Insights into retinoic acid deficiency and the induction of craniofacial malformations and microcephaly in fetal alcohol spectrum disorder.

Fetal Alcohol Spectrum Disorder (FASD) is a set of neurodevelopmental malformations caused by maternal consumption of alcohol during pregnancy. FASD sentinel facial features are unique to the disorder, and microcephaly is common in severe forms of FASD. Retinoic acid deficiency has been shown to cause craniofacial malformations and microcephaly in animal models reminiscent of those caused by prenatal alcohol exposure. Alcohol exposure affects the migration and survival of cranial neural crest cells, which are required for proper frontonasal prominence and pharyngeal arch development. Defects in craniofacial development are further amplified by the many downstream pathways that are transcriptionally controlled retinoic acid target genes, including Shh signaling. Recent evidence shows that alcohol exposure itself is sufficient to induce retinoic acid deficiency in the embryo. These data suggest that retinoic acid deficiency is an important underlying etiology of FASD. In disorders like Vitamin A Deficiency, FASD, DiGeorge (22q11.2 Deletion Syndrome), CHARGE, Smith-Magenis, Matthew-Wood, and Congenital Zika Syndromes, evidence is accumulating to link reduced retinoic acid signaling with developmental defects like craniofacial malformations and microcephaly. Research focus on characterizing the effects of retinoic acid deficiency during early development and on understanding the downstream signaling pathways involved in aberrant head, and craniofacial development will reveal underlying etiologies of these disorders.

Acetaldehyde inhibits retinoic acid biosynthesis to mediate alcohol teratogenicity.

Alcohol consumption during pregnancy induces Fetal Alcohol Spectrum Disorder (FASD), which has been proposed to arise from competitive inhibition of retinoic acid (RA) biosynthesis. We provide biochemical and developmental evidence identifying acetaldehyde as responsible for this inhibition. In the embryo, RA production by RALDH2 (ALDH1A2), the main retinaldehyde dehydrogenase expressed at that stage, is inhibited by ethanol exposure. Pharmacological inhibition of the embryonic alcohol dehydrogenase activity, prevents the oxidation of ethanol to acetaldehyde that in turn functions as a RALDH2 inhibitor. Acetaldehyde-mediated reduction of RA can be rescued by RALDH2 or retinaldehyde supplementation. Enzymatic kinetic analysis of human RALDH2 shows a preference for acetaldehyde as a substrate over retinaldehyde. RA production by hRALDH2 is efficiently inhibited by acetaldehyde but not by ethanol itself. We conclude that acetaldehyde is the teratogenic derivative of ethanol responsible for the reduction in RA signaling and induction of the developmental malformations characteristic of FASD. This competitive mechanism will affect tissues requiring RA signaling when exposed to ethanol throughout life.