Normal and Abnormal Lymphocyte 
Growth Regulation 
Owen N. Witte, M.D. — Investigator 
Dr. Witte is also Professor of Microbiology and Molecular Genetics at the University of California, Los 
Angeles. He holds the President's Chair in Developmental Immunology. He received his B.S. degree in mi- 
crobiology from Cornell University and his M.D. degree from Stanford University, where he trained with 
Irving Weissman in the Medical Scientist Training Program. Dr. Witte completed postdoctoral training in 
molecular virology with David Baltimore at the Massachusetts Institute of Technology before joining the 
faculty at UCLA. Last year he was awarded the Milken Family Medical Foundation Award in Basic Cancer 
Research. 
OUR ability to resist a wide range of infectious 
agents depends on the normal function of 
the immune system. The humoral portion of this 
system is responsible for the production of spe- 
cific antibody molecules from B lymphocytes. 
Millions of B lymphocytes are produced daily in 
the fetal liver or adult bone marrow from primi- 
tive stem cells, and a careful balance between 
growth rate and development must be main- 
tained for the stem cells and their progeny. Too 
low a growth rate can result in immunodefi- 
ciency; too high a growth rate, in various types of 
leukemia or lymphoma. Our laboratory has con- 
centrated on defining the growth control mecha- 
nisms that regulate these stem cells and their 
production of B lymphocytes under normal cir- 
cumstances and in various disease states. 
The ABL Oncogene in Human and 
Murine Leukemias 
The ABL oncogene was first isolated as the ac- 
tive genetic element of the Abelson murine leu- 
kemia virus. This agent is capable of causing a 
wide range of leukemias in mice, including those 
of immature cell types within the B cell lineage. 
The biological properties of the ABL gene prod- 
uct depend on its activity as a tyrosine-specific 
protein kinase. 
The human homologue of the ABL gene has 
now been strongly implicated in the pathogene- 
sis of a family of human leukemias that harbor a 
specific cytogenetic abnormality called the Phila- 
delphia chromosome or Ph 1 . This chromosome 
translocation uses messenger RNA splicing to join 
portions of chromosome 22 sequences encoding 
a gene called BCR to a portion of the coding se- 
quences of the ABL gene on chromosome 9. The 
tyrosine kinase activity of the chimeric BCR/ABL 
gene product is evoked under these circum- 
stances and strongly correlates with the transfor- 
mation activity of the protein. 
Two different forms of BCR/ABL protein can 
occur, depending on the precise position of the 
chromosomal breakpoints. In human chronic my- 
elogenous leukemia, a larger protein product 
called P2 1 0 BCR/ABL is produced, and in cases of 
Phi -positive acute lymphocytic leukemia, a 
PI 85 BCR/ABL protein product is commonly 
found. Surprisingly, the precise contribution of 
BCR sequences determines the efficiency of tyro- 
sine kinase activity of the ABL segment and the 
malignant potential of the gene product. 
Detailed site-directed mutagenesis studies on 
molecularly cloned copies of the BCR/ABL genes 
show that BCR sequences are essential to activate 
the ABL tyrosine kinase activity and produce a 
functional oncogene. The precise function of 
BCR in the normal cell is not known, but several 
lines of evidence, including sequence homolo- 
gies, autophosphorylation activity, and inactiva- 
tion with nucleotide analogues, suggest that BCR 
may be a protein kinase itself. We are evaluating 
the possibility that the enzymatic action of BCR is 
required to activate the enzyme function of ABL 
in the chimeric oncogene. 
The normal ABL oncogene products are ex- 
pressed in many cell types, but their role in mam- 
malian cell physiology is unknown. Gross struc- 
tural changes can activate their oncogenic 
potential. It has been difficult to identify more 
subtle mutations that might activate ABL in other 
types of leukemias because the normal ABL gene 
can be toxic to most rodent fibroblast cell types 
when highly expressed via gene transfection tech- 
niques. The precise mechanism of toxicity is not 
established, but probably relates to a cell cycle 
blocking effect. 
To circumvent this problem, we have used a 
new approach to preparing retroviral expression 
stocks developed by Dan Littman and his col- 
leagues (HHMI, University of California, San Fran- 
cisco). Full-length cDNA copies of the cellular 
ABL genes are cloned into a retroviral vector that 
has been modified to allow amplification in an 
acute transfection system. Retroviral particles are 
produced that can transmit the cellular ABL gene 
at high efficiency to a wide variety of cell types. 
Using this system, we have been able to select for 
transformed clones that harbor new classes of ac- 
tivating mutations in ABL. We will further ana- 
483 
