Propionic acidemia is an autosomal recessive disorder caused by a deficiency in 
the mitochondrial enzyme propionyl-CoA carboxylase (PCC). PCC is composed of two 
subunits, alpha and beta, encoded by the PCCA and PCCB genes, respectively. We 
analyzed mutations of the PCCA gene using patients' fibroblasts diagnosed with 
alpha subunit deficiency. By RT-PCR, four of 12 cell lines examined appeared to 
have a larger transcript present at a level comparable with that of the expected 
normal species. Sequencing of the larger transcriptrevealed an 84 bp insertion 
at nt 1209 of the codingsequence. Its incorporation in the transcript results in 
translation termination due to the presence of two in-frame stop codons. The 84 
bp insertion was found to originate from the intron between nt 1209 and 1210. 
Consensus splice donor and acceptor sites were found at the 3'- and 5'-ends of 
the insertion, respectively. The insertion was also found in the remaining eight 
cell lines as well as in normal cells, but at a muchreduced level compared with 
the normal lengthsequence. Mutation analysis of the four cell lines showing 
seemingly elevated levels of the insertion sequence revealed one nonsense 
mutation (R288X), two frameshift deletions (700del5 and 1115del4) and one splice 
mutation (1671IVS+5G-->C) as expressed alleles. We conclude that the common 
characteristic of the four cell lines is that they contain mRNA destabilizing 
mutations that reduce the mRNA level of the normal length sequence. 
Consequently, the low levels of cryptic mRNAs become detectable at a level 
similar to that of the residual level of the normal length mRNA. We suggest that 
screening for an increased proportion of the 84 bp insertion by RT-PCR can be 
used as a rapid assay for RNA destabilizing mutations. Our results suggest 
caution in associating such mutations with aberrant mRNA species, such as 
cryptic splice products, which may instead be part of the 'background noise' of 
the splicing machinery.