HP1γ function is required for male germ cell survival and spermatogenesis

Background: HP1 proteins are conserved components of eukaryotic constitutive heterochromatin. In mammals, there are three genes that encode HP1-like proteins, termed HP1 alpha, HP1 beta and HP1 gamma, which have a high degree of homology This paper describes for the first time, to our knowledge, the physiological function of HP1 gamma using a gene-targeted mouse. Results: While targeting the Cbx3 gene (encoding the HP1 gamma protein) with a conditional targeting vector, we generated a hypomorphic allele (Cbx3(hypo)), which resulted in much reduced (barely detectable) levels of HP1 gamma protein. Homozygotes for the hypomorphic allele (Cbx3(hypo/hypo)) are rare, with only 1% of Cbx3(hypo/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 Cbx3(hypo/hypo) testes is greater than that for wildtype testes, indicating that L1 retrotransposon silencing is reversed, leading to ectopic expression of ORF1p in Cbx3(hypo/hypo) germ cells. Conclusions: The Cbx3 gene product (the HP1 gamma protein) has a non-redundant function during spermatogenesis that cannot be compensated for by the other two HP1 isotypes. The Cbx3(hypo/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 HP1 gamma in transposon silencing and parental imprinting.