Ponaxen 45 mg Tablets (Ponatinib)

How does BCR-ABL and T315I mutation testing work for CML?

BCR-ABL testing actually happens in two distinct phases of CML care — diagnosis/monitoring versus resistance investigation — and T315I specifically only becomes relevant in the second phase.

Phase 1: Diagnosis — confirming the BCR-ABL fusion gene

CML is defined by the Philadelphia chromosome, a translocation between chromosomes 9 and 22 that creates the BCR-ABL1 fusion gene. This fusion gene produces an abnormal protein that drives uncontrolled white blood cell growth — and it’s the target that all BCR-ABL TKIs (imatinib, dasatinib, nilotinib, ponatinib) are designed to block.

At diagnosis, this is typically confirmed using:

• Karyotyping (cytogenetics) — examines chromosomes under a microscope to detect the Philadelphia chromosome directly
• FISH (fluorescence in situ hybridization) — uses fluorescent probes to detect the BCR-ABL fusion at the DNA level, faster and more sensitive than karyotyping
• RT-PCR (reverse transcription PCR) — detects and quantifies BCR-ABL transcript levels in blood, the most sensitive method and the one used for ongoing monitoring

Phase 2: Ongoing monitoring — tracking treatment response

Once on a TKI, quantitative RT-PCR becomes the routine monitoring tool, typically done every 3 months initially. Results are reported on the International Scale (IS), expressed as a percentage — for example, “BCR-ABL ≤0.1% IS” represents a major molecular response (MMR), a key treatment milestone.

This monitoring is what triggers the next phase of testing — if BCR-ABL levels rise unexpectedly or fail to drop as expected, that’s the signal to look for a resistance mutation.

Phase 3: Resistance testing — looking for T315I and other mutations

When response is inadequate or disease progresses on a TKI, mutation analysis (often called “kinase domain mutation testing”) is performed:

• Sanger sequencing has traditionally been used, though it requires a relatively high proportion of mutant cells to detect
• Next-generation sequencing (NGS) is increasingly preferred — it’s far more sensitive and can detect low-level mutant clones before they become dominant, which matters because T315I can emerge as a small subpopulation under treatment pressure and then expand

T315I specifically is a single point mutation in the BCR-ABL kinase domain that physically blocks imatinib, dasatinib, and nilotinib from binding — which is why none of those drugs work once it’s present, regardless of dose or switching between them.

Sample source

Like FLT3 testing in AML, this is done on peripheral blood (most common for monitoring) or bone marrow aspirate, since CML cells circulate freely — no tissue biopsy is needed.

Timeline

Initial diagnostic testing (karyotype/FISH/PCR) typically takes 1–2 weeks. Routine quantitative PCR monitoring usually returns in about 1 week. Mutation analysis for T315I, when triggered by loss of response, can take 2–3 weeks depending on whether Sanger sequencing or NGS is used.

What the result means for treatment

One thing worth understanding for your own monitoring: the trend in your BCR-ABL numbers over successive tests often matters more than any single result. A gradual rise across two or three consecutive tests is a stronger signal than one slightly elevated reading.