BRAF, NRAS, and ARAF variants

The genomic basis of LCH was advanced by a 2010 report of an activating variant of the BRAF oncogene (V600E) that was detected in 35 of 61 cases (57%).[1] Multiple subsequent reports have confirmed the presence of BRAF V600E variants in 50% or more of LCH cases in children.[2-4] Other BRAF variants that result in signal activation have been described.[3,5] ARAF variants are infrequent in LCH but, when present, can also lead to RAS-MAPK pathway activation.[6]

The presence of the BRAF V600E variant in blood and bone marrow was studied in a series of 100 patients, 65% of whom tested positive for the BRAF V600E variant by a sensitive quantitative polymerase chain reaction technique.[2] Circulating cells with the BRAF V600E variant could be detected in all high-risk patients and in a subset of low-risk multisystem patients. The BRAF V600E allele was detected in circulating cell-free DNA in 100% of patients with risk-organ–positive multisystem LCH, 42% of patients with risk-organ–negative LCH, and 14% of patients with single-system LCH.[7]

The myeloid dendritic cell origin of LCH was confirmed by finding CD34-positive stem cells with the variant in the bone marrow of high-risk patients. In those with low-risk disease, the variant was found in more mature myeloid dendritic cells, suggesting that the stage of cell development at which the somatic variant occurs is critical in defining the extent of disease in LCH.

Pulmonary LCH in adults was initially reported to be nonclonal in approximately 75% of cases,[8] while a later study of BRAF variants showed that 25% to 50% of adult patients with lung LCH had evidence of BRAF V600E variants.[8,9] Another study of 26 pulmonary LCH cases found that 50% had BRAF V600E variants and 40% had NRAS variants.[10] Approximately the same number of variants are polyclonal as are monoclonal. It has not been determined whether clonality and BRAF pathway variants are concordant in the same patients, which might suggest a reactive rather than a neoplastic condition in smoker's lung LCH and a clonal neoplasm in other types of LCH.

In a study of 117 patients with LCH, 83 adult patients with pulmonary LCH underwent molecular analysis. Nearly 90% of these patients had variants in the MAPK pathway.[11][Level of evidence C3] Of the 69 patients who had their biopsy samples further analyzed using a next-generation sequencing panel of 74 genes, 36% had BRAF V600E variants, 29% had BRAF N486-P490 deletions, 15% had MAP2K1 variants or deletions, and 4% had NRAS variants. Only one patient had a KRAS variant. Additionally, 11 patients had their biopsy samples analyzed using whole-exome sequencing. An average of 14 variants were found per patient, which is markedly higher than the average of one variant found per pediatric patient.[12] There were no clinical correlates, including presence of a BRAF V600E variant and smoking status. Of the 117 patients with LCH, 60% experienced a relapse.

The RAS-MAPK signaling pathway (see Figure 8) transmits signals from a cell surface receptor (e.g., a growth factor) through the RAS pathway (via one of the RAF proteins [A, B, or C]) to phosphorylate MEK and then the extracellular signal-regulated kinase (ERK), which leads to nuclear signals affecting cell cycle and transcription regulation. The V600E variant of BRAF leads to continuous phosphorylation, and thus activation, of MEK and ERK without the need for an external signal. Activation of ERK occurs by phosphorylation, and phosphorylated ERK can be detected in virtually all LCH lesions.[1,13]

In a mouse model of LCH, the BRAF V600E variant was shown to inhibit a chemokine receptor (CCR7)–mediated migration of dendritic cells, forcing them to accumulate in the LCH lesion.[14] This variant also causes an increased expression of BCL2L1, which results in resistance to apoptosis. This process leads to the cells being less responsive to chemotherapy. The BRAF V600E variant also causes growth arrest of hematopoietic progenitor cells and a senescence-associated secretory phenotype that further promotes accumulation of the pathological cells.[15]

Another mouse model with the BRAF V600E variant under control of Scl or Map17 gene promoters added additional insights into the biology of neurodegenerative LCH.[16] These studies confirmed the hematopoietic origin of CD11a-positive macrophages with BRAF V600E variants. This process disrupts the blood-brain barrier and causes loss of Purkinje cells and progressive neurodegeneration by resistance to apoptosis and production of senescent associated secretory proteins, which include inflammatory cytokines IL-1, IL-6, and matrix metalloproteinases. Treatment with a MAP kinase inhibitor and a senolytic agent (navitoclax) decreased the pathogenic cell numbers and led to clinical improvement in the mice.

In summary, LCH is now considered a myeloid neoplasm primarily driven by activating variants of the MAPK pathway. Fifty percent to 60% of the activating variants are caused by BRAF V600E variants, which are enriched in patients with multisystem risk organ–positive LCH and in patients with neurodegenerative-disease LCH.[17] Ongoing studies are assessing whether low-level variant detection in peripheral blood can be used as a minimal residual disease marker to assist in therapeutic decisions.