Cutaneous manifestations in Costello syndrome: HRAS p.Gly12Ser affects RIN1-mediated integrin trafficking in immortalized epidermal keratinocytes

Heterozygous germline missense variants in the HRAS gene underlie Costello syndrome (CS). The molecular basis for cutaneous manifestations in CS is largely unknown. We used an immortalized human cell line, HaCaT keratinocytes, stably expressing wild-type or CS-associated (p.Gly12Ser) HRAS and defined RIN1 as quantitatively most prominent, high-affinity effector of active HRAS in these cells. As an exchange factor for RAB5 GTPases, RIN1 is involved in endosomal sorting of cell-adhesion integrins. RIN1-dependent RAB5A activation was strongly increased by HRASGly12Ser, and HRAS-RIN1-ABL1/2 signaling was induced in HRASWT- and HRASGly12Ser-expressing cells. Along with that, HRASGly12Ser expression decreased total integrin levels and enriched β1 integrin in RAB5- and EEA1-positive early endosomes. The intracellular level of active β1 integrin was increased in HRASGly12Ser HaCaT keratinocytes due to impaired recycling, whereas RIN1 disruption raised β1 integrin cell surface distribution. HRASGly12Ser induced co-localization of β1 integrin with SNX17 and RAB7 in early/sorting and late endosomes, respectively. Thus, by retaining β1 integrin in intracellular endosomal compartments, HRAS-RIN1 signaling affects the subcellular availability of β1 integrin. This may interfere with integrin-dependent processes as we detected for HRASGly12Ser cells spreading on fibronectin. We conclude that dysregulation of receptor trafficking and integrin-dependent processes such as cell adhesion are relevant in the pathobiology of CS.

Attenuated phenotype of Costello syndrome and early death in a patient with an HRAS mutation (c.179G>T; p.Gly60Val) affecting signalling dynamics

Costello syndrome (CS) is caused by heterozygous germline HRAS mutations. Most patients share the HRAS mutation c.34G>A (p.Gly12Ser) associated with the typical, relatively homogeneous phenotype. Rarer mutations occurred in individuals with an attenuated phenotype. Although many disease-associated HRAS alterations trigger constitutive activation of HRAS-dependent signalling pathways, additional pathological consequences exist. An infant with failure-to-thrive and hypertrophic cardiomyopathy had a novel de novo HRAS mutation (c.179G>T; p.Gly60Val). He showed subtle dysmorphic findings consistent with attenuated CS and died from presumed cardiac cause. Functional studies revealed that amino acid change p.Gly60Val impairs HRAS binding to effectors PIK3CA, phospholipase C1, and RAL guanine nucleotide dissociation stimulator. In contrast, interaction with effector rapidly accelerated fibrosarcoma (RAF) and regulator NF1 GTPase-activating protein was enhanced. Importantly, expression of HRAS p.Gly60Val in HEK293 cells reduced growth factor sensitivity leading to damped RAF-MAPK and phosphoinositide 3-kinases-AKT signalling response. Our data support the idea that a variable range of dysregulated HRAS-dependent signalling dynamics, rather than static activation of HRAS-dependent signal flow, may underlie the phenotypic variability in CS.