Cytogenetic Studies Continue to Yield Important Ca Research Data

SEATTLE--With advanced molecular genetic techniques now availableto researchers for the identification and study of genes involvedin cancer, the question arises: Is the study of chromosomal translocations,additions, and deletions still a worthwhile pursuit?

Janet Rowley, MD, DSc, speaking at the American Society of Hematology'ssubcommittee on pediatric hematology program at the ASH annualmeeting, gave an emphatic yes and cited new research to back upher enthusiasm.

Dr. Rowley, professor of medicine, University of Chicago, andthe members of her laboratory at the University's Franklin McLeanMemorial Research Institute have been involved in characterizinga newly identified, ETS-family transcription factor called TEL,which has been implicated in acute lymphoid leukemia (ALL).

The gene coding for TEL (translocation ETS leukemia protein) wasdiscovered by pursuing the observation that the cells of manyALL patients carry a particular translocation involving chromosome12p.

[The research on TEL as a fusion partner for ABL was formallypresented at a scientific session of the ASH meeting (abstract1043).]

"There are several hundred translocations still out there,waiting to have their genetic breakpoints defined," Dr. Rowleysaid.

"It's important to further identify the rearrangements thatare consistently associated with each of the leukemias. Also,we need to look at the diagnosis, and whether a patient has only,say, a t(8;21) translocation, or a t(15;17)--or are there otherchanges already present in the karyotype of the cells?"

Cytogenetic studies are still very important for defining regionsof consistent chromosome gain or loss, she said. "We're stillbasically nowhere, in a genetic sense, as far as understandingthe secondary changes that accompany these translocations."

She said that a number of chromosomal regions are known to beconsistently deleted in leukemia, but researchers don't know whichgenes are involved. "We need a great deal more definitionof these regions, and cytogenetic studies are a critical resourcefor that information," she said.

Dr. Rowley also stressed the importance of collaboration in thesearch for clinically important breakpoints. "There are patientswith rare translocations," she noted, "and the onlyway we're going to be able to identify genes from those patientsis by cooperative efforts among individual researchers, each ofwhom may have DNA from only one or two patients. If we collaborateworldwide, we'll make far faster progress."

Poor Prognosis Infant ALL

Cytogenetic studies can also provide insight into why a particularcancer in a particular group may have a poor prognosis, Dr. Rowleysaid. She cited infant ALL, which has an extremely poor prognosis,particularly if it develops in the first 6 months.

Two different studies showed that the MLL gene was rearrangedin about 67% of childhood ALL cases overall, but in almost allof the 0- to 6-month-old infant ALL cases.

A t(4;11) translocation was found to be the major translocationin these very young infants. Dr. Rowley said that it has beenknown for some time that this translocation is associated witha very poor prognosis.

In a number of these cases, she said, the translocation was identifiedonly by the use of molecular analysis, highlighting the valueof combining the two approaches (see box below).

Combining Cytogenetics With Molecular Techniques

Although enthusiastic about the uses of cytogenetics in cancerresearch, Dr. Janet Rowley, in an ASH presentation, did acknowledgeits limitations. She called molecular methods, such as Southernblots and PCR "extremely valuable adjuncts to the more traditionalcytogenetic techniques of karyotyping and FISH."

Dr. Rowley cited an example from her own lab in which moleculartechniques revealed a more complete picture of a translocationthan did cytogenetics alone.

The TEL gene (discussed in the story above) is involved in a numberof translocation gene fusions, one of which is with the AML1 gene.

"One of the surprises," she said, "has been theapparent frequency of this fusion, as detected by molecular techniques.It is present in around 25% of children with B-cell ALL, whichis an enormous percentage and one that we would not have predictedon a cytogenetic basis."

Dr. Rowley said that combining molecular and cytogenetic techniquesallowed her group to determine that most of the B-cell ALL patientswith the TEL/AML1 fusion also carry a deletion of the normal TELgene.

This would seem to indicate that two events are required for leukemo-genesisin these cells: the formation of the abnormal fusion gene andthe loss of the normal gene.