From N-of-1 to versatility in propionic acidemia: Antisense oligonucleotide-mediated skipping of a constitutive PCCA pseudoexon

Propionic acidemia is a rare autosomal recessive disorder caused by mutations in the PCCA or PCCB gene, resulting in deficient propionyl-CoA carboxylase activity. We identified a unique homozygous deep-intronic PCCA variant, NM_000282.4:c.1285-1358C>G, in an individual with neonate-onset propionic acidemia. Fibroblasts from this individual expressed only PCCA mRNA containing an 84-bp pseudoexon, which is present at low levels in healthy controls, leading to the loss of PCCA and PCCB proteins and severely reduced propionyl-CoA carboxylase activity. Transfection of fibroblasts with chemically synthesized antisense oligonucleotides (ASOs) designed to skip the pseudoexon restored productive PCCA splicing, rescued PCCA protein expression, and markedly increased propionyl-CoA carboxylase activity above wild-type levels. The efficacy of the ASOs was further evaluated in fibroblasts from 7 additional individuals with propionic acidemia carrying mutations in PCCA or PCCB . ASO treatment successfully restored enzymatic activity, particularly in fibroblast lines, with residual activity exceeding 1% of normal. These findings suggest that ASO-mediated splicing correction targeting the 84-bp pseudoexon can restore mRNA, protein, and enzymatic function in individuals with deep intronic mutations, as well as in other individuals with propionic acidemia, indicating the feasibility of ASO therapy as a molecular treatment strategy for a subset of individuals with propionic acidemia.