EGFR mutations in GBM cluster in the extracellular (EC) domain and include in-frame deletions (such as the common “variant III” del 6-273) and missense mutations (A289V, A289D, T263P, G598V). Mutations involving residue R108 have been reported in GBM, frequently occurring with other EGFR mutations at amino acids A289, P596, and G598. In vitro studies have shown that R108K mutation leads to increased ligand-binding affinity and shows anchorage-independent growth and tumorigenic potential when stably expressed in NIH-3T3 cells. The predictive and prognostic significance of this mutation at R108 needs further elucidation. Correlation with other clinical and laboratory findings is recommended.

Somatic mutations in the tyrosine kinase domain of the epidermal growth factor receptor (EGFR) gene are present in approximately 80% of the lung adenocarcinomas that respond to first and second generation EGFR inhibitors (eg, gefitinib, erlotinib and afatinib). Two types of mutations account for approximately 80-90% of all EGFR mutations: short in-frame deletions in Exon 19 and a point mutation in exon 21 at codon 858 (L858R). Other less common mutations in exons 18, 20, and 21 are found in 10-20% of EGFR-mutated cases. EGFR Exon 19 deletions, EGFR Exon 21 L858R and EGFR Exon 18 G719 mutations correlate strongly with sensitivity to specific EGFR inhibitors and the response rate to therapy with TKIs has been reported to be up to 80% in such cases. The T790M mutation in exon 20 is associated with resistance to some EGFR inhibitors. However, third generation TKI (eg, osimertinib) can specifically target T790M. EGFR R108K (C.323G>A) is a rare missense mutation in Exon 3. In a study of 1006 lung carcinomas, R108K mutation was found concomitantly with other EGFR mutations - most notably p.L858R (Illei et.al). However, its prognostic and therapeutic significance remains to be fully elucidated.