ARID1A genomic alterations driving microsatellite instability through somatic MLH1 methylation with response to immunotherapy in metastatic lung adenocarcinoma: a case report.

BACKGROUND: Tumor molecular screening allows categorization of molecular alterations to select the best therapeutic strategy. AT-rich interactive domain-containing 1A (ARID1A) gene mutations are present in gastric, endometrial, and clear cell ovarian tumors. Inactivation of this gene impairs mismatch repair (MMR) machinery leading to an increased mutation burden that correlates with microsatellite instability (MSI), associated with tumor-infiltrating lymphocytes and programmed death ligand 1 (PD-L1) expression. This is the first case report in lung adenocarcinoma of ARID1A gene alterations leading to sporadic MSI, through somatic mutL homolog 1 (MLH1) promoter methylation, with an MLH1 gene mutation as the second somatic hit. CASE PRESENTATION: A 50-year-old never-smoker Bulgarian woman, with no comorbidities and no family history of cancer, was diagnosed with metastatic lung adenocarcinoma. PD-L1 immunohistochemistry (IHC) of tissue biopsies on right groin adenopathies resulted in 30% positivity. Liquid biopsy test reported actionable alterations in ARID1A gene, rearranged during transfection (RET) gene fusions, epidermal growth factor receptor (EGFR) gene R776H mutation, breast cancer (BRCA) genes 1/2, and cyclin-dependent kinase inhibitor 2A (CDKN2A) gene mutations. The patient was treated with immunotherapy, and showed a treatment response lasting for 19 months until a new metastasis appeared at the right deltoid muscle. Genomic analysis of a sample of this metastasis confirmed PD-L1 positivity of greater than 50% with CD8+ T cells expression and showed MSI with a deleterious c.298C>T (p.R100*) MLH1 gene mutation. Multiplex ligation-dependent probe amplification (MLPA) of this sample unveiled MLH1 gene promoter methylation. The MLH1 gene mutation and the MLH1 gene methylation were not present at the germline setting. CONCLUSIONS: In this particular case, we show that ARID1A gene mutations with sporadic MSI due to somatic MLH1 gene promoter methylation and MLH1 gene mutation could change the prognosis and define the response to immunotherapy in a patient with lung adenocarcinoma. Comprehensive solid and liquid biopsy tests are useful to find out resistance mechanisms to immune checkpoint inhibitors. Our data encourages the development of new therapies against ARID1A mutations and epigenomic methylation when involved in MSI neoplasms.