SMAD4 and gastrointestinal polyp: ENG mutations cause severe pulmonary AVMs and a higher incidence of gastrointestinal bleeding, ACVRL1 mutations correlate to milder phenotype with fewer pulmonary complications but significant epistaxis and gastrointestinal issues, while SMAD4 mutations are associated with additional complications, including gastrointestinal polyps and an increased risk of colorectal cancer.