The pathogenic role of the overactivated ABL1 tyrosine kinase (TK) pathway is well recognized in some forms of BCR-ABL1 like acute lymphoblastic leukemia (ALL); TK inhibitors represent a useful therapeutic choice in these patients who respond poorly to conventional chemotherapy. Here we report a novel peptide biosensor (PABL)-ELISA assay to investigate ABL1 activity in four immortalized leukemic cell lines with different genetic background. The PABL sequence comprises an ABL1 tyrosine (Y) phosphorylation site and a targeting sequence which increases the specificity for ABL1; additional peptides (Y-site-mutated (PABL-F) and fully-phosphorylated (PPHOSPHO-ABL) biosensors) were included in the assay. After incubation with whole cell lysates, average PABL phosphorylation was significantly increased (basal versus PABL phosphorylation: 6.84 ± 1.46% versus 32.44 ± 3.25%, p-value < 0.0001, two-way ANOVA, Bonferroni post-test, percentages relative to PPHOSPHO-ABL in each cell line). Cell lines expressing ABL1-chimeric proteins (K562, ALL-SIL) presented the higher TK activity on PABL; a lower signal was instead observed for NALM6 and REH (p<0.001 and p<0.05 versus K562, respectively). Phosphorylation was ABL1-mediated, as demonstrated by the specific inhibition of imatinib (p<0.001 for K562, NALM6, ALL-SIL and p<0.01 for REH) in contrast to ruxolitinib (JAK2-inhibitor), and occurred on the ABL1 Y-site, as demonstrated by PABL-F whose phosphorylation was comparable to basal levels. While requiring further optimization and validation in leukemic blasts to be of clinical interest, the PABL-based ELISA assay provides a novel in vitro tool for screening both the aberrant ABL1 activity in BCR-ABL1 like ALL leukemic cells and their potential response to TK inhibitors.

BACKGROUND: Rhabdomyosarcoma (RMS) is an aggressive malignancy, and 20% of children present with metastases at diagnosis. Patients presenting with disseminated disease very occasionally have no clear evidence of a primary tumor mass. Since these patients have rarely been investigated, we report on a series of patients with RMS and unknown primary tumor site registered in the MTS 2008 protocol (October 2008 - December 2016) coordinated by the European pediatric Soft tissue sarcoma Study Group. METHODS: Patients were administered 9 cycles of induction chemotherapy, and 48 weeks of maintenance chemotherapy. Surgery and/or radiotherapy was planned after the first assessment of tumor response, and implemented after six cycles of chemotherapy. If feasible, radiotherapy to all sites of metastasis was recommended. RESULTS: We identified 10 patients with RMS and unknown primary site, most of them adolescents (median age 15.8 years, range 4.6-20.4). Nine had fusion-positive alveolar RMS. Multiple organ involvement was identified in 7 patients, 2 only had bone marrow disease, and 1 only had leptomeningeal dissemination. All patients were given chemotherapy, 4 were irradiated, and none had surgery. Three patients underwent allogeneic bone marrow transplantation. At the time of this analysis, only 2 patients are alive in complete remission: 1 had received radiotherapy; and 1 had a bone marrow transplant. CONCLUSIONS: RMS with unknown primary tumor occurs mainly in adolescents and is typically fusion-positive alveolar. Radiotherapy may be important, but survival is poor and patients should be offered enrollment in investigational trials.