Nature Genetics
- 33, 426 - 429 (2003)
Published online: 18 February 2003; | doi:10.1038/ng1098
Amnionless, essential for mouse gastrulation, is mutated in recessive hereditary megaloblastic anemiaStephan M. Tanner1, Maria Aminoff2, Fred A. Wright1, Sandya Liyanarachchi1, Mervi Kuronen2, Anne Saarinen2, Orit Massika3, Hanna Mandel3, Harald Broch4 & Albert de la Chapelle11
Human Cancer Genetics Program, Comprehensive Cancer Center, The Ohio State University, Columbus, Ohio 43210, USA. 2
Folkhälsan Institute of Genetics, Department of Medical Genetics, University of Helsinki, Helsinki, Finland. 3
Metabolic Unit, Department of Pediatrics, Rambam Medical Center, Haifa, Israel. 4
Department of Pediatrics, Vestfold Central Hospital, Toensberg, Norway.
Correspondence should be addressed to Albert de la Chapelle delachapelle-1@medctr.osu.edu 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 amnion2,
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.
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