Susceptibility of alpha1 antitrypsin deficiency variants to polymer blocking therapy

: The Z variant (Glu342Lys) causes alpha1 antitrypsin (AAT) to self-assemble into polymer chains that accumulate within hepatocytes causing liver disease and exposing a cryptic epitope recognised by the 2C1 monoclonal antibody (mAb). They can be blocked by the small molecule '716 that stabilises an intermediate on the polymerisation pathway. We have characterised 23 mutants of AAT in a cellular model to establish: (i) their ability to form intracellular polymers; (ii) whether polymer formation could be prevented by '716; and (iii) whether the polymers expose the 2C1 cryptic epitope. Most of the variants, including Mprocida (Leu41Pro), Mherleen (Pro369Leu), Mduarte (Asp256Val), Lfrankfurt (Pro255Thr), Yorzinuovi (Pro391His), Mwurzburg (Pro369Ser) and p.289S accumulated as intracellular polymers. Eleven formed polymers that were resistant to '716, including Mprocida, Mmalton (ΔPhe51), Lfrankfurt, Mduarte, S (Glu264Val), Mherleen, and Yorzinuovi. The '716 resistant mutants localise to a region of the AAT molecule separate from the binding site of the small molecule and form polymers that are less well-recognised by the 2C1 mAb. They are fully recognised by a novel 8A7 mAb that we developed to have a broader specificity. Our data suggest that individuals with these mutations are unlikely to benefit from treatment with '716 or its derivatives.