Uncommon EGFR Mutations: G719X, S768I and L861Q
Beyond exon 19 deletions, L858R and exon 20 insertions, EGFR-mutant lung cancer includes a set of uncommon mutations, chiefly G719X, S768I and L861Q. These are sensitising to some EGFR inhibitors but with different data behind them, and they often occur as compound mutations.
Quick Answer
Beyond exon 19 deletions, L858R and exon 20 insertions, EGFR-mutant lung cancer includes a set of uncommon mutations, chiefly G719X, S768I and L861Q. These are sensitising to some EGFR inhibitors but with different data behind them, and they often occur as compound mutations.
The Main Uncommon Mutations
G719X refers to substitutions at codon 719 in exon 18 (G719A, G719S, G719C). S768I sits in exon 20 but, unlike exon 20 insertions, retains sensitivity to some inhibitors. L861Q is in exon 21, near L858R.
Together these account for roughly 7 to 10 percent of EGFR-mutant non-small-cell lung cancer.
Drug Sensitivity
These mutations activate EGFR but the drug-binding pocket is affected differently from classical mutations. Observational and pooled analyses suggest the second-generation irreversible inhibitor afatinib has useful activity, and it carries a specific label for G719X, S768I and L861Q in some regions.
Third-generation osimertinib also has activity against these mutations in trials and case series, though the evidence base is smaller than for classical mutations.
Compound Mutations
Uncommon EGFR mutations frequently occur together, for example G719X with S768I, or G719X with L861Q. The response of a compound mutation can differ from either component alone.
A compound mutation that includes a classical sensitising mutation generally behaves like the classical mutation; combinations of only uncommon mutations are less predictable.
Detection
Broad next-generation sequencing detects these mutations reliably. Some targeted PCR panels include the common uncommon mutations but not all, so a negative limited panel does not fully exclude them.
The exact amino-acid change matters, so reports should give the specific variant rather than only the category.
Interpretation Notes
S768I in isolation should not be confused with an exon 20 insertion; despite both being in exon 20, their drug sensitivity differs.
Treatment choice for uncommon mutations is individualised and based on a smaller evidence base than for classical EGFR mutations.
Key Takeaways
- ·G719X, S768I and L861Q are sensitising EGFR mutations distinct from exon 19 deletions and L858R.
- ·Afatinib has a specific indication for them; osimertinib also shows activity.
- ·They often occur as compound mutations, which changes the expected response.
- ·S768I is not an exon 20 insertion despite its location.
Put these genes in pathway context
Frequently asked questions
What is the key idea in Uncommon EGFR Mutations: G719X, S768I and L861Q?
Beyond exon 19 deletions, L858R and exon 20 insertions, EGFR-mutant lung cancer includes a set of uncommon mutations, chiefly G719X, S768I and L861Q. These are sensitising to some EGFR inhibitors but with different data behind them, and they often occur as compound mutations.
What should be kept with the result or mechanism?
Afatinib has a specific indication for them; osimertinib also shows activity. They often occur as compound mutations, which changes the expected response. S768I is not an exon 20 insertion despite its location.
References
- 1Afatinib in Untreated Stage IIIB/IV Lung Adenocarcinoma with Major Uncommon EGFR Mutations (G719X/L861Q/S768I). Targeted Oncology, 2023. PubMed
- 2Updated Molecular Testing Guideline for the Selection of Lung Cancer Patients for Treatment With Targeted Tyrosine Kinase Inhibitors. Archives of Pathology & Laboratory Medicine, 2018. PubMed
- 3EGFR and HER2 exon 20 insertion mutations in lung cancer: a narrative review of approved targeted therapies. Translational Lung Cancer Research, 2023. PubMed
- 4Mechanisms of acquired resistance to targeted cancer therapies. Nature Reviews Cancer, 2016. PubMed
Continue Reading
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