Hereditary persistence of alpha-fetoprotein and H19 expression in liver of BALB/cJ mice is due to a retrovirus insertion in the Zhx2 gene.

The alpha-fetoprotein (AFP) and H19 genes are transcribed at high levels in the mammalian fetal liver but are rapidly repressed postnatally. This repression in the liver is controlled, at least in part, by the Afr1 gene. Afr1 was defined >25 years ago when BALB/cJ mice were found to have 5- to 20-fold higher adult serum AFP levels compared with all other mouse strains; subsequent studies showed that this elevation was due to higher Afp expression in the liver. H19, which has become a model for genomic imprinting, was identified initially in a screen for Afr1-regulated genes. The BALB/cJ allele (Afr1(b)) is recessive to the wild-type allele (Afr1(a)), consistent with the idea that Afr1 functions as a repressor. By high-resolution mapping, we identified a gene that maps to the Afr1 interval on chromosome 15 and encodes a putative zinc fingers and homeoboxes (ZHX) protein. In BALB/cJ mice, this gene contains a murine endogenous retrovirus within its first intron and produces predominantly an aberrant transcript that no longer encodes a functional protein. Liver-specific overexpression of a Zhx2 transgene restores wild-type H19 repression on a BALB/cJ background, confirming that this gene is responsible for hereditary persistence of Afp and H19 in the livers of BALB/cJ mice. Thus we have identified a genetically defined transcription factor that is involved in developmental gene silencing in mammals. We present a model to explain the liver-specific phenotype in BALB/cJ mice, even though Afr1 is a ubiquitously expressed gene.