Amnionless, essential for mouse gastrulation, is mutated in recessive hereditary megaloblastic anemia

Abstract

The amnionless gene, Amn, on mouse chromosome 12 encodes a type I transmembrane protein that is expressed in the extraembryonic visceral layer during gastrulation¹. Mice homozygous with respect to the amn mutation generated by a transgene insertion have no amnion^2,3. The embryos are severely compromised, surviving to the tenth day of gestation but seem to lack the mesodermal layers that normally produce the trunk⁴. The Amn protein has one transmembrane domain separating a larger, N-terminal extracellular region and a smaller, C-terminal cytoplasmic region. The extracellular region harbors a cysteine-rich domain resembling those occurring in Chordin, found in Xenopus laevis embryos, and Sog, found in Drosophila melanogaster. As these cysteine-rich domains bind bone morphogenetic proteins (Bmps), it has been speculated that the cysteine-rich domain in Amn also binds Bmps⁴. We show that homozygous mutations affecting exons 1–4 of human AMN lead to selective malabsorption of vitamin B₁₂ (a phenotype associated with megaloblastic anemia 1, MGA1; OMIM 261100; refs. 5,6) in otherwise normal individuals, suggesting that the 5′ end of AMN is dispensable for embryonic development but necessary for absorption of vitamin B₁₂. When the 5′ end of AMN is truncated by mutations, translation is initiated from alternative downstream start codons.