It is possible that
VHL mutation alone may not sufficient to develop VHL-associated neoplasms. The mutations can be altering the less-understood HIF-independent pathways (
Champion et al., 2008) or other oncogenes or tumor suppressor genes (
Chan et al., 2002). It has been shown that the loss of VHL gene does not promote tumor growth in primary cells (
Mack et al., 2003) while it has been shown in renal cell carcinoma (RCC) cell lines, the loss of
VHL gene promotes growth and the re-introduction of wild type
VHL gene suppresses tumor growth (
Gnarra et al., 1996;
Iliopoulos et al., 1996). Additional mutations in the RCC could be responsible for this growth advantage in the setting of VHL deficiency. Studies on cytogenetic patterns of chromosomal loss and gain in human CCRCC tumor tissue samples have been done. The cytogenetic profiles were from various stages of tumor progression show that −3p, +5q, −14q, +7 and −8p are the most frequent alterations. The 3p loss is often associated with 5q gain due to unbalanced translocations and this is usually followed by continual deletions on 3p secondary to genome instability (
Pei et al., 2010;
Zhang et al., 2010;
Bhat Singh and Amare Kadam, 2012).
pVHL’s contribution to tumorigenesis may be due to a parallel dysregulation of its HIF-independent functions. A well-known clinical example of this hypothesis is that patients with Class IIC VHL develop pheochromocytoma yet they have a functioning VHL-HIF axis (
Hoffman et al., 2001). Pheochromocytoma-associated VHL disease results from an accumulation of JUNB, which is an inhibitor of the pro-apoptotic molecule JUN. JUN is implicated in modulating excessive growth of sympathetic neurons and may be protective from the formation of pheochromocytoma (
Lee et al., 2005). Another example involves renal and genital tract cyst formation in VHL disease. Studies have shown that tubular epithelial cells that encase these cysts have lost pVHL function. Loss of pVHL can lead to microtubule instability and subsequent defective ciliary function and cyst formation (
Thoma et al., 2007a,
b). Research in CCRCC shows that deficient pVHL prevents β-catenin for degredation leading to dysregulation of Wnt signaling and subsequent contribution to tumorigenesis (
Linehan et al., 2009). Finally, pVHL has recently been shown to have a novel tumor suppressor function. It marks Skp2 for degradation in an E3 ubiquitin ligase independent manner resulting in increased p27/kip1, which results in S-phase arrest and prevents cell proliferation in the context of DNA damage. In pVHL deficient RCC tissue, Skp2 levels are pathologically elevated with low p27/kip1 (
Roe et al., 2011).
It is also important to consider the origin from which VHL associated tumors arise. Since the VHL-associated tumor cells are actually the stromal component of the mass (
Chan et al., 1999;
Vortmeyer et al., 2003), they may arise from the arrested hemangioblast progenitor cells (
Wilkinson et al., 1990;
Vortmeyer et al., 1997;
Huber et al., 2004;
Chan et al., 2005). It has been shown, in CNS and retinal hemangioblastomas, that they express cell markers of mesoderm-derived hemangioblasts and hematopoietic stem cells. These cells have been shown to differentiate into either endothelial or hematopoietic cells when cultured under the appropriate conditions (
Park et al., 2007). Interestingly, it appears that HIF2α maintains cell pluripotency on the OCT4 transcription factor (
Chan et al., 2005;
Covello et al., 2006). Other factors may also be required. Whatever the cause is, tumor development from HIF2α-expressing arrested hemangioblasts seems associated with acquired expression of HIF1α, brachyury and other hemangioblast developmental markers (
Shively et al., 2011).
Creating an animal model using knock-in missense mutations in targeted areas of the VHL gene rather than using whole deletions may confer an ocular VHL phenotype. Common VHL point mutations like R167Q in embryonic stem (ES) cells, and more generally, mutations in the alpha domain, causes impairment of elongin C binding to VHL which has been shown to impair HIF2α regulation and intriguingly has a growth advantage over VHL deleted ES cells (
Bonicalzi et al., 2001;
Lee et al., 2009). In
VHL deficient tumors, blocking HIF2α suppresses tumor progression and reintroduction of degradation resistant HIF2α causes tumor progression (
Kondo et al., 2003;
Zimmer et al., 2004). It is also possible that there are unidentified neighboring genes in chromosome 3p that may be tumorigeneic. Deletions may provide a measure of protection from tumor formation whereas point mutations would not affect these genes. Brk1 maps near the
VHL gene and it functions as a regulator of the actin cytoskeleton. Loss of this gene is protective against tumors and causes defects in migration in RCC and other tumors (
Escobar et al., 2010). To reiterate from before, the study reported by Wong and colleagues has showed a distinct genotype-phenotype correlation in VHL patients with retinal hemangioblastomas. Those with complete deletions have better visual acuity and decreased incidence of retinal hemangioblastomas. Patients with missense mutations had higher incidence of ocular disease. The most frequent mutation was found at codon 167 (
Wong et al., 2007;
Mettu et al., 2010). Retinal hemangioblastomas begin to occur in much younger VHL patients and so it is within the realm of possibility that its pathogenesis may very well not be as complicated as CCRCC’s. A missense mutation may be adequate to produce an ocular VHL phenotype but it is always possible that a background of additional engineered mutations and/or microenvironment factors are also needed.
VHL is a complicated systemic disease that has important functions for the entire body mainly through HIF-dependent and HIF-independent pathways. Parsing out each pathway that is tumorigenic for the eye remains a challenge. However, it is a necessary task in order to understand the disease and to treat patients with VHL-associated retinal hemangioblastoma.