The Impact of PRPF8 Mutations That Cause Retinitis Pigmentosa on Spliceosome Assembly and Activation

Autosomal dominant Retinitis Pigmentosa (adRP) is a leading cause of blindness.  Several genes implicated in adRP code for essential proteins of the spliceosome, a ribonucleoprotein (RNP) complex that catalyzes pre-mRNA splicing.  These proteins are constituents of the U4/U6.U5 tri-snRNP, suggesting a common mechanism may underlie adRP. The tri-snRNP, which contains a U4/U6 snRNA duplex, associates with U1 and U2 snRNPs to form the precatalytic spliceosome. Brr2, an essential DExD/H ATPase affected in some adRP cases, unwinds U4/U6 snRNAs to facilitate the mutually exclusive U2/U6 snRNA base-pairing necessary for catalysis. Brr2’s unwinding activity is regulated by another essential tri-snRNP protein, Prp8. AdRP mutations in PRPF8, the human gene that encodes Prp8, are clustered in the region that interacts with Brr2. Previous studies of splicing factor-associated adRP suggest tri-snRNP assembly defects underlie adRP.  However, recent findings from our lab suggest defective U4/U6 unwinding as an alternative mechanism.  The severity of unwinding defects in budding yeast carrying Brr2 adRP mutations correlated with the onset age of adRP in humans with analogous mutations.  Consistent with our hypothesis, we now show some Prp8 adRP mutations also impair U4/U6 unwinding in tri-snRNPs. We will characterize the effects of additional adRP mutations in PRP8 by assaying for defects in U4/U6 unwinding, pre-mRNA splicing, and growth at a range of temperatures in budding yeast.  We hypothesize these mutations will confer unwinding defects whose severity correlates with adRP onset age in patients carrying analogous mutations.  These analyses will also be extended to other spliceosomal adRP genes in the future.