Ubiquitin Ligases of the N-End Rule Pathway: Assessment of Mutations in UBR1 That Cause the Johanson-Blizzard Syndrome

Abstract

BackgroundJohanson-Blizzard syndrome (JBS; OMIM 243800) is an autosomal recessive disorder that includes congenital exocrine pancreatic insufficiency, facial dysmorphism with the characteristic nasal wing hypoplasia, multiple malformations, and frequent mental retardation. Our previous work has shown that JBS is caused by mutations in human UBR1, which encodes one of the E3 ubiquitin ligases of the N-end rule pathway. The N-end rule relates the regulation of the in vivo half-life of a protein to the identity of its N-terminal residue. One class of degradation signals (degrons) recognized by UBR1 are destabilizing N-terminal residues of protein substrates.Methodology/Principal FindingsMost JBS-causing alterations of UBR1 are nonsense, frameshift or splice-site mutations that abolish UBR1 activity. We report here missense mutations of human UBR1 in patients with milder variants of JBS. These single-residue changes, including a previously reported missense mutation, involve positions in the RING-H2 and UBR domains of UBR1 that are conserved among eukaryotes. Taking advantage of this conservation, we constructed alleles of the yeast Saccharomyces cerevisiae UBR1 that were counterparts of missense JBS-UBR1 alleles. Among these yeast Ubr1 mutants, one of them (H160R) was inactive in yeast-based activity assays, the other one (Q1224E) had a detectable but weak activity, and the third one (V146L) exhibited a decreased but significant activity, in agreement with manifestations of JBS in the corresponding JBS patients.Conclusions/SignificanceThese results, made possible by modeling defects of a human ubiquitin ligase in its yeast counterpart, verified and confirmed the relevance of specific missense UBR1 alleles to JBS, and suggested that a residual activity of a missense allele is causally associated with milder variants of JBS.

The N-end rule pathway counteracts cell death by destroying proapoptotic protein fragments

“…Johanson-Blizzard syndrome (JBS) is caused by inactivating mutations in both copies of UBR1, which encodes an N-recognin (Fig. 1A) (23,39). Clinical features of JBS are consistent with our finding that even a partial ablation of the Arg/N-end rule pathway sensitizes cells to apoptosis (Fig.…”

“…For example, nasal wing aplasia (near-absence of nostrils) in JBS patients is likely to result from apoptosis of cells that normally form nasal wings. Insufficiency and inflammation of exocrine pancreas in JBS are likely to be caused by apoptosis of acinar cells (17,23). The antiapoptotic activity of the Arg/ N-end rule pathway suggests that its up-regulation may facilitate malignant phenotypes in mammalian cell lineages.…”

“…The main determinant of an N-degron is a destabilizing N-terminal residue of a substrate protein. Recognition components of the N-end rule pathway, called N-recognins, are specific E3 Ub ligases that can target N-degrons (17,23). The N-end rule pathway consists of two branches, the Ac/N-end rule and the Arg/N-end rule pathways.…”

Clinical utility gene card for: Johanson–Blizzard syndrome

“…Fifty-nine different mutations are known (including published mutations [1][2][3][4][5][6][7][8] and unpublished mutations identified in our lab). These include nonsense mutations (15), splice site mutations (14), small deletions and duplications/insertions causing frameshift (9), small inframe deletions (3) and missense mutations (18).…”

“…7 This does not allow the conclusion that missense mutations generally lead to a less-severe clinical expression! Mutations in affected individuals occur in either homozygous or compound-heterozygous state.…”