This online seminar outlines a recent example of the use of molecular barcoding in combination with next-generation sequencing to detect somatic mosaicism in cancer patients. 

Ioannis Ragoussis, associate professor in the Department of Human Genetics at McGill University, will discuss the study, which confirmed that somatic mosaicism was the cause of a rare cancer syndrome in four children. 

The children presented with multiple primary tumors associated with DICER1 syndrome, an inherited disorder characterized by pleuropulmonary blastoma along with other rare childhood sarcomas and dysplasias. The patients were each found to carry a specific DICER1 “hotspot” RNase IIIb mutation in multiple tumor biopsies from different sites. However, the mutations were not detected in the patients’ germline using conventional technology, leading to the suspicion of somatic mosaicism. 

Dr. Ragoussis and colleagues investigated and characterized the suspected mosaic origin of the DICER1 RNase IIIb mutations by deep sequencing both tumor and normal tissues from the four patients. 

An in-house, custom made DICER1 PCR-based array and a HaloPlex^HS panel incorporating molecular barcodes were used prior to next generation sequencing on the Illumina HiSeq at high coverage. Using the molecular barcoding technology, the relative abundance of the previously identified mutations was assessed between tumor and non-tumor samples from the respective patients with the aim of confirming or refuting the hypothesis of a mosaic origin. 

Using this approach, the team confirmed its hypothesis that DICER1 RNase IIIb mosaicism was the cause of the rare DICER1-associated tumors in these children. Dr. Ragoussis will provide details of how the sensitivity of the Haloplex^HS panel improved the ability to detect rare low-frequency mosaic mutations.