Increased Cdx protein dose effects upon axial patterning in transgenic lines of mice

Abstract

To investigate the link between Cdx protein concentration and axial patterning in embryos, we made lines of mice OE1, OE2 and OE4 that overexpress each of the Cdx genes Cdx1, Cdx2 and Cdx4, respectively. The lines carry Cdx transgenes under the transcriptional control of their own promoter/enhancer elements. Transgenic embryos show Cdx transcription at 8.5 to 8.7 days within normal spatial domains for Cdx expression (primitive streak/tailbud), yet, overall, they contain elevated levels of Cdx proteins. Increased doses of Cdx proteins result in homeotic shifts in vertebral types along most of the vertebral column, with transformations being most obvious within the cervical region. Most of the shifts are anterior-to-posterior transformations and the anterior limits of these are commonly skull/vertebra 1 (v1) for OE1, v1/v2 for OE2 and v7 for OE4. OE embryos display anterior shifts in the expression of a Hoxa7/lacZ reporter within neural, paraxial and lateral plate mesoderm tissues. Hoxa7/lacZ expression commences at the normal time in OE1 and OE4 embryos. OE2 embryos display a forward shift in the gradient of Cdx2 protein along the axis, suggesting that a Cdx morphogen gradient model could account, at least in part, for the homeotic shifts in vertebral types. OE mice display additional defects: forelimb deficiencies in OE1, multiple tail axes, vertebral mis-alignments and axial truncations in OE2.

Epigenetic regulations in hematopoietic Hox code

“…Hox expression boundaries respond to Cdx proteins in a dose-dependent manner, and Cdx gradients might serve as instructional (morphogen) gradients for setting Hox expression patterns. In this model, temporal colinearity and Cdx gradients operate together in the establishment of Hox boundaries (Gaunt et al, 2004(Gaunt et al, , 2008. Cdx protein concentration may regulate the rate and extent of sequential Hox gene activation (temporal colinearity).…”

Origins of Cdx1 regulatory elements suggest roles in vertebrate evolution

“…Cdx1 knockout (Subramanian et al, 1995;van den Akker et al, 2002) or over-expresser (Gaunt et al, 2008) mice display homeotic mutations within the axial skeleton, particularly in cervical and anterior thoracic regions. The axial defects in Cdx1 mutants extend forward to the skull/atlas level.…”

“…The axial defects in Cdx1 mutants extend forward to the skull/atlas level. The other reported anatomical defects in Cdx1 mutants are in forelimb bud size (Gaunt et al, 2008) and, when combined with Cdx2 mutation, in spinal ganglia (van den Akker et al, 2002).…”

“…RA response reporter studies show that RA is present in the embryonic regions known to express Cdx1: the primitive streak of 7.5 day mouse embryos and, by 8.5 days, the developing somites (Rossant et al, 1991). RA administered to pregnant mothers induces homeotic mutations in the axial skeleton (Kessel and Gruss, 1991), and the range of these resembles those induced by Cdx1 protein over-expression (Gaunt et al, 2008).…”

“…own promoter elements) (Gaunt et al, 2008) only the urogenital systems of OE1 and OE2 mice show obvious defects. About 16% of both OE1 (10/62) and OE2 (4/25) heterozygous mice show defects in their reproductive tracts.…”