IRS1 and SHP2 Signaling in Myelodysplastic Syndrome and Acute Myeloid Leukemia.

Myelodysplastic syndrome (MDS) encompasses a heterogeneous group of clonal hematopoietic stem cell disorders characterized by ineffective hematopoiesis, refractory cytopenia and a tendency to progress towards acute myeloid leukemia (AML). The occurrence of acute leukemia results from a combination of mutations and changes in protein functions that confer the ability of proliferation, defects in cell differentiation and apoptosis. The deregulation of multiple cellular signaling pathways in MDS and acute leukemia hinders the development of an effective drug for treatment. SHP2 is a cytoplasmic tyrosine phosphatase encoded by the gene PTPN11. The SHP2 signaling pathway is involved in important functions as regulation of proliferation, differentiation and cell migration and has been described as overexpressed and constitutively phosphorylated in acute and chronic leukemic cells. The IGF1/IRS1 signaling pathway has an important role in the development of various cancers such as breast, colon and prostate cancer and multiple myeloma. Increased expression of IRS1 has been described in neoplastic transformation, and reduction of IRS1 expression has been related to the level of differentiation of neoplastic cells and with a poor prognosis in solid tumors. In acute lymphoblastic leukemia (ALL), IRS1 expression was noted to be overexpressed and correlated with prognosis. The expression of SHP2 and IRS1 in MDS has not yet been elucidated, as well the expression of IRS1 in AML. The aim of the present study was to investigate the gene and protein expression of IRS1 and SHP2 in MDS and AML patients, and also PTPN11 exon 3 mutations.

We studied 06 healthy donors, 26 patients with MDS (18 low-risk [RA/RARS] and 07 high-risk [RAEB/RAEBt] according to FAB classification), 24 with AML and 5 with ALL. Samples were obtained from bone marrow. Gene expression was evaluated by real time RT-PCR in total cells. Western blot of mononuclear cells was performed for protein expression. PTPN11 mutations were analyzed by PCR with specific primers and sequencing.

Real time RT-PCR demonstrated a reduced expression of IRS1 in MDS cells (0.13 [0.01-1.19], P=0.01) and AML cells (0.10 [0.009-7.14], P=0.02), when compared to normal cells (0.62 [0.10-1.41]). On the other hand, IRS1 expression was significantly increased in LLA samples (2.35[0.73-11.64] P=0.03) when compared to normal cells. Western blot analysis confirmed the gene expression pattern: IRS1 expression and phosphorylation were down-regulated in MDS and AML and upregulated in ALL when compared to normal hematopoietic cells. Regarding PTPN11 expression, no difference was detected in MDS (1.03 [0.07-4.96], P >0.05) and AML cells (0.48 [0.17-2.29], P >0.05) when compared with normal hematopoietic cells (0.56 [0.2-1.0]). Interesting, PTPN11 expression was increased in low risk MDS (1.35 [0.12-4.96], P=0.048) or total MDS (1.03 [0.07-4.96], P =0.039) when compared to AML cells. Western blot analysis confirmed the SHP2 expression pattern. However, SHP2 phosphorylation was marked increased in both MDS and AML compared to normal hematopoietic cells. PTPN11 mutation was not detected in the samples.

Our results show that IRS1 expression and phosphorylation is down-regulated in MDS and AML patients compared to normal hematopoietic cells, suggesting that IGF1/IRS1 signaling pathway is differently regulated in normal cells, MDS, AML and ALL. IRS1 overexpression in ALL is in agreement with previous authors and indicates IRS1 as a possible target in ALL, thought not MDS and AML. SHP2 activation is increased in MDS and AML, and was independent from PTPN11 mutation. The increased activation of SHP2 in MDS, not previously described, indicates that this tyrosine phosphatase may play a role in MDS pathophysiology and leukemia transformation.

Keywords: IRS1, SHP2, leukemia, myelodysplasia

No relevant conflicts of interest to declare.