A genetic approach to H1 histone function

Abstract

The mouse H1 histone gene family encodes a group of at least seven basic protein variants that bind to DNA and facilitate the formation of higher order chromatin structures. Interestingly, among the histones the H1 class displays the most complex pattern of subtypes including differentiation-specific and tissue-specific members. The large number and different patterns of expression among the H1 subtypes suggests that these H1 variants are in part responsible for the wide variations in chromatin condensation that exist within the genome and in different cell types. However, proof for a functional difference for the H1 variants is not available.;The H1{dollar}\sp\circ{dollar} subtype accumulates in several tissues during the first few weeks of postnatal mouse development and is found at highest concentrations in terminally differentiated tissues with a low rate of cell proliferation. In addition, we have shown that the H1{dollar}\sp\circ{dollar} gene is very rapidly induced at the transcriptional level during the precommitment period of mouse erythroleukemia (MEL) cell differentiation. To investigate the role of H1{dollar}\sp\circ{dollar} in mouse development, I have disrupted the single-copy H1{dollar}\sp\circ{dollar} gene by homologous recombination in mouse embryonic stem (ES) cells and transmitted this mutation to the mouse germ line. Surprisingly, mice lacking H1{dollar}\sp\circ{dollar} protein exhibited no apparent anatomical or phenotypic abnormalities and were born within the expected range predicted by Mendelian genetics. Furthermore, both male and female homozygotes were fertile and could be bred to produce additional homozygous progeny. Northern and Western analysis confirmed the absence of H1{dollar}\sp\circ{dollar} RNA and protein respectively in the mutant animals. Chromatin from H1{dollar}\sp\circ{dollar}-deficient animals showed no significant change in the relative proportions of other H1 subtypes or in the stoichiometry between linker histones and nucleosomes, suggesting that other H1 histones can compensate for the deficiency in H1{dollar}\sp\circ{dollar} by occupying sites normally containing H1{dollar}\sp\circ.{dollar} These results argue that H1{dollar}\sp\circ{dollar} function is dispensable for normal embryogenesis and demonstrate that other members of the H1 histone family can compensate for the loss of H1{dollar}\sp\circ.{dollar} Mice homozygous for disruptions at the H1c and H1e loci have also been generated and are presented in the thesis as well.