Constitutively activating MEK1 mutations drive treatment refractory histiocytic neoplasms but confer response to ERK inhibition. Constitutively activating MEK1 mutations drive treatment refractory histiocytic neoplasms but confer response to ERK inhibition

Histiocytic neoplasms are clonal disorders of the monocyte/macrophage lineage defined by mutations activating MAP kinase signaling, including BRAFV600E mutations in ~50% of patients, which confer robust responses to BRAF inhibition. More recently, the MEK inhibitor cobimetinib was FDA-approved for BRAFV600-wild-type histiocytosis patients. Here following genomic analysis of 500 pediatric and adult patients with diverse histiocytoses, the most common alteration following BRAFV600E was MEK1E102_I103 in-frame deletion. MEK1E102_I103del and related RAF-independent MEK mutants were associated with an aggressive multi-system clinical phenotype with worse progression-free survival with MEK inhibition as compared to patients with other classes of MEK1/2 mutations. Given that cancer-associated MEK1/2 mutations have not been modeled in vivo and the clinical importance of MEK1 mutations in histiocytoses, we generated MEK1E102_I103del conditional knock-in mice. Using three distinct hematopoietic Cre drivers, these mice developed a 100% penetrant, lethal disorder by 6 weeks of age with expansion of classical and non-classical monocytes, macrophages, and Cd11b+ conventional dendritic cells which infiltrated hematopoietic organs, liver, and skin. MEK1-mutant mice were sensitive to single-agent treatment with the oral ERK inhibitor ulixertinib. We consequently treated five MEK1E102_I103del-mutant patients with ulixertinib on prospective IRB-approved Single Patient Use Expanded Access protocols, four of whom had disease refractory to MEK inhibition. Four of five patients experienced objective clinical and/or radiological responses to ulixertinib. These data reveal the impact of oncogenic MEK kinase mutations in vivo, identify patients with likelihood of resistance to MEK inhibition, and nominate ERK inhibition as a therapeutic approach to overcome resistance to MEK inhibition in histiocytoses. Overall design: Sorted live cells were harvested from frozen/thawn bone marrow and spleen samples from Mx1-cre-Map2k1-E102_I103del mutant (n=3) and control mice (n=3) mice for 3' scRNA sequencing.