Introduction
Colorectal cancer (CRC) has been an public health problem for many years
because of increasing incidence and high mortality rate despite some
overall improvements in diagnosis and treatment [1, 2]. It is a
common form of cancer, with more than 1.5 million new patients diagnosed
every year worldwide [3] and more common in developed than
developing countries [4]. Previously published studies have
indicated that age over 60 years, alcohol consumption and tobacco habits
may contribute to sporadic CRC risk [5, 6]. Moreover, the
development of CRC is widely considered as a multi-step, multi-factorial
process involving gene-gene and gene-environment interactions [7].
It has been suggested that inter-individual differences including single
nucleotide polymorphism (SNP) might influence CRC risk [8, 9] .
The cytochrome P450 enzymes (CYPs) take part in the metabolism of
carcinogenic compounds in the human body [10]. CYP1A1 is expressed
in the large bowel [11]. Because of the involvement of CYP1A1 in the
metabolism of PAHs and estrogens, and perhaps cruciferous vegetables and
heterocyclic amines [12], it might be expected that CYP1A1 variants
might influence the risk of colorectal cancer. A commonly studied SNP in
the CYP1A1 gene has been indicated to associate with CRC risk, which was
localized on chromosome 15q22 [13]. The commonly reported the
3801T>C (also referred to as 2A, m1, or rs4646903) and
A2455G polymorphisms (also known as E7, *2C, m2, rs1048943) with CRC
risk [14]. The T3801C and A2455G polymorphisms might change the
level of gene expression or messenger RNA stability, resulting in a
highly inducible activity of the enzyme [15].
To date, 37 publications (supplemental Table 2) and seven previous
meta-analyses [16-22] have been reported to investigate the
correlation between the CYP1A1 T3801C and A2455G polymorphisms
with CRC risk. However, the results were inconsistent or even
contradictory. In addition, a lot of original studies did not involve in
previously published meta-analyses (supplemental Table 1). Moreover,
previously published meta-analyses also did not evaluate positive
results to identify multiple comparisons. Therefore, we performed an
updated meta-analysis and systematically evaluated the published
meta-analyses on the association between CYP1A1 T3801C and A2455G
polymorphism with CRC susceptibility.