Analysis of ALK-1 and endoglin in newborns from families with hereditary hemorrhagic telangiectasia type 2

ALK-1 (activin receptor-like kinase-1), a type I receptor of the transformi ng growth factor (TGF)-beta superfamily, is the gene mutated in hereditary hemorrhagic telangiectasia type 2 (HHT2) while endoglin is mutated in HHT1. Using a novel polyclonal antibody to ALK-1, we measured ALK-1 expression o n human umbilical vein endothelial cells (HUVEC) of newborns from HHT famil ies whose affected members had normal endoglin levels. ALK-1 levels were sp ecifically reduced in three HUVEC with ALK-1 missense mutant codons, and no rmal in two newborns not carrying the missense mutations present in the cli nically affected relatives. Levels were also normal in a HUVEC with deletio n of S232 in the ATP binding site of ALK-1, Thus HHT2 appears to be associa ted with a loss of function of the mutant allele due to a reduction in eith er protein level or activity. We also report three new ALK-1 missense mutat ions leading to G48E/A49P, C344Y and E407D substitutions. In COS-1 transfec ted cells, ALK-1 was found in the TGF-beta 1 and -beta 3 receptor complexes in association with endoglin and T beta RII, but not in activin receptor c omplexes containing endoglin. In HUVEC, ALK-1 was not detectable in the TGF -beta 1 or -beta 3 receptor complexes. However, in the absence of ligand, A LK-1 and endoglin interactions were observed by immunoprecipitation/western blot in HUVEC from normal as well as HHT1 and HHT2 patients. Our data sugg est a transient association between these two proteins of the TGF-beta supe rfamily, both required at a critical level to ensure vessel wall integrity.