Loss of the region of chromosome 3p spanning the VHL gene is the most frequent genomic change in clear cell renal carcinoma, which is the most common form of kidney cancer. The next two most common genomic abnormalities in kidney cancer are chromosome 5q amplification and chromosome 14q loss. Moreover, we show here that kidney cancer has the highest rate of chromosome 3p loss, chromosome 5q amplification, and 14q loss amongst a wide variety of tumor types.
loss leads to increased abundance of HIF1α and HIF2α and deregulation of HIF-dependent transcription is a signature abnormality in kidney cancer. Mounting evidence suggest that HIF2α, rather than HIF1α, promotes pVHL-defective renal carcinogenesis. Indeed, many pVHL-defective renal carcinomas produce low, or undetectable, levels of HIF1α (6
) and restoring HIF1α expression in a VHL−/−
renal carcinoma line was shown before to suppress tumorigenesis (10
). We confirmed this later finding and showed that HIF1α, at levels comparable to those seen in when pVHL function is impaired, suppresses renal carcinoma proliferation and tumor growth.
Prompted by this knowledge we asked whether HIF1α, which resides on chromosome 14q23.2, might be one of the genes targeted by 14q deletions in kidney cancer. Indeed, we found that the vast majority of 14q deletions detected in renal carcinoma encompass HIF1α. Moreover, we documented that HIF1α, but not neighboring genes on chromosome 14q, is often subject to focal deletions in kidney cancer cell lines. Some of these deletions led to the production of aberrant mRNAs and proteins that compromised HIF1α’s ability to suppress VHL−/−
renal carcinoma proliferation and tumorigenesis. We also discovered that downregulation of wild-type HIF1α promotes renal carcinoma growth in vivo
. Finally, we showed that somatic, presumably pathogenic, HIF1α
mutations in human clear cell carcinomas enfeeble HIF1α as a tumor suppressor in cell proliferation assays. Collectively, these genetic and functional data credential HIF1α as a clear cell renal carcinoma suppressor gene. Hence loss of pVHL simultaneously leads to activation of an oncoprotein (HIF2α) and a tumor suppressor protein (HIF1α). This would explain the frequent loss of chromosome 14q in kidney cancer and is consistent with the observation that loss of HIF1α protein in preneoplastic lesions in the kidneys of VHL patients heralds further malignant transformation (9
In a recent study the percentage of clear cell renal carcinomas with low HIF1α expression approximated the frequency of 14q loss for this tumor type (8
) and 14q loss was enriched amongst the HIF1α negative tumors (Kate Nathanson-personal communication). Moreover, we confirmed that HIF1α transcriptional activity is indeed decreased in VHL−/−
kidney tumors that have sustained 14q deletions encompassing HIF1α
compared to those that have not. Although homozygous HIF1α
deletions appear to be common in clear cell renal carcinoma cell lines we have not, however, documented a similarly high frequency in primary renal tumors. Although this discrepancy might be due to technical factors it raises the possibility that HIF1α haploinsufficiency is sufficient to promote primary tumor growth in vivo
, and that reduction to nullizgyosity is selected for during tumor progression in vivo
or the propagation of clear cell carcinoma lines in vitro
HIF1α is usually thought to promote tumor growth but there is precedence for it functioning as a tumor suppressor. For example, loss of HIF1α enhances tumor formation by embryonic stem cell-derived teratoma cells and by murine astrocytes (37
). In the context of renal carcinoma, HIF1α might act as a tumor suppressor specifically by antagonizing HIF2α. For example, transactivation by one of the two HIFα transactivation domains (the C-terminal transactivation domain or CTAD) is inhibited by the asparaginyl hydroxylase FIH1. HIF2α is less sensitive than HIF1α to FIH1-mediated inhibition (40
). Competitive displacement of HIF2α by HIF1α from HIF-responsive promoters that depend upon the CTAD for full activation would therefore potentially decrease promoter activity. Moreover, HIF1α can suppress HIF2α levels via as yet unclear mechanisms in some contexts (10
). In short, loss of HIF1α might, paradoxically, increase the activity of certain HIF-responsive promoters in pVHL-defective tumor cells. Experimental evidence exists to support this contention (10
) and (C.S. and W.G.K.-data not shown).
In addition to quantitative differences on shared HIF-responsive promoters, there are a number of qualitative differences between HIF1α and HIF2α that might relate to HIF1α scoring as a tumor suppressor protein. For example, some genes that are regulated by HIF1α are not regulated by HIF2α and vice versa. Conceivably, some of the genes that are preferentially activated by HIF1α decrease renal carcinoma cell fitness. In this regard, 3 of the genes found in our HIF1α signature, TXNIP, KCTD11
, have been implicated as tumor suppressors in other contexts (42
). Moreover, HIF1α and HIF2α differ in terms of their ability to engage collateral signaling pathways such as those involving c-Myc and Notch. For example, HIF1α, via a variety of mechanisms, can inhibit c-Myc activity in certain settings whereas HIF2α does not (8
The 14q deletions in kidney cancer are typically very large and usually span HIF1α, as noted above. Nonetheless, rare tumors with small deletions had pinpointed 14q31-ter as the likely location for a kidney cancer tumors suppressor gene (23
). The simplest reconciliation of these findings would be the existence of multiple kidney cancer tumor suppressor genes on 14q, in addition to HIF1α
, with perhaps some acting through haploinsufficiency. Alternatively, these small deletions might have been passenger, rather than driver, mutations.
It will be of interest to determine whether pVHL-defective clear cell renal carcinomas that retain wild-type HIF1α expression utilize alternative mechanisms to circumvent HIF1α’s tumor suppressor activity. We note, for example, that PLAGL1
maps to a region of 6q that is frequently deleted in VHL-associated neoplasms and sporadic clear cell renal carcinomas (27
). Moreover, it will be important to determine whether retention of HIF1α expression alters the response of pVHL-defective tumors to targeted agents that directly or indirectly target HIF. In this regard, it is possible that the salutary effects of rapamycin-like mTOR inhibitors (rapalogs) in kidney cancer are partially mitigated by their ability to downregulate HIF1α (48
), especially in light of a recent report predicting that HIF2α would be relatively resistant to such agents (49