Characterization and mutation analysis of human LEFTY A and LEFTY B, homologues of murine genes implicated in left-right axis development

Members of the transforming growth factor (TGF)-beta family of cell-signali ng molecules have been implicated recently in mammalian left-right (LR) axi s development, the process by which vertebrates lateralize unpaired organs (e.g., heart, stomach, and spleen). Two family members, Lefty1 and Lefty2, are expressed exclusively on the left side of the mouse embryo by 8.0 days post coitum. This asymmetry is lost or reversed in two murine models of abn ormal LR-axis specification, inversus viscerum (iv) and inversion of embryo nic turning (inv). Furthermore, mice homozygous for a Lefty1 null allele ma nifest LR malformations and misexpress Lefty2. We hypothesized that Lefty m utations may be associated with human LR-axis malformations, We now report characterization of two Lefty homologues, LEFTY A and LEFTY B, separated by similar to 50 kb on chromosome lq42. Each comprises four exons spliced at identical positions. LEFTY A is identical to ebaf, a cDNA previously identi fied in a search for genes expressed in human endometrium. The deduced amin o acid sequences of LEFTY A and LEFTY B are more similar to each other than to Lefty1 or Lefty2. Analysis of 126 human cases of LR-axis malformations showed one nonsense and one missense mutation in LEFTY A. Both mutations li e in the cysteine-knot region of the protein LEFTY A, and the phenotype of affected individuals is very similar to that typically seen in Lefty1(-/-) mice with LR-axis malformations.