Gene Expression Profiling — Tumour RNA Activity Testing

While standard genomic sequencing examines the static DNA code to uncover permanent structural mutations, gene expression profiling evaluates the dynamic transcriptome—measuring which genes are actively being transcribed into functional mRNA molecules. This active expression level reflects how the tumour is behaving in real time, indicating biological processes such as cellular proliferation, vascular invasion, and resistance to apoptosis.

Techniques such as microarrays, quantitative reverse transcription-polymerase chain reaction (RT-PCR), and next-generation RNA sequencing (RNA-seq) analyze tumour tissue extracted from surgical biopsies. Sophisticated bioinformatic algorithms calculate composite multi-gene recurrence scores, stratifying patients into distinct prognostic and predictive risk groups that guide systemic therapy decisions.

Gene expression profiling is instrumental in sparing thousands of cancer patients from unnecessary, toxic treatments. In early-stage breast cancer, for example, patients with low genomic risk scores can frequently omit adjuvant cytotoxic chemotherapy, relying safely on endocrine therapy alone without compromising their survival outcomes. Conversely, discovering a high-risk gene expression profile identifies patients who derive substantial life-preserving benefit from adding chemotherapy to their treatment regimen. This tailored approach ensures treatments are proportioned to actual biological risk rather than tumour size alone.

Gene expression profiling is conducted in certified molecular pathology laboratories on tumour tissue previously removed during surgery or a core needle biopsy. The tissue is typically retrieved from formalin-fixed, paraffin-embedded (FFPE) blocks. Laboratory technicians extract and purify the tumour RNA, verify its quality, and perform multi-gene amplification and analysis. The final report delivers a standardized numerical risk score along with a clear prognostic and predictive interpretation.

This testing is most established in early-stage, hormone receptor-positive, HER2-negative breast cancer using validated commercial assays such as Oncotype DX, MammaPrint, and Prosigna. It is also applied in colon cancer (e.g., Oncotype DX Colon) to evaluate recurrence risk in stage II disease, in prostate cancer (such as Decipher and Prolaris) to guide active surveillance versus radical treatment, and in cutaneous melanoma.

A low-risk gene expression score gives oncologists and patients the clinical confidence to de-escalate treatment by omitting adjuvant chemotherapy, thereby avoiding associated side effects like hair loss, neutropenic infections, and neuropathy. A high-risk score indicates an aggressive underlying tumour biology, prompting recommendations for adjuvant chemotherapy followed by extended targeted or endocrine therapy. In prostate cancer, favourable profiles support safe active surveillance, avoiding radical surgery or radiotherapy.