Molecular Genetics of Normal and Leukemic 
Hematopoiesis 
Alan Bernstein, Ph.D. — International Research Scholar 
Dr. Bernstein is Head of the Division of Molecular and Developmental Biology and Associate Director of 
the Samuel Lunenfeld Research Institute at Mount Sinai Hospital, Toronto. He received his Ph.D. degree 
in medical biophysics from the University of Toronto and trained as a postdoctoral fellow at the Imperial 
Cancer Research Fund Laboratories, London. Before moving to Mount Sinai he was Senior Scientist in 
the Division of Biological Research at the Ontario Cancer Institute. He is the first incumbent of the Anne 
Tanenbaum Chair in Molecular and Developmental Biology and was recently elected a Fellow of the Royal 
Society of Canada. 
MY laboratory is interested in the molecular 
mechanisms that govern the orderly produc- 
tion of hematopoietic cells in the adult mammal. 
It is clear from earlier work involving the genera- 
tion of unique clonal markers (either visible cyto- 
genetic markers or the chromosomal integration 
sites of retroviral vectors) that hematopoiesis in- 
volves the proliferation and differentiation of 
pluripotent stem cells. These cells have the devel- 
opmental and proliferative capacity to produce 
the millions of myeloid and lymphoid cells that 
are continuously required in adult life. Thus the 
hematopoietic system has served as an attractive 
and important experimental model of develop- 
mental processes. Defects in hematopoiesis can 
also lead to disease, including anemias, leuke- 
mias, and such genetic disorders as sickle cell 
anemia and thalassemia, making the understand- 
ing of this system important to both medicine and 
biology. 
We are taking two main approaches to eluci- 
date the mechanisms that control the regulation 
of hematopoietic stem cells. The first approach 
involves the analysis of mouse mutations that 
disrupt hematopoiesis, while the second involves 
the study of the multistage erythroleukemias in- 
duced by the various forms of Friend murine leu- 
kemia virus, a murine retrovirus. 
The white-spotting (IT) and Steel (^Sl) Loci 
Mutations at either the dominant white- 
spotting {W} or Steel (SI) loci can lead to coat 
color defects, severe macrocytic anemia, and ste- 
rility. This similarity in pleiotropic phenotype of 
W^and SI mutants is striking, particularly as there 
is no common developmental origin of cells that 
give rise to melanocytes, blood cells, and germ 
cells. Furthermore, the Wand SI loci are clearly 
distinct genes, as they map on mouse chromo- 
somes 5 and 10, respectively. 
Initial insights into their mechanism of action 
came from in vivo cell-mixing experiments, in- 
volving either bone marrow transplantation or 
aggregation chimeras with animals of different ge- 
notypes. These early studies established that the 
developmental defects in W mutant mice result 
from a cell-autonomous, intrinsic stem cell de- 
fect, whereas the cellular defect in 5/ mutant 
mice is in the microenvironment in which these 
cells develop in the embryo and function in the 
adult animal. 
Several years ago, we showed that H^was allelic 
with the proto-oncogene c-kit, a member of the 
family of genes that includes c-fms and pdgfr, 
which encode transmembrane receptors with ty- 
rosine kinase activity. Different W alleles are ei- 
ther deletions or point mutations in the cytoplas- 
mic tyrosine kinase domain that result in partial 
or complete loss of tyrosine kinase activity and a 
consequent reduction in the signaling capability 
of the Kit receptor. Thus the W mutant mouse 
provided the first example of a germline muta- 
tion in a mammalian proto-oncogene. This work 
is supported by a grant from the National Insti- 
tutes of Health. 
These findings predicted that SI might encode 
the ligand for the Kit receptor and that this ligand 
might be a potent growth or survival factor for 
melanoblasts, germ cells, and hematopoietic 
stem cells. The microenvironmental nature of the 
defects in SI mutant mice also predicted that the 
5/ protein product might be membrane bound. 
All three predictions have turned out to be 
correct. Thus the W-Sl gene pair has provided 
strong molecular genetic evidence for the impor- 
tant role that cell-cell interactions play in the de- 
velopment and function of hematopoietic and 
other stem cells. Their protein products are strik- 
ingly similar in design and function to those of 
the sev and boss genes, which control the devel- 
opment of the R7 photoreceptor cell in the com- 
pound eye of the fruit fly Drosophila. 
Friend Leukemia Virus 
In the second approach, we are identifying and 
analyzing cellular genes important in the induc- 
tion of the multistage erythroleukemias induced 
by Friend leukemia virus. Several years ago, we 
showed that the p53 gene was inactivated by de- 
letion or retroviral insertion in approximately 30 
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