Clonal Hematopoiesis in Erdheim-Chester Disease

Rationale: In patients with Erdheim-Chester disease (ECD), accumulation of foamy histiocytes leads to multi-systemic disease with various organs involvement. During the past few years, we showed that Langerhans-cell histiocytosis (LCH) and other hemopathies were frequently associated with ECD. These findings, as well as the fact that BRAF^V600Emutation is found in as much as 70% of ECD tissues, led to the reclassification of ECD as a myeloid neoplasm.

Methods: We conducted a systematic next-generation sequencing (NGS) using bone marrow samples obtained from 69 patients with ECD (49 males, median age at the time of NGS 62.68 years SD 13.33, median time of evolution since ECD diagnosis 4.2 years). We performed molecular analysis using high-throughput sequencing focused on 36 genes recurrently mutated in myeloid malignancies. Libraries were prepared using the Ampliseq System according to the manufacter's instructions and run on Ion Proton (Thermofisher). Raw data were analyzed with both Torrent Browser (Thermofisher) and SeqNext (JSI Medical System).

Results: Overall, 75 somatic mutations other than BRAF^V600E in myeloid cancer candidate genes were present in 29 patients (42%), in a limited number of genes. Seventeen of these 29 patients (59%) had multiple mutations (range: 2-7 mutations). The most frequently mutated genes were TET2 , ASXL1 , DNMT3A and NRAS (in 16, 7, 5 and 4 patients respectively). CBL , KRAS , JAK2 and SRSF2 were mutated in 3 patients each. These genes accounted for 77% of all mutation-positive patients. Both the presence and the number of mutations per patients positively correlated with age (p=0.0055 and p=0.0069 respectively). Some patients harbored multiple mutations in the same genes, including TET2 , ASXL1 and NRAS . Six out of 41 patients (15%) carried karyotypic abnormalities. Twelve patients (17%) had a definite myeloproliferative neoplasm other than ECD (mainly CMML in 7 patients). Additionally, BRAF^V600E was present in tissues histiocytes of 40/61 patients (not determined in 8 others) and in 13/60 patients in bone marrow (not determined in 9), some of the latter being treated with BRAF inhibitors. Two patients had a BRAF^V600E mutation detectable in bone marrow but not in tissues. The presence of at least one TET2 mutation was associated with the presence of the BRAF^V600E mutation in tissues (p=0.04). Among the 40 patients with the absence of somatic mutation in the panel of myeloid genes, only 17 were BRAF^V600E Wild-Type and therefore showed no evidence of clonal hematopoiesis. These patients were characterized by less associated LCH (0 versus 23%; p=0.03), less vascular (41 versus 73%; p=0.02) and less cardiac (29 versus 69%; p=0.005) involvements.

Conclusions: A subset of genes that are mutated in patients with myeloid neoplams is frequently mutated in ECD. These mutations may represent early events in the development of hematologic malignancies.