Deletion of the VHL gene, which is located on chromosome 3p, is known to be involved in the development of clear cell renal cell carcinoma (ccRCC). The second most common cytogenetic abnormality associated with ccRCC is thought to be an amplification of chromosome 5q (Gunawan et al., 2001; Yoshimoto et al., 2007; Toma et al., 2008; Chen et al., 2009; Zhang et al., 2010), however, very little is known about its exact effects. A recent study by Wondergem et al. (2012) has now suggested a role for the proto-oncogene pituitary tumour transforming gene 1 (PTTG1) in ccRCC, which is found on chromosome 5q.
The authors used a SNP microarray to confirm that a deletion of chromosome 3p was the most frequent chromosomal aberration observed in 91% of the 43 ccRCC samples tested. An amplification of chromosome 5q was the next most common abnormality, noted in 60% of the cases analysed. There was a large variability in the size of the amplification, with some samples showing a gain of the entire chromosome, and others a smaller terminal fraction of chromosome 5q. Interestingly, FNIP1 is situated on chromosome 5q. However, PTTG1 was located in the minimal region of amplification in every case of 5q gain studied.
Fluorescent in situ hybridisation confirmed the amplification of PTTG1 in 9 of the 11 ccRCC samples examined. When using gene expression microarrays, a significant increase in the levels of PTTG1 mRNA could be seen in the ccRCC specimens (when compared to the normal kidney controls). Moreover, this increased level of PTTG1 mRNA was significantly associated with a higher grade of tumour and decreased cancer-specific survival. Subsequently, immunohistochemical analysis showed that high levels of PTTG1 protein were also detected in ccRCC sections when compared to normal kidney tissue (with the highest levels observed in the high grade tumour samples).
When compared to 2 non-tumour cell lines, the levels of PTTG1 protein were also greater in 7 ccRCC cell lines. To assess the functional significance of PTTG1, shRNA knockdown in SN12C and 786-O cells led to a significant decrease in matrigel invasion and colony formation in soft agar (as opposed to controls). Additionally, xenograft experiments in nude mice demonstrated that shRNA knockdown of PTTG1 in these two cell lines led to a ~50% decrease in mean tumour volume.
Gene expression microarray analysis was then conducted on 786-O and Caki-1 cells treated with PTTG1 siRNA. Several genes were affected by this downregulation, in particular epithelial cell-transforming sequence 2 (ECT2), which is a guanine nucleotide exchange factor for the Rho family of small GTPases. A decrease in expression was also observed at both the mRNA and protein level using qRT-PCR and western blot respectively. ECT2 expression was associated with a higher tumour grade and poor cancer-specific survival. There was also a significant correlation between PTTG1 and ECT2 expression levels in ccRCC tumours, and in HK-2 cells, shRNA knockdown of ECT2 rescued the effects of PTTG1 overexpression in both cell proliferation and colony formation.
Even though the exact association between PTTG1 and ECT2 is undetermined, they both could be used as targets in the development of novel anti-cancer drugs. Furthermore, in addition to its roles in B-cell development (see here and here) and autophagy (see here), could amplifications of FNIP1 in some cases of 5q gain also be involved in renal tumourigenesis? Finally, with regards to BHD syndrome, maybe a dysregulation of FNIP1 (through the loss of FLCN) plays a part in the kidney phenotype observed?
- Chen M, Ye Y, Yang H, Tamboli P, Matin S, Tannir NM, Wood CG, Gu J, & Wu X (2009). Genome-wide profiling of chromosomal alterations in renal cell carcinoma using high-density single nucleotide polymorphism arrays. International journal of cancer. Journal international du cancer, 125 (10), 2342-8 PMID: 19521957
- Gunawan B, Huber W, Holtrup M, von Heydebreck A, Efferth T, Poustka A, Ringert RH, Jakse G, & Füzesi L (2001). Prognostic impacts of cytogenetic findings in clear cell renal cell carcinoma: gain of 5q31-qter predicts a distinct clinical phenotype with favorable prognosis. Cancer research, 61 (21), 7731-8 PMID: 11691785
- Toma MI, Grosser M, Herr A, Aust DE, Meye A, Hoefling C, Fuessel S, Wuttig D, Wirth MP, & Baretton GB (2008). Loss of heterozygosity and copy number abnormality in clear cell renal cell carcinoma discovered by high-density affymetrix 10K single nucleotide polymorphism mapping array. Neoplasia (New York, N.Y.), 10 (7), 634-42 PMID: 18592004
- Wondergem B, Zhang Z, Huang D, Ong CK, Koeman J, Van’t Hof D, Petillo D, Ooi A, Anema J, Lane B, Kahnoski RJ, Furge KA, & Teh BT (2012). Expression of the PTTG1 oncogene is associated with aggressive clear cell renal cell carcinoma. Cancer research PMID: 22805307
- Yoshimoto T, Matsuura K, Karnan S, Tagawa H, Nakada C, Tanigawa M, Tsukamoto Y, Uchida T, Kashima K, Akizuki S, Takeuchi I, Sato F, Mimata H, Seto M, & Moriyama M (2007). High-resolution analysis of DNA copy number alterations and gene expression in renal clear cell carcinoma. The Journal of pathology, 213 (4), 392-401 PMID: 17922474
- Zhang Z, Wondergem B, & Dykema K (2010). A Comprehensive Study of Progressive Cytogenetic Alterations in Clear Cell Renal Cell Carcinoma and a New Model for ccRCC Tumorigenesis and Progression. Advances in bioinformatics PMID: 20671976