# Ras Isoforms from Lab Benches to Lives--What Are We Missing and How Far Are We?
## 4 Four Decades of Research and Why Ras Remains Undruggable?
The second functional cluster is in HF, DH, and PH domains (p.lysine 170 to glutamic acid, p.tyrosine 337 to cysteine, p.isoleucine 437 to threonine, p.cysteine 441 to tyrosine, p.serine 548 to arginine, p.leucine 550 to proline, p.arginine 552 to glycine/threonine/methionine/lysine/serine, p.leucine 554 to methionine 558 -deletion-insertion of lysine, p.methionine 422 to valine, p.arginine 497 to glutamine, p.threonine 549 to lysine) [97]. The common missense mutations are p.arginine 552 to serine and p.arginine 552 to threonine) [98]. Arginine-552 is a crucial residue that interacts with aspartic acid 140 and 169 and any missense mutation in this residue hampers the interaction of HF and DH domain essentially affecting the autoinhibition of SOS1 [99]. The third cluster of residues (p.phenylalanine 623 to isoleucine, p.tyrosine 702 to histidine) are in the REM domain which interacts with Cdc25. The conserved residue phenylalanine-623 is crucial for the appropriate orientation and hence catalytic functioning of SOS1 [97]. Thus, these mutations primarily affect the residues that regulate the autoinhibition of SOS1. Hence such gain-of-function mutation of SOS1 keeps it on in its active state and hence the signal flow continues. The underlying effect of these mutations is enhancing the binding of SOS to the \begin{table}
\begin{tabular}{c c c} \hline \hline
**S. No.** & **Subtype of NS** & **Mutated Gene** \\ \hline
1 & NS1 & PTPN11 \\
2 & NS2 & LZTR1 \\
3 & NS3 & KRAS \\
4 & NS4 & SOS1 \\
5 & NS5 & RAF1 \\
6 & NS6 & NRAS \\
7 & NS7 & BRAF \\
8 & NS8 & RT1 \\
9 & NS9 & SOS2 \\
10 & NS10 & LZTR1 \\
11 & NS11 & MRAS \\
12 & NS12 & RRAS2 \\
13 & NS13 & MAPK1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Genetic heterogeneity in [PERSON] syndrome with the name of the mutated gene in each subtype. The table is compiled using data from OMIM- Online Mendelian Inheritance in Man®®. [[https://www.omim.org/](https://www.omim.org/)]([https://www.omim.org/](https://www.omim.org/)) (Online Mendelian Inheritance in Man, OMIM®. McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University (Baltimore, MD), World Wide Web URL: [[https://omim.org/](https://omim.org/)]([https://omim.org/](https://omim.org/))) (accessed on 30 May 2021). plasma membrane, which leads to sustained activation. This enhanced activation of SOS in turn leads to the hyperactivation of Ras. Heterozygous mutations in SOS2 (chromosome 14q21) lead to NS9. All mutated residues (p.threonine 264 to lysine, p.methionine 267 to arginine, and p.threonine 376 to serine) lie in the DH domain. Mutations in these residues also hamper the autoinhibitory interaction of DH with REM and result in an unstable SOS2 in the state of rest. The GEF activity in cells expressing the above-mentioned mutations was higher. As this gain-of-function, the mutants showed higher activation of Ras and as well as enhanced MEK and ERK activation [100]. In addition to NS, an insertional mutation (ins) (3248_3249 insC) induced frameshift results in a shorter SOS1 protein that lacks a proline-rich region (PR) involved in autoinhibition. Hence a constitutively active SOS1-Ras axis gives rise to hereditary gingival fibromatosisis (HGF) [101]. In pure mucosal neuroma syndrome, characterized by mucosal neuromas, SOS1 is mutated as well. In addition to frameshift mutation c.3248 dup(duplication); p. (Arg1084 Lysfs*23) (arginine 1084 to lysine resulting in frameshift) [101], two more frameshift mutations (c.3266 dup and c.3254 dup) were identified that also result in gingival hypertrophy [102]. In Costello syndrome (CS) as well as Leopard syndrome (LPRD) SOS1 is mutated at HD and DH domains respectively [97]. SOS1 gene alterations are also identified in a spectrum of sporadic tumors. Methionine 269 to isoleucine/valine, glycine 434 to arginine, arginine 552 to serine/lysine/glycine/methionine, glutamic acid 846 to lysine, alanine 90 to valine/threonine, asparagine 233 to tyrosine/serine were identified in 1% of lung adenocarcinoma and uterine carcinomas [27] (Figure 4).