An essential SMN interacting protein (SIP1) is not involved in the phenotypic variability of spinal muscular atrophy (SMA)

The survival motor neuron (SMN) protein and the SMN interacting protein 1 ( SIP1) are part of a 300 kD protein complex with a crucial role in snRNP bio genesis and pre-mRNA splicing. Both proteins are colocalised in nuclear str uctures called gems and in the cytoplasm. Approximately 96% of patients wit h autosomal recessive spinal muscular atrophy (SMA) show mutations in the S MN1 gene, while about 4% fail to show any mutation, despite a typical SMA p henotype. Additionally, sibs with identical 5q13 homologs and homozygous ab sence of SMN1 can show variable phenotypes which suggest that SMA is modifi ed by other, yet unknown factors. Since both genes, SMN1 and SIP1, belong t o the same pathway and are part of the same protein complex, it is obvious to ask whether mutations within SIP1 are responsible for both the phenotypi c variability and the appearance of non-SMN mutated SMA patients. First, we identified the chromosomal location of SIP1 and assigned it to chromosomal region 14q13-q21 by fluorescence in situ hybridisation. No SMA related dis order has yet been assigned to this chromosomal region. Next, we determined the exon-intron structure of the SIP1 gene which encompasses 10 exons and identified five transcription isoforms. We sequenced either RT-PCR products or genomic DNA covering the complete coding region from 23 typical SMA pat ients who had failed to show any SMN1 mutation. No mutation and no polymorp hism was found within SIP1. Additionally, we sequenced RT-PCR products or g enomic fragments of the entire SIP1 coding region from 26 sibs of 11 SMA fa milies with identical genotypes (Delta 7SMN/Delta 7SMN or Delta 7SMN/other mutation) but different phenotypes and again no mutation was found. Finally , we performed quantitative analysis of RT-PCR products from the same 26 si bs. No difference in expression level of the five isoforms among phenotypic ally variable sibs was observed. Based on these data, we suggest that neith er the phenotypic variability nor the 5q-unlinked SMA are caused by mutatio ns within SIP1.