To date, mutations in two genes have been linked to HHT, accounting for most but not all clinical cases. Mutations of the ENG gene, localized to the long arm of chromosome 9 (9q33-q34.1), cause HHT1; mutations of the gene encoding activin receptor-like kinase 1, ALK1 (also known as ACVRL1), localized on the long arm of chromosome 12 (12q11-q14), cause HHT2. It is not clear whether genotype-phenotype correlations can be established in HHT, but there may be a higher prevalence of pulmonary AVMs in HHT1.

ENG and ALK1 encode receptor proteins, which are members of the transforming growth factor-beta (TGF p) superfamily. Therefore, HHT is caused by a disturbance in the

TGF p signaling pathway, which is an important pathway involved in cellular proliferation, differentiation, adhesion, and migration. However, the exact mechanism of how a disturbance of this pathway leads to HHT remains unclear.

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