Genome code for most common form of pediatric brain cancer cracked

WASHINGTON - The genetic code for medulloblastoma, the most common pediatric brain cancer and a leading killer of children with cancer, have been decoded by scientists at the Johns Hopkins Kimmel Cancer Center.

The genetic “map” is believed to be the first reported of a pediatric cancer genome.

Notably, the findings show that children with medulloblastoma have five- to tenfold fewer cancer-linked alterations in their genomes compared with their adult counterparts, the scientists said.

“These analyses clearly show that genetic changes in pediatric cancers are remarkably different from adult tumors. With fewer alterations, the hope is that it may be easier to use the information to develop new therapies for them,” said Victor Velculescu, associate professor of oncology at the Johns Hopkins Kimmel Cancer Center.

The Johns Hopkins team used automated tools to sequence hundreds of millions of individual chemicals called nucleotides, which pair together in a preprogrammed fashion to build DNA and, in turn, a genome.

Combinations of these nucleotide letters form genes, which provide instructions that guide cell activity. Alterations in the nucleotides, called mutations, can create coding errors that transform a normal cell into a cancerous one. The scientists at Johns Hopkins have previously mapped genome sequences for pancreatic, adult brain, breast and colon cancers with similar methods.

For the study, scientists sequenced nearly all protein-encoding genes in 22 samples of pediatric medulloblastoma and compared these sequences with normal DNA from each patient to identify tumor-specific changes or mutations. Each tumor sample had an average of 11 mutations. There were 225 mutations in all.

Then, the investigators searched through a second set of 66 medulloblastomas, including some samples from adults, to find how these mutations altered the proteins made by the genes.

The team found that most of the mutations congregate within a few gene families or pathways. The most prevalent pathway ordered the way long strands of DNA, that make up chromosomes, are twisted and shaped into dense packets that open and close depending on when genes need to be activated. Such a process is regulated by chemicals that operate outside of genes, termed “epigenetic” by scientists.

Within the epigenetic pathway, two commonly mutated genes were both involved in how molecules called histones wrap around DNA.

“These epigenetic changes may be more important than we thought in childhood cancers,” said Will Parsons, an assistant professor at Texas Children’s Cancer Center and Baylor College of Medicine.

Mutations in MLL2 and MLL3 were identified in 16 percent of the entire set of 88 medulloblastoma samples. Add to this three other epigenetic alterations found by the scientists in the genome scan, and the total set accounts for 20 percent of mutations in all the brain cancer samples.

Second to epigenetic pathways were gene mutations in pathways such as Hedgehog and Wnt that control tissue and organ development in humans and other animals. Both pathways have previously been linked to childhood medulloblastoma.

The study has been published online in the Science Express. (ANI)