High mobility group 2 protein (Hmgb2) is a member of the HMGB protein family, which includes the ubiquitous Hmgb1 and the embryo-specific Hmgb3. The three proteins are more than 80% identical at the amino acid level and their biochemical properties are indistinguishable. Hmgb1 is an abundant component of all mammalian nuclei and acts as an architectural factor that bends DNA and promotes protein assembly on specific DNA targets. Cells that lack Hmgb1 can survive, although mutant mice die shortly after birth. As Hmgb2 is present in all cultured cells and is abundant in thymus, the preferred source for HMGB proteins, it was considered a ubiquitous variant of Hmgb1. We show that in adult mice Hmgb2 is restricted mainly to lymphoid organs and testes, although it is widely expressed during embryogenesis. Mice that lack Hmgb2 are viable. However, male Hmgb2(-/-) mice have reduced fertility, that correlates with Sertoli and germ cell degeneration in seminiferous tubules and immotile spermatozoa. Significantly, Hmgb2 is expressed at very high levels in primary spermatocytes, while it is barely detectable in spermatogonia and elongated spermatids. This peculiar pattern of expression and the phenotype of mutants indicate that Hmgb2 has a specialised role in germ cell differentiation.
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