A Study to Evaluate ALN-6400 in Healthy Volunteers and Patients With Hereditary Hemorrhagic Telangiectasia (HHT)

The purpose of this study is to: * evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of single ascending doses of ALN-6400 in healthy volunteers * evaluate the efficacy, safety, tolerability and PD of multiple doses of ALN-6400 in adult patients with HHT

Inclusion Criteria - Part A: * Is a healthy adult volunteer Part B: * Is an adult patient with a clinical diagnosis of HHT Exclusion Criteria - Part A: * Has alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \> upper limit of normal (ULN) * Has known human immunodeficiency virus (HIV) infection; or known current or chronic hepatitis C virus or hepatitis B virus infection * Has an estimated glomerular filtration (eGFR) of \2×ULN * Has total bilirubin \>1.5×ULN * Has eGFR of \