Effector domain mutations dissociate p21ras effector function and GTPase-activating protein interaction

KrasP34R and KrasT58I mutations induce distinct RASopathy phenotypes in mice

Somatic KRAS mutations are highly prevalent in many cancers. In addition, a distinct spectrum of germline KRAS mutations causes developmental disorders called RASopathies. The mutant proteins encoded by these germline KRAS mutations are less biochemically and functionally activated than those in cancer. We generated mice harboring conditional Kras^LSL-P34Rand Kras^LSL-T58I knock-in alleles and characterized the consequences of each mutation in vivo. Embryonic expression of Kras^T58I resulted in craniofacial abnormalities reminiscent of those seen in RASopathy disorders, and these mice exhibited hyperplastic growth of multiple organs, modest alterations in cardiac valvulogenesis, myocardial hypertrophy, and myeloproliferation. By contrast, embryonic Kras^P34R expression resulted in early perinatal lethality from respiratory failure due to defective lung sacculation, which was associated with aberrant ERK activity in lung epithelial cells. Somatic Mx1-Cre–mediated activation in the hematopoietic compartment showed that Kras^P34R and Kras^T58I expression had distinct signaling effects, despite causing a similar spectrum of hematologic diseases. These potentially novel strains are robust models for investigating the consequences of expressing endogenous levels of hyperactive K-Ras in different developing and adult tissues, for comparing how oncogenic and germline K-Ras proteins perturb signaling networks and cell fate decisions, and for performing preclinical therapeutic trials.

Mouse models are developed to accurately recapitulate multiple features of RASopathy disorders caused by germline KRASP34R and KRAST581 mutations.

Germline KRAS mutations cause aberrant biochemical and physical properties leading to developmental disorders

The KRAS gene is the most common locus for somatic gain-of-function mutations in human cancer. Germline KRAS mutations were shown recently to be associated with developmental disorders, including Noonan syndrome (NS), cardio-facio-cutaneous syndrome (CFCS), and Costello syndrome (CS). The molecular basis of this broad phenotypic variability has in part remained elusive so far. Here, we comprehensively analyzed the biochemical and structural features of ten germline KRAS mutations using physical and cellular biochemistry. According to their distinct biochemical and structural alterations, the mutants can be grouped into five distinct classes, four of which markedly differ from RAS oncoproteins. Investigated functional alterations comprise the enhancement of intrinsic and guanine nucleotide exchange factor (GEF) catalyzed nucleotide exchange, which is alternatively accompanied by an impaired GTPase-activating protein (GAP) stimulated GTP hydrolysis, an overall loss of functional properties, and a deficiency in effector interaction. In conclusion, our data underscore the important role of RAS in the pathogenesis of the group of related disorders including NS, CFCS, and CS, and provide clues to the high phenotypic variability of patients with germline KRAS mutations.

Biochemical and functional characterization of germ line KRAS mutations

Germ line missense mutations in HRAS and KRAS and in genes encoding molecules that function up- or downstream of Ras in cellular signaling networks cause a group of related developmental disorders that includes Costello syndrome, Noonan syndrome, and cardiofaciocutaneous syndrome. We performed detailed biochemical and functional studies of three mutant K-Ras proteins (P34R, D153V, and F156L) found in individuals with Noonan syndrome and cardiofaciocutaneous syndrome. Mutant K-Ras proteins demonstrate a range of gain-of-function effects in different cell types, and biochemical analysis supports the idea that the intrinsic Ras guanosine nucleotide triphosphatase (GTPase) activity, the responsiveness of these proteins to GTPase-activating proteins, and guanine nucleotide dissociation all regulate developmental programs in vivo.

An unexpected new role of mutant Ras: perturbation of human embryonic development

The Ras signaling pathway controls important cellular responses to growth factors, and somatic mutations in RAS genes and other components of the Ras pathway, such as PTPN11 (encoding the protein-tyrosine phosphatase SHP-2) and BRAF, are found in human malignancies. Ras proteins are guanosine nucleotide-binding proteins that cycle between active guanosine triphosphate (GTP)-bound and inactive guanosine diphosphate (GDP)-bound conformations. Neoplasia-associated Ras mutations frequently affect amino acids G12, G13, or Q61 and decrease the intrinsic guanosine triphosphatase (GTPase) activity by ten- to twentyfold. The GTPase activity is crucial for Ras inactivation by hydrolysis and release of a phosphate group from Ras·GTP to produce Ras·GDP. We and others have recently discovered germline mutations in the KRAS gene in individuals diagnosed with Noonan and cardio–facio–cutaneous (CFC) syndrome, two clinically overlapping disorders characterized by short stature, distinct facial anomalies, heart defects, and other abnormalities. Noonan syndrome-associated mutations V14I and T58I K-Ras activate Ras but have milder biochemical effects than somatic mutations encountered in cancers, offering an explanation why these K-Ras lesions are tolerated during embryonic development. Together with recent findings of BRAF, MEK1, and MEK2 mutations in CFC syndrome and HRAS mutations in Costello syndrome, another clinically related disorder, it has now become clear that Noonan-like features (short stature, relative macrocephaly, facial anomalies, learning difficulties) that are found in these three related disorders are a result of constitutive activation of the Ras–Raf–extracellular signal-regulated and mitogen-activated protein kinase pathway.

Expansion of the genotypic and phenotypic spectrum in patients with KRAS germline mutations

BACKGROUND

Noonan syndrome, cardio-facio-cutaneous syndrome (CFC) and Costello syndrome constitute a group of developmental disorders with an overlapping pattern of congenital anomalies. Each of these conditions can be caused by germline mutations in key components of the highly conserved Ras-MAPK pathway, possibly reflecting a similar pathogenesis underlying the three disorders. Germline mutations in KRAS have recently been identified in a small number of patients with Noonan syndrome and CFC.

METHODS AND RESULTS

260 patients were screened for KRAS mutations by direct sequencing. Overall, we detected KRAS mutations in 12 patients, including three known and eight novel sequence alterations. All mutations are predicted to cause single amino acid substitutions. Remarkably, our cohort of individuals with KRAS mutations showed a high clinical variability, ranging from Noonan syndrome to CFC, and also included two patients who met the clinical criteria of Costello syndrome.

CONCLUSION

Our findings reinforce the picture of a clustered distribution of disease associated KRAS germline alterations. We further defined the phenotypic spectrum associated with KRAS missense mutations and provided the first evidence of clinical differences in patients with KRAS mutations compared with Noonan syndrome affected individuals with heterozygous PTPN11 mutations and CFC patients carrying a BRAF, MEK1 or MEK1 alteration, respectively. We speculate that the observed phenotypic variability may be related, at least in part, to specific genotypes and possibly reflects the central role of K-Ras in a number of different signalling pathways.

Hypothermic stress leads to activation of Ras-Erk signaling

The small GTPase Ras is converted to the active, GTP-bound state during exposure of vertebrate cells to hypothermic stress. This activation occurs more rapidly than can be accounted for by spontaneous nucleotide exchange. Ras-guanyl nucleotide exchange factors and Ras GTPase-activating proteins have significant activity at 0 degrees C in vitro, leading to the hypothesis that normal Ras regulators influence the relative amounts of Ras-GTP and Ras-GDP at low temperatures in vivo. When hypothermic cells are warmed to 37 degrees C, the Raf-Mek-Erk protein kinase cascade is activated. After prolonged hypothermic stress, followed by warming to physiologic temperature, cultured fibroblasts assume a rounded morphology, detach from the substratum, and die. All of these biologic responses are attenuated by pharmacologic inhibition of Mek. Previously, it had been found that low temperature blocks acute growth factor signaling to Erk. In the present study, we found that this block occurs at the level of Raf activation. Temperature regulation of Ras signaling could help animal cells respond appropriately to hypothermic stress, and Ras-Erk signaling can be manipulated to improve the survival of cells in cold storage.

Interaction of activated Ras with Raf-1 alone may be sufficient for transformation of rat2 cells

v-H-ras effector mutants have been assessed for transforming activity and for the ability of the encoded proteins to interact with Raf-1-, B-Raf-, byr2-, ralGDS-, and CDC25-encoded proteins in the yeast two-hybrid system. Transformation was assessed in rat2 cells as well as in a mutant cell line, rv68BUR, that affords a more sensitive transformation assay. Selected mutant Ras proteins were also examined for their ability to interact with an amino-terminal fragment of Raf-1 in vitro. Finally, possible cooperation between different v-H-ras effector mutants and between effector mutants and overexpressed Raf-1 was assessed. Ras transforming activity was shown to correlate best with the ability of the encoded protein to interact with Raf-1. No evidence for cooperation between v-H-ras effector mutants was found. Signaling through the Raf1-MEK-mitogen-activated protein kinase cascade may be the only effector pathway contributing to RAS transformation in these cells.

Identification of neurofibromin mutants that exhibit allele specificity or increased Ras affinity resulting in suppression of activated ras alleles

Neurofibromin plays a critical role in the downregulation of Ras proteins in neurons and Schwann cells. Thus, the ability of neurofibromin to interact with Ras is crucial for its function, as mutations in NF1 that abolish this interaction fail to maintain function. To investigate the neurofibromin-Ras interaction in a systematic manner, we have carried out a yeast two-hybrid screen using a mutant of H-ras, H-rasD92K, defective for interaction with the GTPase-activated protein-related domain (GRD) of NF1. Two screens of a randomly mutagenized NF1-GRD library led to the identification of seven novel NF1 mutants. Characterization of the NF1-GRD mutants revealed that one class of mutants are allele specific for H-raSD92K. These mutants exhibit increased affinity for H-raSD92K and significantly reduced affinity for wild-type H-ras protein. Furthermore, they do not interact with another H-ras mutant defective for interaction with GTPase-activating proteins. Another class of mutants are high-affinity mutants which exhibit dramatically increased affinity for both wild-type and mutant forms of Ras. They also exhibit a striking ability to suppress the heat shock sensitive traits of activated RAS2G19v in yeast cells. Five mutations cluster within a region encompassing residues 1391 to 1436 (region II). Three NF1 patient mutations have previously been identified in this region. Two mutations that we identified occur in a region encompassing residues 1262 to 1276 (region I). Combining high-affinity mutations from both regions results in even greater affinity for Ras. These results demonstrate that two distinct regions of NF1-GRD are involved in the Ras interaction and that single amino acid changes can affect NF1's affinity for Ras.

RAS signalling is abnormal in a c-raf1 MEK1 double mutant

A mutant rat cell clone that suppresses the transformation defects of RAS effector loop substitutions is heterozygous for mutations in c-raf1 and MEK1. The mutant cells can be transformed by many otherwise defective RAS effector mutants, including RAS genes with the effector regions of distantly related GTPases, even though the encoded RAS proteins do not interact with either the mutant or wild-type RAF in Saccharomyces cerevisiae. While the significance of the c-raf1 mutation is unclear, the MEK1 mutation increases MEK1 activity and leads to activation of mitogen-activated protein kinase. The mutant MEK1 is coupled to the epidermal growth factor pathway but exhibits decreased physical interaction with RAF. When overexpressed, the MEK1 mutation is transforming and causes hyperphosphorylation of RAF. Signalling from RAS to MEK1 may be mediated by something other than RAF alone, but signalling through MEK1 is probably sufficient for RAS transformation.

Transformation and pp60v-src autophosphorylation correlate with SHC-GRB2 complex formation in rat and chicken cells expressing host-range and kinase-active, transformation-defective alleles of v-src

The biochemical properties of several pp60v-src substrates believed to participate in src-mediated transformation were examined in cells expressing a kinase-active, transformation-defective v-src allele (v-src-F172 delta/Y416F) and its parental allele, v-src-F172 delta, a host-range--dependent allele that transforms chicken cells to a fusiform morphology, but does not transform rat cells. Because pp60v-src-F172 delta is dependent on autophosphorylation for transforming ability, these alleles provide a unique opportunity to examine the role of pp60v-src autophosphorylation in regulating substrate interactions. Increased pp125FAK tyrosine phosphorylation and high levels of pp60v-src-associated phosphotidylinositol-3' kinase activity were detected specifically in chicken cells exhibiting round, refractile transformation but not in cells transformed to a fusiform morphology. Increased pp125FAK kinase activity, but not increased pp125FAK tyrosine-phosphorylation correlated with pp60v-src autophosphorylation and increased anchorage-independent growth. Thus, pp125FAK and PI3'K may participate in morphological transformation by v-src. Furthermore, association of phosphorylated SHC with the adapter GRB2 correlated with increased anchorage-independent growth (and autophosphorylation) in both rat and chicken cells independent of the morphological phenotype induced. Therefore, host-range dependence for transformation may be regulated through association of SHC with GRB2, thus implicating SHC as a crucial substrate for src-dependent transformation.

Transcriptional activation of a ras-like gene (kir) by oncogenic tyrosine kinases

We report the characterization of a member of the ras gene family that is overexpressed in cells transformed by abl tyrosine kinase oncogenes. The gene, named kir (for kinase-inducible ras-like), is induced at the transcriptional level. kir mRNA has a rapid turnover and encodes a protein of 33 kDa with guanine nucleotide-binding activity but undetectable intrinsic GTPase activity. kir was cloned by differential screening of genes present in fully malignant versus growth factor-independent cell lines expressing wild-type or mutant forms of BCR/ABL. BCR/ABL and v-Abl induce transcription of the kir gene via specific signaling pathway(s), but kir overexpression alone is not sufficient to mediate transformation.