A cryptic promoter in the first exon of the SPG4gene directs the synthesis of the 60-kDa spastin isoform

Abstract

Background: Mutations in SPG4 cause the most common form of autosomal dominant hereditary spastic paraplegia, a neurodegenerative disease characterized by weakness and spasticity of the lower limbs due to degeneration of the corticospinal tract. SPG4 encodes spastin, a microtubulesevering ATPase belonging to the AAA family. Two isoforms of spastin, 68 and 60 kDa, respectively, are variably abundant in tissues, show different subcellular localizations and interact with distinct molecules. The isoforms arise through alternative initiation of translation from two AUG codons in exon 1; however, it is unclear how regulation of their expression may be achieved.

Conditional Mutagenesis of a Novel Choline Kinase Demonstrates Plasticity of Phosphatidylcholine Biogenesis and Gene Expression in Toxoplasma gondii

“…These findings resonate with a previous study showing the presence of a promoter in the first exon of the SPG4 gene, which can govern the synthesis of a short spastin isoform (26). The exon 1 promoter is likely active in T. gondii, as shown by higher abundance of exon 6.…”

A Promoter in the Novel Exon of hPPARγ Directs the Circadian Expression of PPARγ

“…First exons play important roles in basal transcription of some genes 23,24) , and we now show that the first novel exon of PPAR fits this pattern (Fig. 1).…”

Sequence variants in SPAST, SPG3A and HSPD1 in hereditary spastic paraplegia

“…The p.Pro41Leu variant found in this study results in a poor PEST score, below the score for p.Pro45Gln, suggesting that the p.Pro41Leu change might also modify the phenotype in the patient carrying the variant to earlier age of onset or act as a susceptibility factor. Alternatively, the p.Pro41Leu variant may exert its effect by interrupting the cryptic promoter recently found in SPAST [20]. This was shown for the p.Ser44Leu variant which reduced the activity of the cryptic promoter to 50%, while no effect was detected for the p.Pro45Gln variant, indicating that these variants not necessarily act through the same mechanism.…”

“…Reverse transcription using Superscript III Reverse Transcriptase (Invitrogen) and oligo(dT) 20 primer was followed by PCR amplification. To ensure that no signal came from the genomic DNA, at least one of the primers for the amplification was located across two exons.…”

“…Firstly, they have been reported to increase the stability of the long isoform of spastin, by interrupting a PEST motif that serves as a proteolytic signal [18]. Secondly, they might decrease the activity of a cryptic promoter identified in SPAST [20], which has been shown to direct the synthesis of the 60 kDa isoform of spastin. These two suggested pathogenic mechanisms imply either haplo-insufficiency of the short isoform or dominant negative effect caused by increased ratio between the long isoform and the short isoform.…”