Despite improvements in survival rates for children with cancer since the 1960s, progress for many pediatric malignancies has slowed over the past two decades. With the recent advances in our understanding of the genomic landscape of pediatric cancer, there is now enthusiasm for individualized cancer therapy based on genomic profiling of patients’ tumors. However, several obstacles to effective personalized cancer therapy remain. For example, relatively little data from prospective clinical trials demonstrate the selective efficacy of molecular-targeted therapeutics based on somatic mutations in the patient’s tumor. In this commentary, we discuss recent advances in preclinical testing for pediatric cancer and provide recommendations for providing scientific justification and translational relevance for novel therapeutic combinations for childhood cancer. Establishing rigorous criteria for defining and validating druggable mutations will be essential for the success of ongoing and future clinical genomic trials for pediatric malignancies. Cancer Res; 75(24); 1–11. ©2015 AACR.

Large-scale sequencing efforts are uncovering the complexity of cancer genomes, which are composed of causal “driver” mutations that promote tumor progression along with many more pathologically neutral “passenger” events. The majority of mutations, both in known cancer drivers and uncharacterized genes, are generally of low occurrence, highlighting the need to functionally annotate the long tail of infrequent mutations present in heterogeneous cancers. Here we describe a mutation assessment pipeline enabled by high-throughput engineering of molecularly barcoded gene variant expression clones identified by tumor sequencing. We first used this platform to functionally assess tail mutations observed in PIK3CA, which encodes the catalytic subunit alpha of the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) frequently mutated in cancer. Orthogonal screening for PIK3CA variant activity using in vitro and in vivo cell growth and transformation assays differentiated driver from passenger mutations, revealing that PIK3CA variant activity correlates imperfectly with its mutation frequency across breast cancer populations. Although PIK3CA mutations with frequencies above 5% were significantly more oncogenic than wild-type in all assays, mutations occurring at 0.07% to 5.0% included those with and without oncogenic activities that ranged from weak to strong in at least one assay. Proteomic profiling coupled with therapeutic sensitivity assays on PIK3CA variant-expressing cell models revealed variant-specific activation of PI3K signaling as well as other pathways that include the MEK1/2 module of mitogen-activated protein kinase pathway. Our data indicate that cancer treatments will need to increasingly consider the functional relevance of specific mutations in driver genes rather than considering all mutations in drivers as equivalent. Cancer Res; 75(24); 1–14. ©2015 AACR.

Preclinical models revealed that the immune system can mediate rejection of established tumors, but direct evidence in humans has been limited to largely immunogenic tumors, such as melanoma. The recent success of immune checkpoint inhibitors and adoptive T-cell transfer immunotherapy in clinical trials has instilled new hope for the use of T-cell immunotherapy in the treatment of cancer. IL15, a potent immunostimulatory cytokine, both potentiates host T-cells and natural killer (NK) cell immune responses and promotes the generation of long-lived memory T cells with superior functional capacity, with potential use in adoptive T-cell transfer protocols. IL15 has been recently tested in the clinic and showed dramatic effects at the level of responding NK and CD8+ memory T cells. The recent advances in the knowledge of IL15-dependent regulation of T-cell responses, gene expression, and metabolic adaptation have important implications for the use of IL15 in T-cell–based immunotherapy of cancer. Cancer Res; 75(24); 1–7. ©2015 AACR.

Merkel cell carcinoma (MCC) is an uncommon, but highly malignant, cutaneous tumor. Merkel cell polyoma virus (MCV) has been implicated in a majority of MCC tumors; however, viral-negative tumors have been reported to be more prevalent in some geographic regions subject to high sun exposure. While the impact of MCV and viral T-antigens on MCC development has been extensively investigated, little is known about the etiology of viral-negative tumors. We performed targeted capture and massively parallel DNA sequencing of 619 cancer genes to compare the gene mutations and copy number alterations in MCV-positive (n = 13) and -negative (n = 21) MCC tumors and cell lines. We found that MCV-positive tumors displayed very low mutation rates, but MCV-negative tumors exhibited a high mutation burden associated with a UV-induced DNA damage signature. All viral-negative tumors harbored mutations in RB1, TP53, and a high frequency of mutations in NOTCH1 and FAT1. Additional mutated or amplified cancer genes of potential clinical importance included PI3K (PIK3CA, AKT1, PIK3CG) and MAPK (HRAS, NF1) pathway members and the receptor tyrosine kinase FGFR2. Furthermore, looking ahead to potential therapeutic strategies encompassing immune checkpoint inhibitors such as anti-PD-L1, we also assessed the status of T-cell–infiltrating lymphocytes (TIL) and PD-L1 in MCC tumors. A subset of viral-negative tumors exhibited high TILs and PD-L1 expression, corresponding with the higher mutation load within these cancers. Taken together, this study provides new insights into the underlying biology of viral-negative MCC and paves the road for further investigation into new treatment opportunities. Cancer Res; 75(24); 1–7. ©2015 AACR.