Oncogenic MAPK pathway activation induces a congenital and progressively lethal rare neuropathy in mice

RASopathies, rare congenital syndromes affecting multiple organ systems, often include peripheral neuropathy of unknown origin. While RASopathy-associated gene variants are proto-oncogenic, the impact of timing and mosaicism on pathogenicity remains poorly understood. Here, we investigate the links between Braf, a key mitogen-activated protein kinase (MAPK) effector, and peripheral neuropathy. By targeting Braf p.V600E, an oncogenic variant found in mosaic RASopathies, to embryonic Mpz-expressing cells in mice, we induced a congenital Charcot-Marie-Tooth-like degenerative neuropathy. This phenotype was characterized by hyperplastic nerves, hindlimb weakness, and unexpectedly reduced body size. Constitutively active Braf expanded a Jun+ Schwann cell repair state, impairing myelination and nerve homeostasis. To examine relevance to RASopathies, we differentiated patient-derived stem cells bearing the cardio-facio-cutaneous syndrome-associated BRAF p.Q257R variant into Schwann cells. Compared to wild-type controls, CFC-derived cells failed to acquire mature phenotypes, instead exhibiting progenitor or repair-type transcriptional profiles. Our findings implicate somatic mosaicism in the unresolved genetic heterogeneity of neuropathies and expand the candidate gene list for peripheral nerve disorders. Moreover, they reveal a MAPK-dependent mechanism linking neural crest-derived Schwann cell dysfunction to both body growth and nerve homeostasis, providing new insights into the mechanisms in RASopathy-associated neuropathy and potential therapeutic targets.