Time-dependent pleiotropic phenotypes are caused by progressive DNA polymerase α deficiency in zebrafish

The mechanisms underlying variation in cell- and tissue-specific phenotypes caused by mutations are poorly understood. For example, why some cancers show cellular atypia more than others is unclear. As an illustration of phylogenetic variation in phenotype, mutations in a subunit of DNA polymerase α (Pol α), one of the primary eukaryotic replicative DNA polymerases, result in immediate cell cycle arrest in yeast and Arabidopsis; a null mutation in the same subunit results in lethality in 5 to 7 days in zebrafish, in association with striking tissue-specific, time-dependent pleiotropic cellular phenotypes. Here, we describe a zebrafish null mutant in the gene encoding the B subunit of Pol α, huli hutu (hht), in which cell death with nuclear fragmentation is found in the central nervous system where the largest number of cells arise, while cells present in smaller numbers, such as the gastrointestinal epithelial cells, instead show cellular atypia that is associated with hepatic dysgenesis and pancreatic and air bladder agenesis. Wild-type maternal mRNA in genotypically mutant eggs becomes undetectable by 24 hours post-fertilization (hpf). Furthermore, the treatment of wild-type larvae with DNA synthesis inhibitors aphidicolin or hydroxyurea from 24 hpf phenocopies hht. Taken together, the data are consistent with a model in which reduced capacity for cell proliferation caused by either genetic or chemical mechanisms of DNA synthesis beginning at about 24 hpf results in cell death in tissues with large cell number, and abnormal nuclear morphology in tissues with fewer cells, including the gastrointestinal tract.