RAS and CSF3R mutations in severe congenital neutropenia

To the editor:

Severe congenital neutropenia (CN), a congenital disorder of hematopoiesis and a preleukemic predisposition, has been used as a model for multistep pathogenesis of leukemia. CSF3R gene mutations are regarded as an early marker of malignant transformation in CN.^1,2  A commonly accepted model is that the acquisition of CSF3R mutations, which confer a growth advantage to myeloid progenitors,³  is an early and specific event, typically followed by other nonspecific genetic changes necessary for complete transformation. Common genetic abnormalities like acquired clonal cytogenetic alterations^4,5  or activating RAS mutations⁵  have also been observed to be associated with CN-related myelodysplastic syndrome (MDS)/leukemia. The assumption of RAS mutations as frequent genetic aberrations in the malignant transformation of CN^6,7  is mainly based on the original study on RAS mutations in CN by Kalra et al⁵  (5/11 patients with CN and MDS/leukemia).

To further investigate the role of RAS mutations in CN, we screened for mutations of KRAS and NRAS in 45 CN patients. The patients were selected for secondary malignancies (n = 15) or for detection of a CSF3R mutation (n = 30). Surprisingly, we detected RAS mutations in only 2 CN patients with malignant transformation (Table 1 no. 9, NRAS c.37G > C, p.G13R; no. 10, KRAS c.182A > C, p.Q61P). Chromosomal abnormalities were present in all 12 patients with secondary malignancies for whom data were available and included monosomy 7 (n = 4), trisomies (n = 3), deletions (n = 2), and translocations (n = 5). Truncating CSF3R mutations²  were detected in 9 of 15 patients with secondary malignancies (Table 1). The group of CN patients with secondary malignancies with CSF3R mutations did not obviously differ from that without CSF3R mutations with regard to the genetic background of CN or to specific chromosomal anomalies.

Because there was an obvious discrepancy between the data of the original study of Kalra et al and the results of our present study (P < .05 using Fisher exact probability test), we asked the authors to send us DNA from their RAS mutation–positive CN patients to screen these specimens for CSF3R mutations. DNA was available for 3 of the 5 patients with RAS mutations described there. Although we could demonstrate that the majority of patients with CN and MDS/leukemia harbor acquired CSF3R mutations (18/23, 78%),²  none of these CN patients with secondary MDS/leukemia and RAS mutation had a CSF3R mutation (Table 1). Interestingly, in a recent study investigating patterns of mutations in de novo acute myeloid leukemia (AML) and AML secondary to CN, the authors also detected a very low percentage of RAS mutations in the group of CN AML/MDS patients (1/14, 7%).⁹  With the exception of one patient with CN and secondary chronic myelomonocytic leukemia the occurrence of CSF3R and RAS mutations was mutually exclusive.

A model of cooperativity of genetic aberrations in leukemogenesis has been proposed postulating at least 2 broad complementation groups of mutations: one class of mutations that confers a proliferative and/or survival advantage to hematopoietic progenitors, and another group of mutations resulting in a differentiation block in hematopoiesis.¹⁰  The results of our study demonstrate that CSF3R and RAS mutations most likely do not act cooperatively in leukemogenesis, as generally agreed so far, but might have overlapping functions and may replace each other in the deregulation of common signaling pathways.