We have identified a variant allele in a let-7 complementary site in the KRAS 3′UTR that alters let-7-mediated regulation of KRAS expression and is associated with a 1.4–2.3 fold increased risk for NSCLC among moderate smokers. This variant allele is found in 18.1–20.3% of lung cancer cases versus in only 5.8% of the world populations. The association of this allele to lung cancer was identified in the NM case-control study and validated in the MA case-control study, making this finding highly significant.
We find that this variant allele at the LCS6 site indeed affects regulation of KRAS
expression in vitro
, allowing increased levels of KRAS
. Interestingly, tumors containing the variant allele had lower let-7
levels than tumors without the variant allele, and as low let-7
has been associated with a poor prognosis in NSCLC,14, 15, 30
these findings raise the possibility that the subset of NSCLC patients harboring the variant allele may be those with an especially poor prognosis, a hypothesis requiring future studies to validate.
While it is not possible at this time to further define the mechanism of cancer predisposition for patients who harbor this variant allele, the finding that the variant allele appears to allow increased KRAS in vitro
suggests that KRAS
overexpression might be one plausible mechanism. An additional hypothesis could be that altered let-7
binding in the KRAS
3′UTR could somehow lower cellular levels of let-7
, perhaps through a feedback loop, and lower let-7
levels could further result in increased cell growth, as let-7
is known to repress cell growth pathways.16
For example, recent evidence indicates that a negative feedback loop involving lin-28
specifically regulates cellular let-7
In the background of the variant allele, a cycle creating lowered let-7
levels and increased KRAS
expression could in concert act as the first steps in oncogenesis. These hypotheses need testing in future studies.
Because lung cancer is so deadly when caught at later stages, screening programs have been initiated in current and ex-smokers: The Early Lung Cancer Action Project (ELCAP) found that a chest computed tomography (CT) scan is three times more sensitive in detecting early-stage lung cancer than a chest X-ray in “high-risk” populations (2.4% versus 0.7%).32, 33
Yet there remains considerable controversy over the use of lung CT scans as a global screening approach for lung cancer, because of the expense (estimated cost 2 billion dollars yearly in the US alone) and the very low yield of cancers detected yearly (1.2%).34
One of the primary problems is that although smoking is the number one risk factor for developing lung cancer, only 10% of smokers ever develop lung cancer. With 44.5 million current smokers in the United States (20.9% of the population) and over 1.3 billion smokers worldwide, there is a clear need to identify markers whose genetic variation is associated with lung cancer risk that would help better prioritize who should be screened clinically and be offered chemopreventive agents.
While numerous studies of carcinogen metabolizing and detoxifying genes and DNA repair genes have identified sequence variations associated with risk for lung cancer, a meta-analysis of such polymorphisms in DNA repair pathways concluded that for any increased risk association between a single SNP and lung cancer, the risk fluctuation would likely be minimal, and only panels evaluating a collection of SNPs would ever successfully predict lung cancer risk.35
Supporting this hypothesis, two SNPs associated with lung cancer risk (OR = 1.19–1.8) were recently identified from three large studies examining over 300,000 SNPs. The mechanism of lung cancer predisposition caused by these SNPs is controversial, but is hypothesized to be through affects on a nicotine receptor.36 37
In contrast, in this study, we have identified the first miRNA binding-site SNP that alone can predict a significant increase in NSCLC risk in people with a moderate smoking history. The mechanism may be due to altered regulation of the KRAS oncogene and possibly cellular let-7 levels as well. These findings give insight into a new paradigm, and support the pursuit of 3′UTR sequencing for similar SNPs in all tumor related genes to better understand their role in genetic cancer risk. This strategy can be a complementary, and likely productive, approach to enhance current efforts to define genetic cancer risk.