Conditional ablation of the RFX4 isoform 1 transcription factor: Allele dosage effects on brain phenotype.

Regulatory factor X4 (RFX4) isoform 1 is a recently discovered isoform of the winged helix transcription factor RFX4, which can bind to X-box consensus sequences that are enriched in the promoters of cilia-related genes. Early insertional mutagenesis studies in mice first identified this isoform, and demonstrated that it was crucial for mouse brain development. RFX4 isoform 1 is the only RFX4 isoform significantly expressed in the mouse fetal and adult brain. In this study, we evaluated conditional knock-out (KO) mice in which one or two floxed alleles of Rfx4 were deleted early in development through the use of a Sox2-Cre transgene. Heterozygous deletion of Rfx4 resulted in severe, non-communicating congenital hydrocephalus associated with hypoplasia of the subcommissural organ. Homozygous deletion of Rfx4 resulted in formation of a single ventricle in the forebrain, and severe dorsoventral patterning defects in the telencephalon and midbrain at embryonic day 12.5, a collection of phenotypes that resembled human holoprosencephaly. No anatomical abnormalities were noted outside the brain in either case. At the molecular level, transcripts encoded by the cilia-related gene Foxj1 were significantly decreased, and Foxj1 was identified as a direct gene target of RFX4 isoform 1. The phenotypes were similar to those observed in the previous Rfx4 insertional mutagenesis studies. Thus, we provide a novel conditional KO animal model in which to investigate the downstream genes directly and/or indirectly regulated by RFX4 isoform 1. This model could provide new insights into the pathogenesis of obstructive hydrocephalus and holoprosencephaly in humans, both relatively common and disabling birth defects.