Improving the Laboratory Diagnosis of M-like Variants Related to Alpha1-Antitrypsin Deficiency.

Alpha1-antitrypsin (AAT) is a serine protease inhibitor that is encoded by the highly polymorphic gene. Mutations in this gene can lead to AAT deficiency (AATD), which is associated with an increased risk of lung and/or liver disease. On the basis of electrophoretic migration, AAT variants are named with capital letters; M (medium) signifies the normal protein. Among pathological variants, the M-like ones represent a heterogeneous group of rare allelic variants that exhibit the same electrophoretic pattern as the M wild-type protein, which makes them difficult to detect with routine methods. In order to avoid their misdiagnosis, the present study defines and validates effective methods for the detection of two pathogenic M-like variants, M and M. Comparison of protein phenotypes using isoelectric focusing of samples that presented the M variant, as revealed by exons 5 sequencing, identified a particular electrophoretic pattern amenable to the M protein. The specific phenotyping pattern was retrospectively validated, thus enabling the detection of 16 patients with M variant among the subjects already tested but not sequenced according to our diagnostic algorithm. The M allele was detected by intron 4 sequencing of SERPINA1 gene. M and M are often misdiagnosed and the introduction of diagnostic improvements can help the clinical management, especially in patients with established lung disease without any other reported risk factors.