Understanding Molecular Subtypes Is Basis for Genomic Medicine in Prostate Cancer
Boston, MA—Incorporating genomics into the practice of medicine requires the demonstration of the ability of biomarkers to impact clinical decision-making, and ensuring that patients receive the best therapy based on genomic findings. Scott A. Tomlins, MD, PhD, Assistant Professor of Pathology and Urology, University of Michigan Medical School, Ann Arbor, reviewed efforts to realize genomic medicine into prostate cancer diagnosis and management at the second Global Biomarkers Consortium annual conference.
Genomic medicine starts with understanding basic molecular subtypes. Numerous microarray studies, array comparative genomic hybridization studies, and the characterization of the prostate cancer transcriptome and genome have identified common alterations. “As in lung cancer, we can define prostate cancer molecular subtypes,” said Dr Tomlins. “The easiest way to do this is to look for the presence of ETS gene fusions. Like an ALK [anaplastic lymphoma kinase] rearrangement, these are chromosomal rearrangements that occur in prostate cancer, but they occur in about 50% of prostate cancers.”
More than 50% of prostate-specific antigen (PSA)-screened prostate cancers harbor fusions between the transmembrane protease serine 2 (TMPRSS2) and the v-ets erythroblastosis virus E26 oncogene homolog (avian [ERG]) genes.
No clear individual driver has been identified in patients with prostate cancer without an ETS fusion.
Exome sequencing of prostate tumor and normal pairs has discovered focal loss in common tumor-suppressor genes such as PTEN, TP53, and RB1. Point mutations have been found in several genes, including TP53, SPOP, PTEN, FOXA1, and MED12, each with a prevalence of <10%.
“We have things in prostate cancer that we know are potentially targetable, but there hasn’t been the momentum to do what’s been done in lung cancer where we looked for these patients to find them,” Dr Tomlins said.
“Interestingly, there are almost no high-level focal amplifications in prostate cancer until the posttreatment setting. There is a high level of genomic aberrations, and we’re seeing these as you start to do whole-genome studies.”
A TMPRSS2:ERG fusion serves as a urine biomarker for prostate cancer. It has been shown to improve on serum PSA for predicting cancer in men undergoing diagnostic biopsy. “It is as specific for prostate cancer as we can get,” Dr Tomlins said. A urine-based test developed at the University of Michigan uses quantification of TPRSS2:ERG and PCA3, along with PSA, to calculate the risk of prostate cancer on needle biopsy.
Rather than provide a positive or negative value, the test will provide the risk of having cancer and high-grade cancer, with estimates of what constitutes low, intermediate, and high risk, said Dr Tomlins.
Integrative profiling of lethal castration-resistant prostate cancer (CRPC) has been conducted at the University of Michigan in an effort to characterize the mutations that are present. The exomes of 50 lethal heavily pretreated metastatic CRPCs obtained at rapid autopsy were sequenced.
“Prostate cancer has a low mutational burden, but you do see patients with high numbers of mutations,” Dr Tomlins said. The exome data revealed 9 genes that were significantly mutated, 6 of which are recurrently mutated in prostate cancer—TP53, AR, ZFHX3, RB1, PTEN, and APC. Of the mutated genes, 3 did not have roles in prostate cancer—MLL2, OR5L1, and CDK12.
Clinical Sequencing Program: Finding What Is Actionable
To translate these findings into potentially targetable actions for individual patients, a clinical sequencing program for advanced cancers was started at the University of Michigan’s Michigan Oncology Sequencing Center. “We do profiling of the normal germline as well. We do full exome and transcriptome studies, and we do the exome of the normal sample, run it through pipelines, and we present the results to a sequencing tumor board [now called a precision medicine tumor board],” said Dr Tomlins. The board discusses actionable results and sends a report to the genetic counselor and the referring physician. A multidisciplinary tumor board determines which biomarkers are actionable.
The University of Michigan has recently initiated one of the first genetically stratified clinical trials (a multi-institution, randomized phase 2 trial) in which ETS gene fusions are assessed (ERG by immunohistochemistry, and ETV1 by fluorescence in situ hybridization), and patients are stratified to treatment with an antiandrogen alone or a poly (ADP-ribose) polymerase inhibitor plus an antiandrogen.