Von Hippel-Lindau disease (VHL): a need for a murine model with retinal hemangioblastoma

Clinical Pathological Features and Current Animal Models of Type 3 Macular Neovascularization

Type 3 macular neovascularization (MNV3), or retinal angiomatous proliferation (RAP), is a distinct type of neovascular age-related macular degeneration (AMD), which is a leading cause of vision loss in older persons. During the past decade, systematic investigation into the clinical, multimodal imaging, and histopathological features and therapeutic outcomes has provided important new insight into this disease. These studies favor the retinal origin of MNV3 and suggest the involvement of retinal hypoxia, inflammation, von Hippel–Lindau (VHL)–hypoxia-inducible factor (HIF)–vascular endothelial growth factor (VEGF) pathway, and multiple cell types in the development and progression of MNV3. Several mouse models, including the recently built Rb/p107/Vhl triple knockout mouse model by our group, have induced many of the histological features of MNV3 and provided much insight into the underlying pathological mechanisms. These models have revealed the roles of retinal hypoxia, inflammation, lipid metabolism, VHL/HIF pathway, and retinoblastoma tumor suppressor (Rb)–E2F cell cycle pathway in the development of MNV3. This article will summarize the clinical, multimodal imaging, and pathological features of MNV3 and the diversity of animal models that exist for MNV3, as well as their strengths and limitations.

The Endothelial Landscape and Its Role in Von Hippel-Lindau Disease

Von Hippel–Lindau disease (VHL) is a rare hereditary disease characterized by the predisposal to develop different types of highly vascularized tumors. VHL patients carry a VHL mutation that causes partial lack of functional VHL protein (pVHL) in all cells, and a total lack thereof in cells harboring a second hit mutation. Absence of pVHL generates a prolonged state of pseudo-hypoxia in the cell due to accumulation of hypoxia inducible factor, an important transcription factor regulating pro-tumorigenic genes. The work here presented focuses on characterizing the endothelium of VHL patients, by means of blood outgrowth endothelial cells (BOECs). Transcriptome analysis of VHL-derived BOECs, further supported by in vitro assays, shows that these cells are at a disadvantage, as evidenced by loss of cell adhesion capacity, angiogenesis defects, and immune response and oxidative metabolic gene downregulation, which induce oxidative stress. These results suggest that the endothelium of VHL patients is functionally compromised and more susceptible to tumor development. These findings contribute to shedding light on the vascular landscape of VHL patients preceding the second hit mutation in the VHL gene. This knowledge could be useful in searching for new therapies for these patients and other vascular diseases.

Rb1/Rbl1/Vhl loss induces mouse subretinal angiomatous proliferation and hemangioblastoma

Von Hippel-Lindau (Vhl) protein inhibits hypoxia-inducible factor (Hif), yet its deletion in murine retina does not cause the extensive angiogenesis expected with Hif induction. The mechanism is unclear. Here we show that retinoblastoma tumor suppressor (Rb1) constrains expression of Hif target genes in the Vhl-/- retina. Deleting Rb1 induced extensive retinal neovascularization and autophagic ablation of photoreceptors in the Vhl-/- retina. RNA-sequencing, ChIP, and reporter assays showed Rb1 recruitment to and repression of certain Hif target genes. Activating Rb1 by deleting cyclin D1 induced a partial defect in the retinal superficial vascular plexus. Unexpectedly, removing Vhl suppressed retinoblastoma formation in murine Rb1/Rbl1-deficient retina but generated subretinal vascular growths resembling retinal angiomatous proliferation (RAP) and retinal capillary hemangioblastoma (RCH). Most stromal cells in the RAP/RCH-like lesions were Sox9+, suggesting a Müller glia origin, and expressed Lgals3, a marker of human brain hemangioblastoma. Thus, the Rb family limit Hif target gene expression in the Vhl-/- retina, and removing this inhibitory signal generates new models for RAP and RCH.

Deletion of the von Hippel-Lindau Gene in Hemangioblasts Causes Hemangioblastoma-like Lesions in Murine Retina

von Hippel-Lindau (VHL) disease is an autosomal-dominant tumor predisposition syndrome characterized by the development of highly vascularized tumors and cysts. LOH of the VHL gene results in aberrant upregulation of hypoxia-inducible factors (HIF) and has been associated with tumor formation. Hemangioblastomas of the central nervous system and retina represent the most prevalent VHL-associated tumors, but no VHL animal model has reproduced retinal capillary hemangioblastomas (RCH), the hallmark lesion of ocular VHL. Here we report our work in developing a murine model of VHL-associated RCH by conditionally inactivating Vhl in a hemangioblast population using a Scl-Cre-ERT2 transgenic mouse line. In transgenic mice carrying the conditional allele and the Scl-Cre-ERT2 allele, 64% exhibited various retinal vascular anomalies following tamoxifen induction. Affected Vhl-mutant mice demonstrated retinal vascular lesions associated with prominent vasculature, anomalous capillary networks, hemorrhage, exudates, and localized fibrosis. Histologic analyses showed RCH-like lesions characterized by tortuous, dilated vasculature surrounded by "tumorlet" cell cluster and isolated foamy stromal cells, which are typically associated with RCH. Fluorescein angiography suggested increased vascular permeability of the irregular retinal vasculature and hemangioblastoma-like lesions. Vhl deletion was detected in "tumorlet" cells via microdissection. Our findings provide a phenotypic recapitulation of VHL-associated RCH in a murine model that may be useful to study RCH pathogenesis and therapeutics aimed at treating ocular VHL.Significance: This study describes a model that phenotypically recapitulates a form of retinal pathogenesis that is driven by genetic loss of the VHL tumor suppressor, providing a useful tool for its study and therapeutic intervention. Cancer Res; 78(5); 1266-74. ©2018 AACR.

Neuroprotective effects respond to cerebral ischemia without susceptibility to HB-tumorigenesis in VHL heterozygous knockout mice

The von Hippel-Lindau (VHL) tumor suppressor gene plays a prominent role in the development of hemangioblastomas (HBs) within specific regions of the human' central nervous system (CNS). Alterations in VHL gene are rarely observed in the more common features of human VHL-related tumors in animal models, and VHL heterozygous knockout (VHL+/-) mice do not develop HBs. We tested whether VHL heterozygous knockout mice exhibited genetic predisposition to the development of HBs and conferred a selective advantage involving growth of blood vessels to its carrier. No differences were observed between wild-type and VHL+/- mice in development ad reproduction. The heterozygous VHL+/- mice did not develop higher genetic susceptibility to CNS-HBs over their lifetime. Furthermore, this recessive VHL gene heterozygosity is relatively stable. Interestingly, we found these heterozygous VHL+/- mice gained an advantage conferring to angiogenic ability in a particular environment, compared with wild-type mice. The heterozygous VHL+/- mice obviously enhanced hypoxia inducible factor-1 (HIF)-dependent and Twist1 angiogenic mechanism in response to acute cerebral ischemia, resulting in decreased cerebral tissue damage and neuroprotective response through neovascularization. Our findings provide evidence of partial loss function of VHL as a novel precise therapeutic target in acute cerebral ischemia.