JMML -LMMJ- (Juvenile myelomonocytic leukemia -JMML-) - Genes PTPN11, NF1, SPECC1, ARHGAP26, NRAS and KRAS

JMML (JMML) is an aggressive hematologic malignancy generated during childhood and is characterized by excessive proliferation and monocyte infiltration and granulocytic cells. This malignancy represents approximately 1% of childhood leukemias and affects very young children, with an average age of 2 years. Affected children have a marked splenomegaly and hepatomegaly, with lymphadenopathy, pallor and rashes. Peripheral blood leukocytosis are common with monocitosis, myeloid / erythroid precursors, anemia and thrombocytopenia.

This may be due to mutations in at least six genes identified: PTPN11, NF1, SPECC1, ARHGAP26, NRAS and KRAS. In a few cases JMML may be predisposed by other conditions, namely the neurofibromatosis type 1 and Noonan syndrome - see Neurofibromatosis type 1-gene NF1 and Noonan syndrome ... -genes PTPN11, SOS1, RAF1, KRAS , NRAS and BRAF.

In 80-90% of individuals affected by juvenile myelomonocytic leukemia can be detected genetic alterations in KRAS, NRAS, NF1 and PTPN11 genes that cause deregulated activation of RAS signaling pathway. Mutations in these genes are mutually exclusive, which clarified the pathogenetic importance of the RAS pathway in JMML. In fact, activation of the RAS pathway is an essential step in the proliferative response of cells to most hematopoietic growth factors.

25-30% of cases of juvenile myelomonocytic leukemia have been detected somatic mutations in NRAS and KRAS genes of. RAS genes (NRAS, KRAS and HRAS although the latter is not involved so far in this disease) encoding G similar proteins involved in signal transduction of growth and differentiation from receptor tyrosine kinases to cells core. The NRAS gene is located on the short arm of chromosome 1 (1p22-p32), while KRAS is on the short arm of chromosome 12 (12p12.1). The coding sequence of the three genes RAS is equally distributed in four exons except KRAS, the fourth exon has two alternative forms transcribed (K-RASA and RASB K-). Under normal conditions, Ras proteins are in equilibrium between the active (GTP - bound) form and the inactive (GDP - bound). However, the mutated forms of Ras lose the ability to hydrolyze GTP and always remain in its active form. The most frequent mutations in ras genes described so far are located in codons 12 and 13 (increase affinity for GTP) and at codons 59 and 61 (inactivating the autocatalytic GTPase function). As a consequence of these mutations, uncontrolled cell proliferation mediated cell cycle in steady state synthesis, favoring tumor development and excessive proliferative response of cells to hematopoietic growth factors appears.

35-40% of cases have been detected somatic mutations distributed by PTPN11, especially in exons 2, 3, 4, 7, 8, 9, 12 and 13. This gene is located on the long arm of chromosome 12 (12q24) and encodes a tyrosine phosphatase Src homology 2 (SHP2) containing two domains of Src homology-2 (SH2 and N-C-SH2) and a phosphatase catalytic domain. Protein tyrosine phosphatase family are known to be signaling molecules that regulate many cellular processes such as cell growth, differentiation, mitotic cycle and oncogenic transformation processes. Disturbances occurring in the sequence encoding these domains cause changes in the interactions between the N-SH2 domains and phosphatase, necessary for maintaining a closed and inactive conformation domains SHP2. Thus, the SHP2 preferably mutated proteins remain in an open and active conformation and are thus able to activate the RAS signaling pathway and leading to overproduction of leukocytes. Mutations are reported in the PTPN11 gene associated with juvenile myelomonocytic leukemia -somáticas- are different from those associated with Noonan syndrome -germinales-. The first confer greater effects on phosphatase activity than the latter, suggesting that somatic mutations occurring in JMML provide a gain of function more severe than germline mutations associated with Noonan syndrome, possibly because some of these somatic mutations could not be tolerated in the germline.

Neurofibromin, a protein of 2818 amino acids encoded by the NF1 gene is located on the long arm of chromosome 17 (17q11.2), it is a protein with a tumor suppression function by downregulating Ras. Genetic alterations distributed over the 60 exons of the gene NF1 He detected over 1,000 different - they are responsible for neurofibromatosis type 1, from which can be generated JMML, as occurs in the 11% of cases. Additionally, 10-15% of cases of juvenile myelomonocytic leukemia can be caused by mutations in the NF1 gene diagnosis without neurofibromatosis type 1. Under normal conditions, neurofibromin has a region with GTPase activity that binds to and modulates the Ras oncogene converting its active form (Ras-GTP) to its inactive (Ras-GDP). Mutations in the NF1 gene cause a decrease of its presence and / or activity, resulting in Ras - mediated cell proliferation.

In addition to the mutations described above, they have identified certain genetic alterations that may be responsible for juvenile myelomonocytic leukemia. Among them, a chromosome aberration involving SPECC1 gene (CYTSB), located on the short arm of chromosome 17 (17p11.2) consisting of a t (5; 17) (q33; p11.2) with PDGFRB; and an alteration in the ARHGAP26 gene, located on the long arm of chromosome 5 (5q31) due to chromosomal translocation (5; 11) (q31; q23) with KMT2A / mLL1. The latter translocation has been identified in the leukemic cells of patients with JMML, also carry inactivating mutations in the second allele.

Tests in IVAMI: in IVAMI perform mutation detection associated with the development of juvenile myelomonocytic leukemia (JMML), by complete PCR amplification of the exons of PTPN11, NF1, SPECC1, ARHGAP26, NRAS and KRAS, respectively, and subsequent sequencing. It is recommended to begin the study by exons 2, 3, 4, 7, 8, 9, 12 and 13 of PTPN11, where they are most mutations associated with this gene, and continue by NRAS and KRAS, of small size, which accumulate between one third and one quarter of cases. If negative, it is suggested to proceed with the remaining seven exons of PTPN11 finally, if desired, by sequencing the NF1, SPECC1 and ARHGAP26 gene. We recall that the mutations are mutually exclusive, so that the detection of an alteration associated with the development of the disease stop the study of genes and / or subsequent exons, thus saving time and costs.

Samples recommended: EDTA blood collected for separation of blood leukocytes, or impregnated sample card with dried blood (IVAMI may mail the card to deposit the blood sample).