EXPRESSION OF P15INK4B IN HEMATOPOIETIC PROGENITORS
Luciana Teofili, Luigi Maria
Larocca*, Giuseppe Leone
Departments of Hematology and *Pathology, Universita' Cattolica,
Rome, Italy
Correspondence: Luciana Teofili, Departments of Hematology,
Universita' Cattolica, Rome, Italy
We read with interest the study of Iolascon et al. recently
published.1 They investigated by means of the reverse
transcRiptase (RT)-PCR assay the expression of some cell cycle
regulatory genes in chronic myelogenous leukemia (CML), either in
chronic phase and in blast crisis. The major findings of this
investigation were the absence of p16INK4A gene
expression especially in lymphoid blast crisis, the increased
silencing of the p57 Kip2 gene in blast crisis as
compared to chronic phase, and the deregulated expression of
p15INK4B gene. In particular, p15INK4B was
not expressed in most samples collected during the chronic phase,
whilst it was frequently up-regulated during the blast crisis.
Moreover the Authors, comparing the gene expression in CD34+ cells
isolated from bone marrow of normal individuals and of patients
with CML, found that p15INK4B is detectable in CD34+
cells from normal subjects and from patients with blast crisis,
but not in CD34+ cells isolated from patients in chronic phase.
The finding of the absence of p15INK4B expression in
CML in chronic phase is in agreement with our previous
observations obtained by flow cytometry on leukemic cells (2). In
a series of 21 patients (3 acute lymphoid leukemias, 11 acute
myeloid leukemias, 2 byphenotypic acute leukemias and 5 CML in
chronic phase) we found p15INK4B expression only in 4
patients (2 poorly differentiated acute myeloid leukemias, 1
byphenotypic leukemia and 1CML). Interestingly, all the patients
with acute leukemia expressing p15INK4B were resistant
to chemotherapy. This finding, together with the observation of
Iolascon et al.1 showing an increased expression of
p15INK4B in blast crisis as compared with chronic
phase, suggests that p15INK4B expression might be a
prognostic indicator in myeloid malignancies. On the contrary,
their findings on normal progenitors are not in agreement with our
experience concerning the p15INK4B expression in normal
hematopoietic cells. In a recent study we found that
p15INK4B is strongly expressed in normal cells owing to
the granulocytic and monocytic lineage, nevertheless we failed to
detect p15INK4B expression in normal bone marrow CD34+
progenitors (both in the CD34+/CD33+ and in the CD34+/CD33-
subfractions) or in peripheral blood CD34+ cells mobilzed by
G-CSF.3 In our experience, the only CD34+ cell
population which appeared p15INK4B positive was
constituted by peripheral blood CD34+ cells mobilized by
chemotherapy (followed or not by G-CSF
administration).3 These results were confirmed by
different approachs: RT-PCR, immunocytochemistry,
immunohistochemistry and flow cytometry. The discrepancy between
the data obained by Iolascon et al.1 and our own
observations, could be related to a different purity of CD34+ cell
samples evaluated. In our opinion, the purity of selected CD34+
cell population is of relevant importance in determining the
results of the RT-PCR assay. In fact, monocytes, granulocytes and
lymphocytes contain p15INK4B m-RNA amounts detectable
by RT-PCR (3). Thus, also a small cell contamination (mostly
constituted by lymphocytes or monocytes) can be responsible for
the p15INK4B detection in normal CD34+ cell samples.
Similarly, it can be supposed that CD34+ progenitors collected
from CML patients in chronic phase can be contaminated by leukemic
p15INK4B negative cells, so that CD34+ cells could
appear p15INK4B negative as well. Unfortunatly,
Iolascon et al. did not show the purity of the CD34+ cell
population examined, neither it is possible to argue from data
presented in their study in which CML patients
(p15INK4B positive or negative?) CD34+ cells have been
investigated. As regards the p15INK4B expression in
CD34+ cells collected in patients with blast crisis, it is not
clear whether these cells can be considered hematopoletic
progenitors or, more probably, leukemic blasts, considering that
very frequently blast crisis of CML express the CD34 antigen. If
so, to examine the CD34+ cell fraction does not add further
informations to the examination of the whole leukemic cell
population which homogeneously express p15INK4B. Recent
studies suggest that alterations of the p15INK4B gene
expression play an important role in the deregulated cell
proliferation in acute myelold leukemias4 and in
myelodisplastic syndrome.5 The study of Iolascon et
al.3 further extends this hypothesis to CML.
Nevertheless, the exact role of this cyclin-dependent kinase
inhibitor in normal hematopoiesis remains to be partly elucidated;
the use of different methodological approachs and the examination
of highly homogeneous cell populations could be greatly usefull to
this purpose.
Acknowledgements
This work was supported by a grant from A.I.R.C.
(Associazione Italiana Ricerca sul Cancro), Italy.
