Molecular basis of acid ceramidase deficiency in a neonatal form of Farber disease: identification of the first large deletion in ASAH1 gene.

Farber disease, also known as Farber's lipogranulomatosis, is a clinically heterogeneous autosomal recessive disease caused by mutations in the ASAH1 gene. This gene codes for acid ceramidase, a lysosomal heterodimeric enzyme that hydrolyzes ceramide into sphingosine and fatty acid. To date, less than 25 distinct mutations have been identified in Farber patients, but no large deletions have yet been reported. In this work, cultured fibroblasts from a Farber patient with the rare neonatal form of Farber disease were studied to elucidate the molecular basis of this extremely severe phenotype. Direct sequencing of ASAH1 genomic DNA revealed the causative heterozygous mutation in the donor splice site consensus sequence of intron 11, g.24491A > G (c.917 + 4A > G), that resulted in the absence of detectable mRNA. Subsequent analysis of ASAH1 mRNA showed total skipping of exons 3 to 5. Long-range PCR and sequencing led to the identification of a gross deletion of ASAH1 gene, g.8728_18197del (c.126-3941_382 + 1358del) predicting the synthesis of a truncated polypeptide, p.Tyr42_Leu127delinsArgfs*10. Accordingly, no molecular forms corresponding to precursor or proteolytically processed mature protein were observed. These findings indicate that any functionally active acid ceramidase is absent in patient cells, underscoring the severity of the clinical phenotype. Molecular findings in the non-consanguineous parents confirmed the compound heterozygous ASAH1 genotype identified in this Farber case. This work unravels for the first time the mutations underlying the neonatal form of Farber disease and represents the first report of a large deletion identified in the ASAH1 gene. Screening for gross deletions in other patients in whom the mutation present in the second allele had not yet been identified is required to elucidate further its overall contribution for the molecular pathogenesis of this devastating disease.