A novel GAB2::BRAF fusion in cutaneous non-Langerhans-cell histiocytosis with systemic involvement.

Several mutations and gene fusions involved in the mitogen-activated protein kinase (MAPK) pathway have been reported in histiocytic neoplasms including Langerhans cell histiocytosis and non-Langerhans-cell histiocytosis (NLCH). We identified a GAB2::BRAF fusion in a cutaneous lesion from a 22-year-old woman who presented with central diabetes insipidus and red/brown papules on her face, oral mucosa, axilla, and groin. Skin biopsy showed a CD68+, S100-, and CD1a- histiocytic proliferation consistent with NLCH, best clinically classified as xanthoma disseminatum. Next-generation sequencing identified a GAB2::BRAF fusion involving exon 2 of GAB and exon 10 of BRAF. This case implicates a novel fusion in the MAPK signaling pathway, not previously reported in histiocytic neoplasms, as a possible driver of NLCH. Our findings underscore the utility of performing molecular studies on skin biopsy specimens with NLCH to help identify potential targets for therapy.