Game changers in pediatric cancer

Although there have been significant improvements in patient outcomes for some forms of pediatric cancer, progress has been painfully slow for others. An increasing understanding of pediatric cancers is highlighting the unique molecular drivers and challenging the assumption that drugs developed in adults can be applied to children and young adults. Here, we discuss game-changing therapeutic advances and a shifting view of childhood cancers.

Unique genomic background

Although pediatric cancers are rare, representing just 1% of all new cancers diagnosed annually in the United States, they are the second leading cause of death in children aged 1 to 14 years. There are many different histological tumor types under the umbrella of childhood cancers, of which the most common are leukemias, central nervous system tumors, and lymphomas (Figure 1).^1,2

Significant progress has been made in the treatment of certain pediatric cancers in recent decades, exemplified by pediatric acute lymphoblastic leukemia (ALL), which has been transformed from a virtually incurable cancer to one in which 5-year survival rates now reach up to 90%. In other forms of pediatric cancer, however, survival rates have stagnated and little progress has been made in the development of effective new therapies.³

Because of their rarity, pediatric cancers are difficult to study and adequate enrollment of children in clinical trials can be challenging. Pharmaceutical companies are often hesitant to test drugs in the pediatric population in patients who often cannot advocate for themselves. As a result, the activity of drugs developed in adult patients has often been inferred in pediatric patients with the same tumor type or molecular aberrations. However, as researchers have gathered more information about pediatric cancers, there has been increasing recognition of their unique attributes and the need for dedicated clinical trials in this patient population.

Pediatric cancers tend to be found in the developing mesodermic tissue, whereas adult cancers are more prevalent in the epithelial tissues. Genome sequencing studies have revealed a much lower mutational burden in pediatric cancers and the mechanisms of oncogenesis are also quite different; adult tumors can develop from a series of acquired gene mutations, but pediatric tumors tend to develop from a single catastrophic event.^4,5

Even the same type of cancer in a pediatric and adult patient can be quite different, with very different underlying molecular mechanisms. In a recent genomic analysis of different types of pediatric cancer by researchers at St Jude’s Children’s Research Hospital, less than half of the identified mutated genes were found to be similar to those found in adult patients.⁶
 

A ‘magic bullet’?

Chromosomal rearrangements are common in pediatric cancers. This type of molecular abnormality can result in a fusion of 2 different genes when the chromosome breaks apart and the pieces join back together in a muddled order. If the genetic code fuses in a manner that is “readable” by the cell, then it can drive aberrant activation of one or both genes.⁷ Gene fusions often involve kinase enzymes that are essential players in cell signaling pathways regulating hallmark cancer processes, such as unchecked cell proliferation. The fusion drives the constitutive activation of the kinase and, thus, these downstream signaling pathways.