ESPE Abstracts

Background: The underlying pathogenic mechanisms governing growth restriction of Noonan syndrome (NS) remain elusive. Monoallelic inactivating LZTR1 gene variants have been implicated as a cause of NS due to hyperactivation of the canonical RAS-MAPK signalling pathway. Missense LZTR1 variants have been associated with defective ubiquitination theoretically leading to increased Ras substrate availability. Ubiquitination is implicated in growth regulation and linked to p53 signalling. We investigated the role of LZTR1 in this pathway.

Methods: Single nucleotide substitutions were created by mutagenesis of an N-terminal MYC tagged- LZTR1 cDNA. WT and variant constructs were expressed in mammalian cells and lysates prepared for phosphoproteomic analysis and immunoblotting. Analysis of transcriptomic data was conducted using Ingenuity Pathway Analysis. Target proteins and pathways of interest were probed by western blotting and immunofluorescence.

Results: Two heterozygous LZTR1 variants which segregated with short stature and features of growth hormone insensitivity (p.K156E, p.G248R) were expressed in a mammalian cell line. Both variants were thermodynamically stable and phosphoproteomic assays revealed upregulation of the histone acetyltransferase inhibitor, NOC2L (NOC2 Like Nucleolar Associated Transcriptional Repressor), in both variants. This finding, consistent upon immunoblotting and immunofluorescence, was associated with impaired acetylation of p53, with reduced levels of acetylated Lysine residue 382 in both mutants. Elevated cytoplasmic levels of pan-Ras and phosphorylated ERK1/2 were observed in both mutants upon confocal microscopy. Furthermore, Aurora B (AURKB), Ataxia Telangiectasia Mutated (ATM) kinase and Checkpoint kinase 1 (CHK1), major effectors of the DNA damage response (DDR), were preferentially activated in LZTR1 variants. In silico prediction modelling suggested that LZTR1 interacts with NOC2L via the central part of the protein.

Conclusion: Aurora B, NOC2L and p53 form a complex which dictates p53 activation. We demonstrate a novel interaction between NOC2L and LZTR1 and hypothesise that LZTR1 is a novel binding factor modulating the activity of this complex. As NOC2L negatively regulates p53, upregulation of this protein would lead to p53-mediated transcription inhibition. LZTR1 attenuation due to genetic mutations associated with NS, potentiate NOC2L activity leading to reduced apoptosis. The key role of NOC2L in cell cycle regulation suggests a plausible mechanism for the growth restriction/GHI phenotype observed in NS and may represent a novel therapeutic target. However additional work is needed to further characterise this novel mechanism.