MSH6 haploinsufficiency at relapse contributes to the development of thiopurine resistance in pediatric B-lymphoblastic leukemia

Survival of children with relapsed acute lymphoblastic leukemia is poor, and understanding mechanisms underlying resistance is essential to developing new therapy. Relapse-specific heterozygous deletions in MSH6, a crucial part of DNA mismatch repair, are frequently detected. Our aim was to determine whether MSH6 deletion results in a hypermutator phenotype associated with generation of secondary mutations involved in drug resistance, or if it leads to a failure to initiate apoptosis directly in response to chemotherapeutic agents. We knocked down MSH6 in mismatch repair proficient cell lines (697 and UOCB1) and showed significant increases in IC50s to 6-thioguanine and 6-mercaptopurine (697: 26- and 9-fold; UOCB1: 5- and 8-fold) in vitro, as well as increased resistance to 6-mercaptopurine treatment in vivo. No shift in IC50 was observed in deficient cells (Reh and RS4;11). 697 MSH6 knockdown resulted in increased DNA thioguanine nucleotide levels compared to non-targeted cells (3070 vs. 1722 fmol/μg DNA) with no difference observed in mismatch repair deficient cells. Loss of MSH6 did not give rise to microsatellite instability in cell lines or clinical samples, nor did it significantly increase mutation rate, but rather resulted in a defect in cell cycle arrest upon thiopurine exposure. MSH6 knockdown cells showed minimal activation of checkpoint regulator CHK1, γH2AX (DNA damage marker) and p53 levels upon treatment with thiopurines, consistent with intrinsic chemoresistance due to failure to recognize thioguanine nucleotide mismatching and initiate mismatch repair. Aberrant MSH6 adds to the list of alterations/mutations associated with acquired resistance to purine analogs emphasizing the importance of thiopurine therapy

Characterization of COVID-19 Disease in Pediatric Oncology Patients: The New York-New Jersey Regional Experience

PURPOSE: Pediatric oncology patients undergoing active chemotherapy are suspected to be at a high risk for severe disease secondary to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection; however, data to support this are lacking. We aim to describe the characteristics of coronavirus disease 2019 (COVID-19) in this population and also its impact on pediatric cancer care in the New York region during the peak of the pandemic. PATIENTS AND METHODS: This multicenter, retrospective study included 13 institutions. Clinical and laboratory information on 98 patients ≤21 years of age receiving active anticancer therapy, who tested positive for SARS-CoV-2 by nasopharyngeal swab polymerase chain reaction (PCR), was collected. RESULTS: Of the 578 pediatric oncology patients tested for COVID-19, 98 were positive, of whom 73 were symptomatic. Most experienced mild disease, 28 required inpatient management, 25 needed oxygen support, and seven required mechanical ventilation. There is a slightly higher risk of severe disease in males and obese patients, though not statistically significant. Persistent lymphopenia was noted in severe cases. Delays in cancer therapy occurred in 67% of SARS-CoV-2-positive patients. Of four deaths, none were solely attributable to COVID-19. The impact of the pandemic on pediatric oncology care was significant, with 54% of institutions reporting delays in chemotherapy, 46% delays in surgery, and 30% delays in transplant. CONCLUSION: In this large multi-institutional cohort, we observed that mortality and morbidity from COVID-19 amongst pediatric oncology patients were low overall, but higher than reported in general pediatrics. Certain subgroups might be at higher risk of severe disease. Delays in cancer care due to SARS-CoV-2 remain a concern