Genetic evidence that oxidative derivatives of retinoic acid are not involved in retinoid signaling during mouse development

Abstract

).Retinoic acid, the active derivative of vitamin A (retinol), is a hormonal signaling molecule that acts in developing and adult tissues 1 . The Cyp26a1 (cytochrome p450, 26) protein metabolizes retinoic acid into more polar hydroxylated and oxidized derivatives 2,3 . Whether some of these derivatives are biologically active metabolites has been debated 4,5 . Cyp26a1 -/-mouse fetuses have lethal morphogenetic phenotypes mimicking those generated by excess retinoic acid administration, indicating that human CYP26A1 may be essential in controlling retinoic acid levels during development 6,7 . This hypothesis suggests that the Cyp26a1 -/-phenotype could be rescued under conditions in which embryonic retinoic acid levels are decreased. We show that Cyp26a1 -/-mice are phenotypically rescued by heterozygous disruption of Aldh1a2 (also known as Raldh2), which encodes a retinaldehyde dehydrogenase responsible for the synthesis of retinoic acid during early embryonic development 8,9 . Aldh1a2 haploinsufficiency prevents the appearance of spina bifida and rescues the development of posterior structures (sacral/caudal vertebrae, hindgut, urogenital tract), while partly preventing cervical vertebral transformations and hindbrain pattern alterations in Cyp26a1 -/-mice. Thus, some of these double-mutant mice can reach adulthood. This study is the first report of a mutation acting as a dominant suppressor of a lethal morphogenetic mutation in mammals. We provide genetic evidence that ALDH1A2 and CYP26A1 activities concurrently establish local embryonic retinoic acid levels that must be finely tuned to allow posterior organ development and to prevent spina bifida.

Cyp26 enzymes generate the retinoic acid response pattern necessary for hindbrain development

“…3E,F). This demonstrates that, as for other phenotypes observed in cyp26a1-mutant fish and mice (Emoto et al, 2005;Niederreither et al, 2002), the posteriorized hindbrain phenotype caused by blocking all Cyp26 activity is due to the accumulation of excess RA and not to the absence of bioactive Cyp26-generated RA derivatives. Although such derivatives have been observed to have significant retinoidal effects in cells and in embryos, and have been postulated to have functions in vivo (Idres et al, 2002;Pijnappel et al, 1993), we see no evidence for their having a role in hindbrain patterning.…”

“…The posterior limit of cyp26a1 expression lies in the anterior hindbrain (Dobbs-McAuliffe et al, 2004;Kudoh et al, 2002;Sirbu et al, 2005), whereas both cyp26b1 and cyp26c1 mark, sequentially, the r4-r5 and r6-r7 boundaries. However, it is important to notice that cyp26a1 is also expressed in the anterior trunk mesoderm near the RA source, where it probably functions to reduce global RA levels (Emoto et al, 2005;Niederreither et al, 2002). We propose that the severe posteriorization of Cyp26-depleted embryos results from the combined effects of depleting segment-restricted Cyp26 activity within the hindbrain and increasing global RA levels due to the loss of Cyp26a1 activity in the anterior trunk mesoderm (Fig.…”

A novel role for the retinoic acid-catabolizing enzyme CYP26A1 in Barrett's associated adenocarcinoma

“…In a mouse model with mutated RALDH2 and CYP26, the hydroxylated derivates of CYP26 were shown to be biologically inactive (Niederreither et al, 2002). We have recently shown that manipulation of RA concentrations has profound effects on differentiation status ex vivo (Chang et al, 2007).…”

“…A member of the cytochrome P450 family, CYP26A1, has recently been identified as a major RA-catabolizing enzyme, which converts RA to biologically inactive hydroxylated retinoid derivatives (Niederreither et al, 2002). CYP26A1 has been shown to be upregulated in human familial adenomatous polyposis adenomas, sporadic colon cancers and primary ovarian cancer (Downie et al, 2005;Shelton et al, 2006).…”

The discovery of new coding alleles of human CYP26A1 that are potentially defective in the metabolism of all-trans retinoic acid and their assessment in a recombinant cDNA expression system

“…CYP26A1 has been reported to be the principal enzyme responsible for the catabolism of at-RA to the inactive polar metabolites, 4-hydroxy-RA (4-OH-RA) , 4-oxo-RA, and 18-OH-RA [6][7][8]. Although some studies have suggested that 4-OH-RA and 4-oxo-RA have biological activity [9][10][11], other recent studies have suggested that these metabolites are principally inactive breakdown products of at-RA [1,5]. Cyp26a1 null mice die during mid-late gestation [7,12] .…”