Deficient brain snRNP70K in patients with Down syndrome

The small nuclear ribonucleoprotein 70K (snRNP 70K; U1-70 kDa) is an integral part of the spliceosome, a large RNA-protein complex catalyzing the removal of introns from nuclear pre-mRNA. snRNP is one of the best-studied essential subunits of snRNPs, is highly conserved and its inactivation was shown to result in complete inhibition of splicing. Applying subtractive hybridization, we found a sequence with 100% identity to snRNP absent in fetal Down syndrome (DS) brain. This observation made us determine snRNP-mRNA steady-state levels and protein levels in brains of adult patients with DS. snRNP-mRNA and protein levels of five individual brain regions of DS and controls each, were determined by blotting techniques. snRNP-mRNA steady state levels were significantly decreased in DS brain. Performing Western blots with monoclonal and human antibodies, snRNP protein levels were decreased in several regions of DS brain, although one monoclonal antibody did not reveal different snRNP-immunoreactivity. Although decreased snRNP-protein could be explained by decreased mRNA-steady state levels, another underlying mechanism might be suggested: snRNP is one of the death substrates rapidly cleaved during apoptosis by interleukin-1-beta-converting enzyme-like (ICE) proteases, which was well-documented by several groups. As apoptosis is unrequivocally taking place in DS brain leading to permanent cell loses, decreased snRNP-protein levels may therefore reflect decreased synthesis and increased apoptosis-related proteolytic cleavage.