Multiplying the Power of MRD Assessment
Assessing minimal residual disease (MRD), either to determine prognosis after treatment or during follow-up to detect disease recurrence at an early stage, is an established part of the treatment of several blood cancers, especially acute lymphoblastic leukemia (ALL).
In the past, MRD detection methods were limited to flow cytometry and allele-specific oligonucleotide polymerase chain reaction (ASO-PCR). More recently, next-generation sequencing (NGS)-based MRD detection and quantification has become available.
Although this newer method has several advantages over the older techniques in terms of standardization, scalability and sensitivity, one of the less-appreciated features is its ability to track multiple cancer clones, which has implications for both patient care and our understanding of disease biology.
A clone is a group of identical cells derived from the same ancestral cell. Cancer starts out as a single clone, i.e., one mutated cell gives rise to a lineage of cells that all contain the same cancer causing mutations. But through generations of replication, new mutations and cellular characteristics can be acquired, giving rise to new clones. By the time a patient has shown clinical signs of the disease and is being treated, multiple cancer clones are likely to be present.
When it comes to measuring MRD in blood cancer, the ability to detect multiple clones is a function of what you’re looking for. Flow cytometry uses cell surface markers to identify cells as normal or abnormal. Studies have shown, however, that different cancer clones can have different cell surface markers, which may lead to false negatives or false positives when evaluating immunophenotyping data. ASO-PCR requires the development of a customized assay for each cancer clone, making it infeasible to follow multiple clones and again raising the possibility of false negative results. In contrast, NGS-based MRD detection uses universal reagents in multiplexed PCR to allow simultaneous assessment of the full repertoire of immunoglobulin and/or T cell receptor genes, thus enabling identification and tracking of multiple cancer clones at multiple receptors and greatly reducing the risk of false negatives.
An example of the importance of following multiple clones can be seen in a T-ALL patient from my own practice (see graph). The NGS-based MRD method identified clones at two receptors in a patient sample taken at relapse: TCRD and TCRG. Had I been following only the dominant TCRD clone, I would have thought my patient’s disease had reached very low levels at the pre-transplant (pre-HCT) time point, and was virtually non-existent post-transplant (post-HCT). However, because the NGS-based MRD technique allowed me to also track the minor TCRG clone that proved resistant to therapy, I could see that in fact my patient was not responding, and that influenced the decisions I made about managing the patient’s immune suppressive medications following transplant.
Dr. Logan is an Assistant Professor at the UCSF School of Medicine. The views expressed here are his own and are not an endorsement of Adaptive or the clonoSEQ® Process.