Exogenous expression of a histone H3.3 isoform causes extranuclear divisions in the C. elegans intestine

Histone proteins condense DNA into chromatin and play significant roles in gene regulation. Mutations that alter histone function can disrupt critical gene expression programs and are implicated in driving cancer and other human diseases. Most mechanistic research on histones has been conducted in cell culture, mitotic tissues, or animal disease models rather than in normal tissue. Caenorhabditis elegans is a model organism with a well-established developmental program and conserved histone mechanisms shared with other metazoans. Prior work generated several single-copy histone reporters expressing exogenous histone protein in the intestine. Surprisingly, the histone H3.3 reporter worms possessed extra nuclei in the anterior intestine. This phenotype was incompletely penetrant and arose early during larval development. Mutations predicted to perturb histone H3.3 reporter expression or deposition into chromatin ameliorated the phenotype. DNA Fluorescence In situ Hybridization (FISH) was used to measure chromosome copy number and revealed approximately half the DNA content in the extra nuclei versus undivided controls, suggesting aberrant nuclear division during normal rounds of intestinal endoreduplication. Thus, these reporters serendipitously identified a distinct function of histone H3.3 in triggering aberrant nuclear division during animal development. Other histone isoforms may have previously unrecognized biological functions in terminally differentiated tissue alongside their roles in mitotic tissue.