Individual somatic H1 subtypes are dispensable for mouse development even in mice lacking the H1(0) replacement subtype

DNA in the nuclei of all eukaryotic cells is packaged into repeating units of nucleosomes that form the basic unit of chromatin. Each nucleosome consists of an octamer core containing two molecules of each of the core histones, H2a, H2b, H3, and H4. H1 linker histones bind to the nucleosome core particle and the linker DNA between nucleosomes to facilitate further compaction of chromatin into a 30-nm fiber. Recent studies have shown that the chromatin complex, especially the nucleosome and its modifications, can have a profound influence on transcription (reviewed in references 9 and 26). Although histones are highly conserved proteins, multicellular organisms contain a variety of subtypes exhibiting significant sequence divergence. Among the histone classes, the H1 linker histones are the most divergent group. In mammals, there are at least eight H1 subtypes, including the somatic H1s, H1a to H1e, germ cell-specific H1t and H1oo, and replacement linker histone H10 (11, 24). These subtypes exhibit distinct patterns of expression during differentiation and development (12, 24). The significance of the diversity present within the H1 family is not understood. The genes for H1a through H1e and H1t are tightly linked on mouse chromosome 13 (25). The H10 gene is located on mouse chromosome 15 (2). H10 is the smallest and most divergent member of the H1 family (27). H10 accumulates in quiescent cells and during terminal differentiation and terminal cell division, reaching levels as high as 30% of the total H1 in certain tissues, such as adult liver. Despite the unique properties and developmental regulation of H10, previous studies in our laboratory showed that mice develop normally without H10 (23). Analysis of chromatin from H10-null mice indicated that the level of the somatic H1s, especially H1c, H1d, and H1e, was increased so as to maintain a normal ratio of H1 to nucleosomes in H10-deficient chromatin. In certain tissues, such as adult liver, H1c, H1d, and H1e accounted for 95% of the remaining H1, suggesting that these subtypes are responsible for compensating for loss of H10. The present study was undertaken with the following two experimental objectives: first, to determine whether or not any one of several H1 subtypes is essential for mouse development; second, to determine whether H1c, H1d, or H1e is responsible for compensating for the loss of H10 in H10−/− mice. To achieve the first goal, we generated null mutations in each of the three somatic H1 genes by homologous recombination in embryonic stem (ES) cells and then produced mice lacking each of these individual subtypes. To achieve the second goal, we bred each of these three H1 knockout mice to H10 null mice and ultimately produced H1c/H10, H1d/H10, and H1e/H10 double-knockout mice.