Our statistical and biological analyses have strongly implicated RGS17 as a candidate for the lung cancer susceptibility locus at 6q23-25. Although we are not able to analyze RGS17 expression levels in our familial lung cancer cases because of limited biospecimen availability, our data did indicate high RGS17 transcript upregulation in sporadic lung tumors and cell lines, and strong effects on cell proliferation through knock-down and over-expression in a lung cancer cell line. We hypothesize that there exists a rare variant or variants, which lie on the same haplotype detected and defined by the significantly associated SNPs. This rare, highly penetrant genetic lesion is postulated to affect RGS17 expression and lung cancer susceptibility.
Our future efforts to identify causal variants are currently focused on a re-sequencing analysis of the LD block 12 (). This 43 kb region contains the core promoter, non-coding exon 1 and part of intron 1. Several CpG islands are clustered around exon 1. We intend to address the methylation status of four CpG islands located around exon 1, in sporadic lung tumor expression. Because expression data cannot be obtained from familial samples due to biospecimen availability, it will be necessary to determine meaningful changes in the RGS17 gene in familial germline DNA using these types of experiments until appropriate familial specimens become available for the analysis of expression. The elucidation of specific mechanisms by which RGS17 confers accelerated growth and other tumor phenotypes using cell and molecular biology studies must be pursued, and will be seminal in influencing the diagnostic, preventive and therapeutic applications of this research. These studies constitute ongoing and long-term goals generated by the work presented here, and will elucidate the mechanism by which RGS17 affects familial lung cancer susceptibility.
Recent reports have linked RGS domain containing genes to cancer. One study describes SNPs in PDZ-RhoGEF, containing an RGS domain, which modulates the risk of lung cancer in Mexican Americans (26
). Another such study describes the identification of a functional polymorphism in the 3′UTR of RGS6 that is associated with bladder cancer risk, and was shown to affect protein translation (27
). There is also evidence that RGS17 reduces dopamine-D2/Gαi
-mediated inhibition of cAMP formation and abolishes thyrotropin-releasing hormone receptor/Gαq
-mediated calcium mobilization (24
). D2 dopamine receptor agonists exhibits anti-proliferative effects in lung tumors and lung cancer cell lines associated with decreased cAMP accumulation (28
). It is possible that RGS17 is involved in addiction/reward signaling pathways, as the D2 dopamine gene (DRD2) appears to be associated with smoking addiction. A recent paper also found that RGS17 may work on opioid receptor function (30
). Previous work has shown that lung cancers of both non-small cell and small cell histologic types can express high affinity opioid receptors including mu receptors, opioid peptides and also nAChR receptors (31
). Lung cancer growth is inhibited and apoptosis induced by opioids including mu agonists while nicotine acting through nAChRs antagonizes this effect providing a growth regulatory loop that is antagonized by nicotine (31
). Recently, PET imaging studies have provided in vivo evidence for the presence of delta and mu opioid receptor types in small cell, squamous and adenocarcinomas of the lung (33
). Because of the negative action of RGS17 on opioid receptors it is possible that over expression of RGS17 could act through inhibiting a growth regulatory pathway provided by opioid receptors. In a recent study, we demonstrate that RGS17 induces CREB phosphorylation and CREB responsive gene expression (James et al. unpublished data). RGS17 also enhances forskolin mediated cAMP production, forskolin induced gene expression and forskolin induced proliferation. Furthermore, PKA inhibition causes growth arrest of lung tumor cells, which is partially restored by RGS17 overexpresion. Thus, RGS17 appears to be a potential oncoprotein promoting proliferation through cAMP-PKA-CREB signaling. The identification of RGS17 as the major candidate gene for familial lung cancer susceptibility on chromosome 6q has important implications for the diagnosis and treatment of lung cancer as well as delineation of the mechanisms underlying both familial and sporadic lung cancer.