HP1γ function is required for male germ cell survival and spermatogenesis

While targeting the Cbx3 gene (encoding the HP1γ protein) with a conditional targeting vector, we generated a hypomorphic allele (Cbx3hypo), which resulted in much reduced (barely detectable) levels of HP1γ protein. Homozygotes for the hypomorphic allele (Cbx3hypo/hypo) are rare, with only 1% of Cbx3hypo/hypo animals reaching adulthood. Adult males exhibit a severe hypogonadism that is associated with a loss of germ cells, with some seminiferous tubules retaining only the supporting Sertoli cells (Sertoli cell-only phenotype). The percentage of seminiferous tubules that are positive for L1 ORF1 protein (ORF1p) in Cbx3hypo/hypo testes is greater than that for wild-type testes, indicating that L1 retrotransposon silencing is reversed, leading to ectopic expression of ORF1p in Cbx3hypo/hypo germ cells.The Cbx3 gene product (the HP1γ protein) has a non-redundant function during spermatogenesis that cannot be compensated for by the other two HP1 isotypes. The Cbx3hypo/hypo spermatogenesis defect is similar to that found in Miwi2 and Dnmt3L mutants. The Cbx3 gene-targeted mice generated in this study provide an appropriate model for the study of HP1γ in transposon silencing and parental imprinting.The presence of methylated lysine 9 of histone H3 (H3K9ME) and structural heterochromatin protein (HP) 1 proteins are characteristic evolutionarily conserved hallmarks of heterochromatin [1]. In mammals, there are three HP1 isotypes, which have a high degree of homology, termed HP1α (encoded by the Cbx5 gene), HP1β (encoded by the Cbx1 gene) and HP1γ (encoded by the Cbx3 gene) [2,3]. Despite the significant degree of sequence conservation shared between the mammalian HP1 isotypes, several studies have indicated that they are likely to have non-redundant functions. First, their nuclear localization patterns are different: HP1α and HP1β are usually found enriched at sites of constitutive heterochromatin, whereas HP1γ has a more uniform distribution [4-6]. Second, biochemical assay