Linking VHL and SETD2 in a common oncogenic pathway that converges on the mitotic spindle
Introduction
Loss of chromosome 3p is a landmark event in clear cell renal cell carcinoma (ccRCC) that results in mono-allelic loss of VHL (von Hippel Lindau) and SETD2 (Set-domain containing 2), and other tumor suppressors co-located on 3p. Second hits in VHL inactivate this key tumor suppressor initiating tumor progression. SETD2, a histone methyltransferase, was previously shown to have a dual function in methylating both histones and microtubules, thereby contributing to both the histone and tubulin codes. Methylation by SETD2 on microtubules occurs at the mitotic spindle and is essential for normal mitosis and cytokinesis, with loss of SETD2 acting as a strong driver of apoptosis. This raises a conundrum of how cancer cells survive early mono-allelic loss of SETD2, escaping cell death.
Methods
Using biochemcial assays including in vitro kinase assays and mass spectrometry, we have assessed the ability of AURKA, a mitotic kinase that is frequently elevated in VHL-null tumors, to phosphorylate and modulate SETD2 activity. In addition, immunofluorescence assays have highlighted the precise spatial localization of these proteins in the cell.
Results
We have identified the mitotic kinase, Aurora kinase A (AURKA), as a regulator of SETD2. Our data uncover SETD2 as a unique substrate for phosphorylation by AURKA, with mass spectrometry identifying serine 2080 (S2080) as the site of phosphorylation on SETD2. Phospho-SETD2 (S2080) localized at the spindle poles during mitosis, and this specific localization was modulated by AURKA enzymatic activity. Initial observations reveal a role for AURKA driven SETD2 phosphorylation at S2080 in nucleocytoplasmic shuttling of this methyltransferase. Our data suggest that phosphorylation of SETD2 by AURKA contributes to both its methyltransferase (i.e. enzymatic) activity and its geo-spatial location with the cell.
Conclusion
Importantly, AURKA expression levels are high in VHL-null cells resulting from an inability of VHL to target AURKA for degradation. Our data now highlight a direct link between VHL and SETD2, two tumor suppressors believed to independently drive RCC pathogenesis. In summary, our data reveal a tumor-specific vulnerability linked to mitotic fragility that can be precisely targeted to ultimately drive mitotic catastrophe.