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 SERPINA1 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 _wurzburg and M _whitstable . Comparison of protein phenotypes using isoelectric focusing of samples that presented the M _wurzburg variant, as revealed by exons 5 sequencing, identified a particular electrophoretic pattern amenable to the M _wurzburg protein. The specific phenotyping pattern was retrospectively validated, thus enabling the detection of 16 patients with M _wurzburg variant among the subjects already tested but not sequenced according to our diagnostic algorithm. The M _whitstable allele was detected by intron 4 sequencing of SERPINA1 gene. M _wurzburg and M _whitstable 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.