Diagnosis of APL

Detection of residual or recurrent APL

Monitoring the level of PML/RARA in APL patients

Acute promyelocytic leukemia (APL) accounts for 5% to 10% of acute

myeloid leukemia, and generally has a good prognosis with current

treatment protocols. APL cells contain a fusion gene comprised of the

downstream sequences of the retinoic acid receptor alpha gene

(RARA) fused to the promoter region and upstream sequences of

one of several genes, the most common (>80%) being the

promyelocytic leukemia gene (PML). The fusion gene is designated

PML/RARA and may be seen in a karyotype as t(15;17)(q22;q12).

Messenger RNA (PML/RARA) produced from the fusion gene can

be detected using a polymerase chain reaction (PCR)-based assay,

and indicates the presence of neoplastic cells. The PCR-based assay

has greater sensitivity than standard methods such as morphology

review, karyotyping, or fluorescence in situ hybridization (FISH).

Recent studies have indicated that sensitive monitoring is important

because the majority of patients who remain PCR-positive, or

become PCR-positive again following treatment, will relapse and

likely benefit from early intervention for residual/recurrent disease.(1)

This quantitative assay allows PML/RARA levels to be monitored rather

than simply detecting the presence or absence of disease.

An interpretive report will be provided.

Each patient value is compared to an "average" patient's

diagnostic value and to any previous monitoring values for the

patient. An "average" patient at diagnosis will have a normalized

ratio of approximately 4 x 10(5). Very low levels of transcript may

produce normalized ratio values <1 x 10(2).

The assay is reported in the form of a normalized ratio, which is

an estimate of the level of PML/RARA RNA present in the specimen,

expressed in relation to the level of RNA from an internal control

gene (beta glucuronidase, designated GUSB). The normalized

ratio has no units but is directly related to the level of PML/RARA

detected (ie, larger numbers indicate higher levels of PML/RARA

and smaller numbers indicate lower levels). A relative expression

value minimizes variability in the RNA levels measured in separate

specimens tested at different times. Although a quantitative PCR

assay is performed, the precision of the assay is such that results

must be considered semiquantitative, and it is recommended that

only log changes be considered significant. Critical results, such as

a change in the status of positivity, should be repeated on a separate

specimen to verify the result.

PML/RARA levels can only be compared reliably if tested in the

same lab using the same procedure each time

The assay will only detect PML/RARA RNA and will not detect

RNA from the less common RARA fusion genes.

The assay may not detect rare, unusual PML/RARA fusions.

Therefore, if the assay is going to be used for monitoring after

treatment, the test should be performed at the time of diagnosis to

ensure that the test gives a positive result.

Grimwade D, Lo Coco F:  Acute promyelocytic leukemia: a model

for the role of molecular diagnosis and residual disease monitoring

in directing treatment approach in acute myeloid leukemia.

Leukemia 2002 October;16(10):1959-1973