Biology subjects

Vittoria, M. A.

Publications and source records attributed to Vittoria, M. A..

5 recordsLinked to original sources

bioRxiv2021

Inactivation of the Hippo Tumor Suppressor Pathway Promotes Melanoma

Human melanomas are commonly driven by activating mutations in BRAF, which promote melanocyte proliferation through constitutive stimulation of the MAPK pathway. However, oncogenic BRAF alone is insufficient to promote melanoma; instead, its expression merely induces a transient burst of proliferation that ultimately ceases with the development of benign nevi (i.e. moles) comprised of growth-arrested melanocytes. The tumor suppressive mechanisms that induce this melanocytic growth arrest remain poorly understood. Recent modeling studies have suggested that the growth arrest of nevus melanocytes is not solely due to oncogene activation in individual cells, but rather due to cells sensing and responding to their collective overgrowth, similar to what occurs in normal tissues. This cell growth arrest is reminiscent of the arrest induced by activation of the Hippo tumor suppressor pathway, which is an evolutionarily conserved pathway known to regulate organ size. Herein, we demonstrate that oncogenic BRAF signaling activates the Hippo pathway in vitro, which leads to inhibition of the pro-growth transcriptional co-activators YAP and TAZ, ultimately promoting the growth arrest of melanocytes. We also provide evidence that the Hippo tumor suppressor pathway is activated in growth-arrested nevus melanocytes in vivo, both from single-cell sequencing of mouse models of nevogenesis and human tissue samples. Mechanistically, we observe that oncogenic BRAF promotes both ERK-dependent alterations in the actin cytoskeleton and whole-genome-doubling events, and that these two effects independently promote Hippo pathway activation. Lastly, we demonstrate that abrogation of the Hippo pathway, via melanocyte-specific deletion of the Hippo kinases Lats1/2, enables oncogenic BRAF-expressing melanocytes to bypass nevus formation, thus leading to the rapid onset of melanoma with 100% penetrance. This model is clinically relevant, as co-heterozygous loss of LATS1/2 is observed in [~]15% of human melanomas. Collectively, our data reveal that the Hippo pathway enforces the stable growth arrest of nevus melanocytes and therefore represents a critical and previously unappreciated barrier to melanoma development.

cancer biology↗

bioRxiv2021

Oncogenic BRAF Induces Whole-Genome Doubling Through Suppression of Cytokinesis

Melanomas and other solid tumors commonly have increased ploidy, with near-tetraploid karyotypes being most frequently observed. Such karyotypes have been shown to arise through whole-genome doubling events that occur during early stages of tumor progression. The generation of tetraploid cells via whole-genome doubling is proposed to allow nascent tumor cells the ability to sample various pro-tumorigenic genomic configurations while avoiding the negative consequences that chromosomal gains or losses have in diploid cells. Whereas a high prevalence of whole-genome doubling events has been established, the means by which whole-genome doubling arises is unclear. Here, we find that BRAFV600E, the most common mutation in melanomas, can induce whole-genome doubling via cytokinesis failure in vitro and in a zebrafish melanoma model. Mechanistically, BRAFV600E causes decreased activation and localization of RhoA, a critical cytokinesis regulator. BRAFV600E activity during G1/S phases of the cell cycle is required to suppress cytokinesis. During G1/S, BRAFV600E activity causes inappropriate centriole amplification, which is linked in part to inhibition of RhoA and suppression of cytokinesis. Together these data suggest that common abnormalities of melanomas linked to tumorigenesis - amplified centrosomes and whole-genome doubling events - can be induced by oncogenic BRAF and other mutations that increase RAS/MAPK pathway activity. Statement of SignificanceWhole-genome doubling is prevalent in many types of solid tumors and important in shaping tumor genomes, yet the causes of whole-genome doubling are not well understood. Here, we discover that oncogenic BRAFV600E can induce whole-genome doubling through suppression of cytokinesis, and BRAFV600E-induced whole-genome doubling can occur in melanocytes and be present in nascent melanoma cells upon tumorigenesis.

cancer biology↗

bioRxiv2020

Whole genome doubling confers unique genetic vulnerabilities on tumor cells

Whole genome doubling (WGD) occurs early in tumorigenesis and generates genetically unstable tetraploid cells that fuel tumor development. Cells that undergo WGD (WGD+) must adapt to accommodate their abnormal tetraploid state; however, the nature of these adaptations, and whether they confer vulnerabilities that can subsequently be exploited therapeutically, is unclear. Using sequencing data from [~]10,000 primary human cancer samples and essentiality data from [~]600 cancer cell lines, we show that WGD gives rise to common genetic traits that are accompanied by unique vulnerabilities. We reveal that WGD+ cells are more dependent on spindle assembly checkpoint signaling, DNA replication factors, and proteasome function than WGD- cells. We also identify KIF18A, which encodes for a mitotic kinesin, as being specifically required for the viability of WGD+ cells. While loss of KIF18A is largely dispensable for accurate chromosome segregation during mitosis in WGD- cells, its loss induces dramatic mitotic errors in WGD+ cells, ultimately impairing cell viability. Collectively, our results reveal new strategies to specifically target WGD+ cancer cells while sparing the normal, non-transformed WGD- cells that comprise human tissue.

cancer biology↗

bioRxiv2018

Identification of STK25 as a direct activator of LATS signaling

The Hippo pathway maintains tissue homeostasis by negatively regulating the oncogenic transcriptional co-activators YAP and TAZ. Though functional inactivation of the Hippo pathway is common in tumors, mutations in core pathway components are rare. Thus, understanding how tumor cells inactivate Hippo signaling remains a key unresolved question. Here, we identify the kinase STK25 as a novel activator of Hippo signaling. We demonstrate that loss of STK25 promotes YAP/TAZ activation and enhanced cellular proliferation, even under normally growth-suppressive conditions. We reveal that STK25 activates LATS via a previously unobserved mechanism, in which STK25 directly phosphorylates the LATS activation loop. This represents a new paradigm in Hippo activation and distinguishes STK25 from all other identified kinase activators of LATS. STK25 is significantly focally deleted across a wide spectrum of human cancers, suggesting STK25 loss may represent a common mechanism by which tumor cells functionally impair the Hippo tumor suppressor pathway.

cell biology↗

bioRxiv2018

A genome-wide miRNA screen identifies regulators of tetraploid cell proliferation

Tetraploid cells, which are most commonly generated by errors in cell division, are genomically unstable and have been shown to promote tumorigenesis. Recent genomic studies have estimated that [~]40% of all solid tumors have undergone a genome-doubling event during their evolution, suggesting a significant role for tetraploidy in driving the development of human cancers. To safeguard against the deleterious effects of tetraploidy, non-transformed cells that fail mitosis and become tetraploid activate both the Hippo and p53 tumor suppressor pathways to restrain further proliferation. Tetraploid cells must therefore overcome these anti-proliferative barriers to ultimately drive tumor development. However, the genetic routes through which spontaneously arising tetraploid cells adapt to regain proliferative capacity remain poorly characterized. Here, we conducted a comprehensive, gain-of-function genome-wide screen to identify miRNAs that are sufficient to promote the proliferation of tetraploid cells. Our screen identified 23 miRNAs whose overexpression significantly promotes tetraploid proliferation. The vast majority of these miRNAs facilitate tetraploid growth by enhancing mitogenic signaling pathways (e.g. miR-191-3p); however, we also identified several miRNAs that impair the p53/p21 pathway (e.g. miR-523-3p), and a single miRNA (miR-24-3p) that potently inactivates the Hippo pathway via downregulation of the tumor suppressor gene NF2. Collectively, our data reveal several avenues through which tetraploid cells may regain the proliferative capacity necessary to drive tumorigenesis.

cancer biology↗