The major significance of this study is that generation of three new pancreatic mouse models of VHL has allowed us to address the cellular origins and biological roles of VHL during pancreatic development and tumorigenesis. In terms of VHL's role during pancreatic development, we demonstrated that deletion of Vhl
in endocrine α-cells and β-cells which terminally differentiated around E12.5 
, does not appear to affect normal functions of endocrine pancreas during embryogenesis, postnatal development, and in adulthood (Glu-Cre;Vhl f/f and Rip-Cre;Vhl f/f animals). In contrast, deletion of Vhl
in pancreatic progenitor cells that give arise to ductal, exocrine and endocrine pancreas resulted in significant postnatal death in mice (Pdx1-Cre;Vhl f/f animals), even though pancreatic organogenesis was not affected during embryonic development. Together, this data suggest that VHL
is functionally important in postnatal ductal and exocrine pancreas, and that pVHL is not essential for mature endocrine pancreatic cells. While this finding uncovers the novel aspect of VHL
in postnatal ductal and exocrine pancreas, it remains to be determined whether HIF-dependent or HIF-independent pathways are involved.
In terms of VHL's role during pancreatic tumorigenesis, little is known about the cellular origins and molecular mechanisms related to VHL-associated pancreatic diseases, even though a large portion of VHL patients are affected by pancreatic manifestations, such as cysts, MCA and neuroendocrine tumors. By specifically inactivating Vhl
in distinct pancreatic cell populations, we reported the first mouse model of VHL that recapitulates some of the clinical features found in the pancreas of VHL patients, and demonstrated that inactivation of Vhl
in the endocrine pancreas is insufficient to initiate tumorigenesis. VHL-associated pancreatic lesions were only found in mice where Vhl
was inactivated in pancreatic progenitor cells (Pdx1-Cre;Vhl f/f mice), but not in mice in which Vhl
was inactivated in mature endocrine cells (Glu-Cre;Vhl f/f and Rip-Cre;Vhl f/f mice). Thus, our data supports the notion of a progenitor cell origin for these VHL-associated pancreatic lesions. In addition, it has been previously suggested by histopathological studies that MCA originated from pancreatic exocrine cells, such as the centroacinar cells 
or ductal cells 
. Moreover, none of the VHL patients (n>108) with PNET disease evaluated at our center had a functional neoplasm by hormone levels or symptoms 
. Together, these data suggest that VHL-associated MCA and PNET originate from pancreatic ductal or exocrine progenitor cells, not differentiated endocrine cells. Analogous to our hypothesis, VHL-associated hemangioblastomas have been demonstrated to derive from embryologic multipotent cells 
. However, further studies will be required to definitively determine if these VHL-associated pancreatic manifestations can be recapitulated when Vhl
is specifically deleted in progenitor cells of ductal, exocrine or endocrine lineage.
Similar to pervious reports on Vhl
knockout mice 
, we also observed variability in phenotypic penetrance of the Vhl
deletion due to differences in mouse genetic backgrounds. The postnatal lethality in Pdx1-Cre;Vhl f/f mice was most severe in A/J and Balb/C background strains, but not as severe in the C57BL/6 background strain. This finding suggests that strain-specific genetic modifiers may provide protection for the survival of these mice with pVHL deficiency in the pancreas. Notably, we showed that pancreatic phenotypes observed in Pdx1-Cre;Vhl f/f mice correlate with a modest upregulation of Hif1α, but not HIf2α. It is thus possible that these strain-specific genetic modifiers may function to interfere with VHL
downstream signaling, and compensate for the loss of pVHL. This does not exclude the possibility that the pancreatic phenotypes observed in Pdx1-Cre;Vhl f/f mice resulted from mechanisms completely independent of HIF signaling pathways, such as VHL's role in regulating extracellular matrix assembly 
. Identification of these modifier genes and further comparative functional analysis utilizing mice deficient in Hif1α and Hif2α will provide insight into the precise molecular mechanisms leading to the development of VHL-associated pancreatic disease.
An unexpected observation during our studies was the small size seen in mice deficient in pVHL in the islet β-cells. While our analyses excluded the idea that alterations in growth hormone and basal glucose levels led to this phenotype in Rip-Cre;Vhl f/f mice, it is described in the literature that pVHL regulates glucose metabolism in liver and kidney cells 
. Establishment of our Rip-Cre;Vhl f/f homozygous mice now allows us to begin investigating whether or not pVHL also plays an important role in glucose sensing in pancreatic islet β-cells, such as via glucose transporter 1 (Glut1). However, phenotypes seen in Rip-Cre;Vhl f/f mice are confounded by the fact that Rip-Cre is expressed at a low level in the hypothalamus 
. Thus, it would be critical to confirm and determine that deletion of Vhl
indeed occurred in particular nuclei within the hypothalamus in Rip-Cre;Vhl f/f homozygous mice. Only when that information is available, will we be able to conclusively interpret the physiological data resulting from the loss of pVHL in pancreatic islet β-cells or in the hypothalamus.
In summary, to decipher the functional significance of pVHL in the pancreas during development and tumorigenesis, we conditionally inactivated Vhl in distinct pancreatic cell populations and reported the first mouse model of VHL that recapitulates clinical features found in the pancreas of VHL patients. Importantly, our data demonstrated that pVHL is functionally important for postnatal ductal and exocrine pancreas, and suggested that pancreatic progenitor cells, not mature endocrine cells, as the cell of origin for VHL-associated pancreatic lesions. The novel mouse model systems reported in this study will provide the foundation for further functional and genetic analysis to advance our understanding of VHL-associated pancreatic manifestations.