the G⁵¹⁶ Polymorphism of Cytochrome P450 2B6 Gene in the Susceptibility of De Novo Acute Myeloid Leukemia.

Abstract 2513

Acute myeloid leukemia (AML) is a heterogeneous disease with well-known clinical and pathological aspects, characterized by the acquisition of somatic mutations in haematopoietic progenitors leading to disruption of differentiation. However, the genetic etiology of AML, which include gene mutations and chromosomal aberrations, is largely unknown. Altered forms of genes that differ by a single nucleotide polymorphisms (SNPs) have been shown to predispose individuals to AML development. Recently it has been reported that interindividual differences based on detoxification genes polymorphisms may contribute to the AML susceptibility. Human cytochrome P450 (CYP) enzymes play a key role as phase I enzymes in the metabolism of drugs and environmental chemicals. Several CYP enzymes metabolically activate procarcinogens to genotoxic intermediates. CYP2B6 blocks the transformation of precarcinogens to their biologically active forms that provoke chromosomal instability and leukemia. CYP2B6 G⁵¹⁶T SNP change the aminoacid sequence (Gln172His), resulting in enzymatic inactivation. Thus, individuals homozygous for the mutant allele (T/T) or heterozygotes (G/T) present decreased enzymatic activity.

We performed a case-control study in a large series of AML patients to investigate the potential relation between genotype of CYP2B6 G⁵¹⁶T SNP and the risk of de novo AML. We also compared the genotypic frequencies in AML patients in respect to chromosome abnormalities, FAB classification and clinical characteristics.

The CYP2B6 G⁵¹⁶T genotyping was performed on 195 de novo AML patients at diagnosis and 215 sex and age matched healthy controls using a PCR-RFLP assay and LightSNP assay. Unstimulated bone marrow cells were used for karyotypic analysis and karyotypes were described according to ISCN. Statistical analysis was performed using Chi-square test and P<0.05 was considered to be statistically significant.

Karyotypic analysis was successfully performed in 97.8% of AML patients at diagnosis. Among them, 136 (69.7%) showed clonal karyotypic abnormalities. The genotypic distribution in patients and healthy groups was statistically significant and showed: homozygous wild type G/G 54,5% vs 67.2%, heterozygotes G/T 40.4% vs 27.5% and homozygous mutant T/T 6.7% vs 5.5% respectively (p<0.05). Stratification of patients according to FAB classification revealed differences in CYP2B6 genotype (p=0.069), which focuses primarily on an increase frequency of heterozygotes G/T in AML-M2 patients (61.4% vs 27.5%, p=0.000) and homozygous mutant T/T in AML-M6 patients (40% vs 5.4%, p=0.006).No differences in genotypic distribution of CYP2B6 genotype was noticed between male and female AML patients or among AML patients and healthy control group. Among females, the frequency of the mutant variant T was significantly increased in AML patients comparing to the healthy subjects (p=0.024).Interestingly, patients with aberrations of chromosomes 5 and/or 7 [−5/del(5q), −7/del(7q)], which confer a poor prognosis, showed a statistically increased frequency of the homozygous mutant T/T genotype in contrast to patients with favorable prognosis chromosome abnormalities such as inv(16), t(8;21), t(15;17) (p=0.044).

The increased frequency of mutant allele leading to reduced enzyme activity suggests that the CYP2B6 gene may be a predisposing factor for the development of AML. Furthermore, the CYP2B6 G⁵¹⁶T polymorphism seems to be associated with the presence of lesions on chromosomes 5 and / or 7 which are poor prognostic lesions in AML. Therefore, the high frequency of mutant genotypes (G/T and T/T) of CYP2B6 G⁵¹⁶T SNP in these cytogenetic groups may be involved in the development of these specific chromosomal abnormalities.