DNA Polymerase and Mismatch Repair Exert Distinct Microsatellite Instability Signatures in Normal and Malignant Human Cells.

Although replication repair deficiency, either by mismatch repair deficiency (MMRD) and/or loss of DNA polymerase proofreading, can cause hypermutation in cancer, microsatellite instability (MSI) is considered a hallmark of MMRD alone. By genome-wide analysis of tumors with germline and somatic deficiencies in replication repair, we reveal a novel association between loss of polymerase proofreading and MSI, especially when both components are lost. Analysis of indels in microsatellites (MS-indels) identified five distinct signatures (MS-sigs). MMRD MS-sigs are dominated by multibase losses, whereas mutant-polymerase MS-sigs contain primarily single-base gains. MS deletions in MMRD tumors depend on the original size of the MS and converge to a preferred length, providing mechanistic insight. Finally, we demonstrate that MS-sigs can be a powerful clinical tool for managing individuals with germline MMRD and replication repair-deficient cancers, as they can detect the replication repair deficiency in normal cells and predict their response to immunotherapy. SIGNIFICANCE: Exome- and genome-wide MSI analysis reveals novel signatures that are uniquely attributed to mismatch repair and DNA polymerase. This provides new mechanistic insight into MS maintenance and can be applied clinically for diagnosis of replication repair deficiency and immunotherapy response prediction.This article is highlighted in the In This Issue feature, p. 995.

Germline-driven replication repair-deficient high-grade gliomas exhibit unique hypomethylation patterns.

Replication repair deficiency (RRD) leading to hypermutation is an important driving mechanism of high-grade glioma (HGG) occurring predominantly in the context of germline mutations in RRD-associated genes. Although HGG presents specific patterns of DNA methylation corresponding to oncogenic mutations, this has not been well studied in replication repair-deficient tumors. We analyzed 51 HGG arising in the background of gene mutations in RRD utilizing either 450 k or 850 k methylation arrays. These were compared with HGG not known to be from patients with RRD. RRD HGG harboring secondary mutations in glioma genes such as IDH1 and H3F3A displayed a methylation pattern corresponding to these methylation subgroups. Strikingly, RRD HGG lacking these known secondary mutations clustered together with an incompletely described group of HGG previously labeled "Wild type-C" or "Paediatric RTK 1". Independent analysis of two comparator HGG cohorts showed that other RRD/hypermutant tumors clustered within these subgroups, suggesting that undiagnosed RRD may be driving some HGG clustering in this location. RRD HGG displayed a unique CpG Island Demethylator Phenotype in contrast to the CpG Island Methylator Phenotype described in other cancers. Hypomethylation was enriched at gene promoters with prominent demethylation in genes and pathways critical to cellular survival including cell cycle, gene expression, cellular metabolism, and organization. These data suggest that methylation arrays may provide diagnostic information for the detection of RRD HGG. Furthermore, our findings highlight the unique natural selection pressures in these highly dysregulated, hypermutant cancers and provide the novel impact of hypermutation and RRD on the cancer epigenome.

Comprehensive Analysis of Hypermutation in Human Cancer.

We present an extensive assessment of mutation burden through sequencing analysis of >81,000 tumors from pediatric and adult patients, including tumors with hypermutation caused by chemotherapy, carcinogens, or germline alterations. Hypermutation was detected in tumor types not previously associated with high mutation burden. Replication repair deficiency was a major contributing factor. We uncovered new driver mutations in the replication-repair-associated DNA polymerases and a distinct impact of microsatellite instability and replication repair deficiency on the scale of mutation load. Unbiased clustering, based on mutational context, revealed clinically relevant subgroups regardless of the tumors' tissue of origin, highlighting similarities in evolutionary dynamics leading to hypermutation. Mutagens, such as UV light, were implicated in unexpected cancers, including sarcomas and lung tumors. The order of mutational signatures identified previous treatment and germline replication repair deficiency, which improved management of patients and families. These data will inform tumor classification, genetic testing, and clinical trial design.

Invasive High-grade Upper Tract Urothelial Carcinoma in a 14-Year-Old Girl.

We present an unusual pediatric case of invasive upper tract urothelial carcinoma with an associated genetic predisposition. A 14-year-old female presented with intermittent right flank pain, and was found to have a poorly functioning hydronephrotic right kidney. Laparoscopic nephrectomy was performed. Pathology demonstrated upper tract urothelial carcinoma, and she subsequently underwent completion ureterectomy. Genetic studies demonstrated a double-hit constitutional deletion of a DNA mismatch repair protein, revealing a rare Lynch syndrome variant known as Constitutional Mismatch Repair Deficiency Syndrome. This disease places her at high risk for multiple malignancies, including upper tract urothelial carcinoma.

Sustained engraftment post bone marrow transplant despite anti-platelet antibodies in Glanzmann thrombasthenia.

BACKGROUND: Patients with Glanzmann thrombasthenia (GT) have normal platelet counts but abnormal platelet aggregation and carry the risk of life-threatening bleeding. We report three patients who received bone marrow transplantation (BMT) for type I GT and discuss the risk and management of anti-platelet antibodies. PATIENTS AND RESULTS: Diagnosis of GT was made through abnormal platelet aggregation studies or the absence of GPIIb/IIIa by flow cytometry. All patients had severe bleeding requiring multiple red blood cell transfusions. One patient received an unrelated donor transplant and two received matched sibling donor transplants following conditioning therapy with busulfan, cyclophosphamide, and fludarabine. Two patients developed an anti-platelet antibody, treated in one with intravenous immune globulin (IVIG). Engraftment of white blood cells and platelets was achieved on day +13 to +14 and +17 to +25, respectively. Complete donor chimerism and GPIIb/IIIa+ platelets are sustained at +22 to +30 months post transplant. CONCLUSIONS: In summary, patients with GT and history of severe hemorrhage can be cured with BMT, but the presence of anti-platelet antibodies should be sought and platelet transfusions minimized prior to transplant. IVIG may be helpful in cases of refractory immune thrombocytopenia related to anti-platelet antibodies. Improvement in transplant-related complications with current transplant regimens allows consideration of BMT for life-threatening non-malignant disorders such as GT.