Methylprednisolone pulse treatment improve ProSP-C trafficking in twins with SFTPC mutation: An isoform story?

Mutations in the gene encoding surfactant protein C (SP-C) cause interstitial lung disease (ILD) and glucocorticosteroid (GC) treatment is the most recognized therapy in children. We aimed to decipher the mechanisms behind successful GC treatment in twins carrying a BRICHOS c.566G>A (p.Cys189Tyr) mutation in SP-C gene (SFTPC). METHODS: The twins underwent bronchoscopy before and after GCs treatment and immunoblotting analysis of proSP-C and SP-C mature in bronchoalveolar fluid (BALF). Total RNA was extracted and analysed using quantitative real-time PCR assays. In A549 cells, the processing of mutated protein C189Y was studied by immunofluorescence and immunoblotting after heterologous expression of eukaryotic vectors containing WT or C189Y mutant cDNA. RESULTS: Before treatment, BALF analysis identified an alteration of the proSP-C maturation process. Functional study of C189Y mutation in alveolar A549 cells showed that pro-SP-CC189Y was retained within the endoplasmic reticulum together with ABCA3. After 5 month of GCs treatment with clinical benefit, the BALF analysis revealed an improvement of proSP-C processing. SFTPC mRNA analysis in twins revealed a decrease in the expression of total SFTPC mRNA and a change in its splicing leading to the expression of a second shorter proSP-C isoform. In A549 cells, the processing and the stability of this shorter wild type proSP-C isoform was similar to that of the longer isoform, but the half-life of the mutated shorter isoform was decreased. These results suggested a direct effect of GCs on proSP-C metabolism through reducing the SFTPC mRNA level and favouriting the expression of a protein isoform less stable.