Molecular Features of JMML

The genomic landscape of JMML is characterized by variants in one of five genes of the RAS pathway: NF1, NRAS, KRAS, PTPN11, and CBL.[405-407] In a series of 118 consecutively diagnosed JMML cases with RAS pathway–activating variants, PTPN11 was the most commonly altered gene, accounting for 51% of cases (19% germline and 32% somatic) (see Figure 5).[405] Patients with NRAS variants accounted for 19% of cases, and patients with KRAS variants accounted for 15% of cases. NF1 variants accounted for 8% of cases, and CBL variants accounted for 11% of cases. Although variants among these five genes are generally mutually exclusive, 4% to 17% of cases have variants in two of these RAS pathway genes,[405-407] a finding that is associated with poorer prognosis.[405,407]

The variant rate in JMML leukemia cells is very low, but additional variants beyond those of the five RAS pathway genes described above are observed.[405-407] Secondary genomic alterations are observed for genes of the transcriptional repressor complex PRC2 (e.g., ASXL1 was altered in 7%–8% of cases). Some genes associated with myeloproliferative neoplasms in adults are also altered at low rates in JMML (e.g., SETBP1 was altered in 6%–9% of cases).[405-408] JAK3 variants are also observed in a small percentage (4%–12%) of JMML cases.[405-408] Cases with germline PTPN11 and germline CBL variants showed low rates of additional variants (see Figure 5).[405] The presence of variants beyond disease-defining RAS pathway variants is associated with an inferior prognosis.[405,406]

A report describing the genomic landscape of JMML found that 16 of 150 patients (11%) lacked canonical RAS pathway variants. Among these 16 patients, 3 were observed to have in-frame fusions involving receptor tyrosine kinases (DCTN1::ALK, RANBP2::ALK, and TBL1XR1::ROS1 gene fusions). These patients all had monosomy 7 and were aged 56 months or older. One patient with an ALK gene fusion was treated with crizotinib plus conventional chemotherapy and achieved a complete molecular remission and proceeded to allogeneic bone marrow transplant.[407]