PiS and PiS-plus alleles. The importance of phasing gene variants in Alpha-1 Antitrypsin Deficiency

Abstract

Alpha-1 Antitrypsin deficiency (AATD) is an inherited condition characterized by reduced levels of serum AAT due to mutations in SERPINA1 gene. Together, PiZ (Glu342Lys) and the PiS (Glu264Val) are the most frequent deficient alleles of AATD and reduce the protein level by 80% and 40% respectively. Genotyping of PiZ and PiS in AATD patients, is an important step in many diagnostic algorithms. However, there are deficient individuals with lower serum levels of AAT than expected for the initial genotype. In these cases sample must be further analyzed by sequencing of coding exons of SERPINA1 gene, and protein phenotyped by isoelectric focusing.

In this study we describe two previously unknown missense variants, both confirmed to be in-cis configuration with the S mutation, producing a null phenotype. These novel variants were functionally characterized by expressing them in a cellular model. Data show that these variants modify the properties of the PiS allele resulting in higher intracellular retention and no extracellular secretion. Plasma sample analysis from patients carrying these combinations confirmed the null-phenotype behavior. These represent a novel type of AATD alleles that we named ‘PiS-plus’ alleles. Biochemically they have more pronounced pathogenic properties than typical deficient PiS alleles, while their clinical consequences need to be further explored. This finding highlights that genetic diagnosis of AATD should consider, in addition to identify gene variants, the phasing of these variants in order to establish a definitive diagnosis. The discovery of different PiS-plus alleles could help explaining the phenotypical variability observed among AATD patients.