Donadieu, J. | Search bioRxiv

bioRxiv2025

Mosaicism of BRAF V600E in Healthy Skin Predicts Neurodegeneration in Histiocytosis

Most histiocytoses harbor an oncogenic somatic alteration activating the MAP kinase pathway, the most frequent of which is BRAFV600E. Targeted therapy with BRAF or MEK inhibitors is highly effective, but neurodegeneration remains a severe complication. Recent data suggest that neurodegeneration may be secondary to mosaicism for BRAFV600E in microglia. In the prospective observational TARGET-HISTIO cohort study, we investigated extra-CNS mosaicism of BRAFV600E in a cohort of 124 adults with histiocytosis. Seeking widespread mosaicism, we analyzed healthy skin biopsies from areas without histiocytosis or melanocytic infiltration using high-sensitivity digital polymerase chain reaction. BRAFV600Ewas detected in healthy skin in 57/67 (85%) patients with BRAFV600Ehistiocytosis and was present in all second and third skin biopsies (n=6). BRAFV600Ewas absent from skin biopsies in all 37 patients with histiocytosis harboring another oncogenic mutation. BRAFV600E was also present in skin biopsies of four /20 patients with histiocytosis of unknown molecular status. Cells harboring BRAFV600E share characteristics with resident dermis macrophages. Only 58% of patients with BRAFV600E in skin also had BRAFV600E in blood leukocytes. Variant allele frequency in skin and blood leukocytes was 16 times lower than in histiocytosis, and frequency in skin was independent of treatment with BRAF/MEK inhibitors. None of the patients with wild type BRAF alleles in skin had neurodegeneration, whereas 22/61 (36%) of patients with BRAFV600E in skin had neurodegeneration. This study demonstrates that mosaicism of BRAFV600Ein healthy skin is frequent in patients with histiocytosis and is strongly associated with neurodegeneration. Key PointsLow allele frequency of the oncogene BRAF V600E is frequent in healthy skin of patients with histiocytosis. Patients without BRAF V600E in skin do not develop neurodegeneration, while 36% of those with BRAF V600E mosaicism do.

cancer biology↗

bioRxiv2024

Mechanism of neurodegeneration mediated by clonal inflammatory microglia

Langerhans cell Histiocytosis (LCH) and Erdheim-Chester disease (ECD) are clonal myeloid disorders, associated with MAP-Kinase activating mutations and an increased risk of neurodegeneration. Surprisingly, we found pervasive PU.1+ microglia mutant clones across the brain of LCH and ECD patients with and without neurological symptoms, associated with microgliosis, reactive astrocytosis, and neuronal loss. The disease predominated in the grey nuclei of the rhombencephalon, a topography attributable to a local proliferative advantage of mutant microglia. Presence of clinical symptoms was associated with a longer evolution of the disease and a larger size of PU.1+ clones (p= 0.0003). Genetic lineage tracing of PU.1+ clones suggest a resident macrophage lineage or a bone marrow precursor origin depending on patients. Finally, a CSF1R-inhibitor depleted mutant microglia and limited neuronal loss in mice suggesting an alternative to MAPK inhibitors. These studies characterize a progressive neurodegenerative disease, caused by clonal proliferation of inflammatory microglia (CPIM), with a decade(s)-long preclinical stage of incipient disease that represent a therapeutic window for prevention of neuronal death.

immunology↗