Autologous Bone Marrow Transplantation - Blog Science Connections

726 Aberrant Gene Expression in AML
important in only a few select forms of leukemia such as Philadelphia
chromosome-positive chronic myelogenous leukemia (Ph 1
+ CML).
A second example of variability in transcript size is present with c-K-ras.
In normal samples we find a small (1.2 kb) transcript predominantly
expressed (Table 1). This transcript is either absent or expressed at only very
low levels in leukemia samples with the exception of chronic lymphocytic
leukemia (CLL) (Lee et at, personal communication).
Limitations of Our Prior Studies
Since nearly all our studies were carried out on mixed populations of
cells, it is impossible to say whether particular oncogenes are expressed in a
certain subpopulation of cells or on all cells present in a sample. We are also
unable to distinguish between a high level of gene expression in a few cells
and a relatively low level of expression in a majority of the cells. Evaluation of
the expression of proto-oncogenes at the level of the single cell, rather than in
whole populations, will be more informative and specific in determining the
proportion of cells within each population that expresses these genes. The
study of proto-oncogene expression at the single-cell level is feasible by using
RNA in situ hybridization techniques, which are currently being used in our
laboratory.
In situ Hybridization
Untreated or Relapsed Acute Myelogenous Leukemia (AML)
At this time, we have studied the expression of three genes, c-myc, c-s/s,
and c-abl, in patients with untreated acute leukemia. Although we anticipated
the overexpression of c-myc in these patients based on northern blot data, we
were surprised at the very high level expression of c-s/s that is often present
(Fig 1). As expected, c-abl expression is at a level similar to that found in
normal peripheral blood cells, although there occasionally appears to be a
greater percentage of cells positive for c-a£>/.
By northern blot analysis we are rarely able to detect expression of either
c-s/s or c-abl. Yet the expression of these two proto-oncogenes, in addition to
c-myc, was easily detectable at the single-cell level. If similar levels of
expression are present in residual leukemic cells from remission marrow, the
cells should be readily discernible.
Remission
Initial studies on unfractionated bone marrow samples obtained from
patients with AML support the presence of a subpopulation of cells that
overexpress c-myc, c-sis, or both at the RNA level (Fig 2). Although we have
no conclusive evidence that this cell population is involved in the leukemic
event, results suggest this possibility.