Characterisation of human peptidyl prolyl isomerase-like protein 1 (PPIL1) mutations

A novel brain malformation syndrome was previously reported in six individuals from four families. Patients presented with profound microcephaly (OFC 7-9 SD < mean), severe cortical dysplasia, profound cerebellar hypoplasia of both vermis and hemispheres and brainstem hypoplasia. In previous work, autozygosity mapping and whole exome sequencing suggested variants in peptidyl-prolyl isomerase 1 (PPIL1) were responsible for the disease. All patient variants were homozygous and cause the following PPIL1 protein changes: p.A99T, p.T107A, p.R131Q and p.G109C;A101_D106dup. PPIL1 is expressed ubiquitously in humans and is part of the spliceosome, a complex RNA and protein enzyme responsible for splicing introns from pre-mRNA. To gain an insight into the mechanism of this disease, PPIL1 WT and mutant proteins have been expressed and purified from E. coli. This work has shown that two PPIL1 patient mutations significantly affect protein stability. Using SPR, a further patient mutation has been shown to abolish an interaction with SKIP, a protein which is thought to recruit PPIL1 to the spliceosome. PPIL1 is also a peptidyl prolyl-isomerase enzyme and using NMR it has been shown that a further patient mutation reduces this enzymatic activity on a model substrate. Currently the biological target of PPIL1 peptidyl prolyl-isomerase enzyme activity in the spliceosome is unknown. By studying published cryo-EM data and using NMR and ITC experiments it has been shown that PRP17, is a plausible biological target of PPIL1 in the spliceosome. Taken together these results confirm the deleterious effect of the identified patient variants and provide novel insights into the role of PPIL1.