Inhibition of RAS Activation Due to a Homozygous Ezrin Variant in Patients with Profound Intellectual Disability

Abstract

Gain-of-function alterations in several components and modulators of the Ras-MAPK pathway lead to dysregulation of the pathway and cause a broad spectrum of autosomal dominant developmental disorders, collectively known as RASopathies. These findings demonstrate the importance of tight multilevel Ras regulation to safeguard signaling output and prevent aberrant activity. We have recently identified ezrin as a novel regulatory element required for Ras activation. Homozygosity mapping and exome sequencing have now revealed the first presumably disease-causing variant in the coding gene EZR in two siblings with a profound intellectual disability. Localization and membrane targeting of the altered ezrin protein appeared normal but molecular modeling suggested protein interaction surfaces to be disturbed. Functional analysis revealed that the altered ezrin protein is no longer able to bind Ras and facilitate its activation. Furthermore, expression of the altered ezrin protein in different cell lines resulted in abnormal cellular processes, including reduced proliferation and neuritogenesis, thus revealing a possible mechanism for its phenotype in humans. To our knowledge, this is the first report of an autosomal recessively inherited loss-of-function mutation causing reduced Ras activity and thus extends and complements the pathogenicity spectrum of known Ras-MAPK pathway disturbances.

TALPID3 controls centrosome and cell polarity and the human ortholog KIAA0586 is mutated in Joubert syndrome (JBTS23)

“…Of the targeted regions, 73.2% were covered at least 20×, and 83.4% were covered at least 5×. To validate the results, we also conducted WES in the likewise affected sibling, MR026-04, analogous to previously described disease gene identification approaches ( Ahmed et al, 2015 ; Riecken et al, 2015 ). 96% of the target sequence were covered at least 20×.…”

The NF2 tumor suppressor merlin interacts with Ras and RasGAP, which may modulate Ras signaling

“…We have previously shown that ezrin is actively involved in Ras activation by directly binding to both Ras and Son of Sevenless (SOS), a major Ras GEF [15, 16]. The F1 subdomain of FERM is an ubiquitin/Raf1-RBD-like domain [17] and the F1–F2 subdomains of ezrin FERM appear to mediate Ras binding [15, 18], suggesting that merlin may do likewise. Indeed, merlin FERM (amino acids [aa] 1–313), but not the α-helical-tail domains (aa 312–595), could pull down purified HRas 1–166 (Fig.…”

Necrotizing Enterocolitis in Two Siblings and an Unrelated Infant with Overlapping Chromosome 6q25 Deletions

“…In vitro functional studies of this missense allele demonstrated normal cell membrane localization compared to the wildtype protein when expressed in fibroblasts, but failure to bind RAS and thus activate the RAS pathway [Riecken et al, 2015]. NEC was not reported in these patients with EZR point mutations; however, it may be that amino acid substitutions are phenotypically different than whole gene deletions.…”

“…A human phenotype associated with the EZR mutation was recently suggested by Riecken et al [2015] who reported 2 siblings with profound intellectual disability and homozygous mutations in EZR , p.Ala129Thr. In vitro functional studies of this missense allele demonstrated normal cell membrane localization compared to the wildtype protein when expressed in fibroblasts, but failure to bind RAS and thus activate the RAS pathway [Riecken et al, 2015].…”