dissected by laser microdissection microscopy and blood. Genotyping was performed by clinically validated Taqman® discrimination assays on the most common alleles for CYP2D6. We also studied the genotype status of these paired samples for CYP2C9, VKORC1, Factor II, Factor V, MTHFR, CYP3A4 and CYP3A5 genes. CYP2D6 gene copy number and gene rearrangement with CYP2D7 pseudogene were also assessed by Taqman copy number assays at 3 three different sites within gene. Genotype-phenotype conversion was performed using an in-house developed, clinically validated genotype-phenotype translator package. Genotype agreement was assessed between the two DNA sources.

Results : Noticeable non-concordant results between DNA from breast tumors and blood were observed in the genotyping of polymorphisms in the CYP2D6 gene. However, strong agreement between DNA from breast tumors and blood was detected in the genotyping of polymorphisms in all other studied genes.

These results suggest that previous publications refuting the association between CYP2D6 genetic polymorphisms and tamoxifen efficacy could have reached an inaccurate conclusion due to genetic bias and study design. Further research in this biomarker area is needed.

Citation Format: Mehdi Dehghani, Anneliese O Gonzalez, Neda Hashemi-Sadraei, Songlin Zhang, Kevin P Rosenblatt. Correlation between germline and tumor CYP450 2D6 gene polymorphisms [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-10-09.