Genotypic and Phenotypic Variables Affect Meiotic Cell Cycle Progression, Tumor Ploidy, and Cancer-Associated Mortality in a brca2-Mutant Zebrafish Model

Abstract

Successful cell replication requires both cell cycle completion and accurate chromosomal segregation. The tumor suppressor BRCA2 is positioned to influence both of these outcomes, and thereby influence genomic integrity, during meiotic and mitotic cell cycles. Accordingly, mutations in BRCA2 induce chromosomal abnormalities and disrupt cell cycle progression in both germ cells and somatic cells. Despite these findings, aneuploidy is not more prevalent in BRCA2-associated versus non-BRCA2-associated human cancers. More puzzlingly, diploidy in BRCA2-associated cancers is a negative prognostic factor, unlike non-BRCA2-associated cancers and many other human cancers. We used a brca2-mutant/tp53-mutant cancer-prone zebrafish model to explore the impact of BRCA2 mutation on cell cycle progression, ploidy, and cancer-associated mortality by performing DNA content/cell cycle analysis on zebrafish germ cells, somatic cells, and cancer cells. First, we determined that combined brca2/tp53 mutations uniquely disrupt meiotic progression. Second, we determined that sex significantly influences ploidy outcome in zebrafish cancers. Third, we determined that brca2 mutation and female sex each significantly reduce survival time in cancer-bearing zebrafish. Finally, we provide evidence to support a link between BRCA2 mutation, tumor diploidy, and poor survival outcome. These outcomes underscore the utility of this model for studying BRCA2-associated genomic aberrations in normal and cancer cells.

Zebrafish Models of Cancer—New Insights on Modeling Human Cancer in a Non-Mammalian Vertebrate

“…Surprisingly, brca2 mutation in females significantly reduced the survival rate after they have developed tumors compared to males with the same genotype. This study further supports the link between brca2 mutation and cancer aneuploidy with poor survival prognosis [47].…”

“…Recently, brca2 mutant zebrafish in a tp53 -/- background were examined for cell cycle arrest and genomic stability. This model, as it is not embryonically lethal compared to many BRCA2 mouse models, allows for in vivo studies in adult animals [47]. In brca2 mutant zebrafish, it was previously shown that there is an increased incidence of benign testicular tumors [48].…”

brca2-mutant zebrafish exhibit context- and tissue-dependent alterations in cell phenotypes and response to injury

“…To determine how injury and regenerative responses affect ocular tumorigenesis in zebrafish, we analyzed tumor development in brca2 m/m;tp53 m/m and tp53 m/m zebrafish that received unilateral ONI at three months of age. The ONI group was compared to brca2 m/m;tp53 m/m and tp53 m/m zebrafish from a previously reported cohort, designated as the control group 27 (see Methods and Table S4 for details). Tumor development in ONI and control groups is summarized in Table 1 .…”

“…Despite this, brca2 homozygous zebrafish derived from heterozygous parents are fully viable and survive to adulthood. Since cancer susceptibility in zebrafish with brca2 mutation alone is low 25 , we use zebrafish with combined mutations in brca2 and tp53 26 for carcinogenesis studies 25,[27][28][29] . The brca2-mutant zebrafish model provides a unique in vivo system for determining how loss of functional BRCA2 affects various developmental, adult, and cancer-associated phenotypes 25,[27][28][29] .…”

“…The response to injury in zebrafish is distinguished by robust regenerative capacity in multiple adult tissues, including heart, tail fin, retina and optic nerve, and others 30,31 . We focused on the optic nerve pathway (ONP) to evaluate the relationship between BRCA2 mutation, injury response, and cancer risk because we recently reported that brca2mutant/tp53-mutant zebrafish are at high risk for cancers in this site 27 . Interestingly, the ONP in adult fish also exhibits several characteristics that promote nerve regeneration and remyelination after injury [32][33][34][35] .…”