IMMUNOHISTOCHEMICAL AND GENETIC-CHARACTERIZATION OF THE M-CAGLIARI ALPHA-1-ANTITRYPSIN MOLECULE (M-LIKE ALPHA-1-ANTITRYPSIN DEFICIENCY)

BACKGROUND: Genetic alpha-1-antitrypsin (AAT) deficiency may be due to defective secretion, intracellular degradation, or lack of synthesis. Defective secretion results in hepatocytic storage and liver disease. These two events occur only with the common deficiency variant, Z AAT , and with a few rare deficiency variants, called M-like. Hepatocytic storage of AAT (either Z or M-like) can be demonstrated in tissue sect ions by specific immunostaining with a polyclonal anti-AAT antibody, t hat recognizes all variants of AAT. A monoclonal antibody capable of s electively and exclusively reacting with Z AAT has been generated and successfully used in both serum and tissue studies. EXPERIMENTAL DESIG N: To determine whether a new M-like variant, M Cagliari, carries a mu tation different from Z AAT, we have compared antigenic properties and DNA sequences of the two variants. Liver tissue sections from PiZ and PiM Cagliari patients were stained with both polyclonal anti-AAT and monoclonal anti-Z AAT antibodies. DNAs were polymerase chain reaction- amplified with AAT-specific primers and sequenced. RESULTS: Liver tiss ue sections from PiZ livers were positively stained with either the po lyclonal or the monoclonal antibody. The PiM Cagliari liver sections r eacted with the polyclonal antibody, but not with the monoclonal anti- Z AAT, thus indicating a difference in antigenicity from Z AAT. Accord ingly, DNA analysis ruled out a Z mutation and revealed a microdeletio n in exon II, identical with M Malton. CONCLUSIONS: A simple immunohis tochemical assay based upon the application of both polyclonal and mon oclonal antibodies represents a reliable test to distinguish Z and non Z AAT deficiencies, thus assisting in the selection of cases worthy of more time-consuming analyses such as DNA sequencing. The same approac h may be used for the characterization of as yet undefined PiM cases w ith AAT liver storage.