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, holding the President's Chair in Developmental Immunology. He received his B.S. degree in 
microbiology 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 
with David Baltimore at the Massachusetts Institute of Technology before joining the UCLA faculty. His 
honors include the Milken Family Medical Foundation Award in Basic Cancer Research and the Rosenthal 
Foundation Award of the American Association for 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. 
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 
concentrated on defining the growth control 
mechanisms that regulate the production of B 
lymphocytes. 
The ABL Oncogene in Murine and 
Human 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. The biological 
properties of the ABL gene product depend on its 
activity as a tyrosine-specific 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. This is a chro- 
mosome translocation that uses mRNA splicing to 
join part of the BCR gene (of unknown function) 
from chromosome 22 to part of the ABL gene 
from chromosome 9, forming the so-called Phila- 
delphia chromosome, or Ph 1 . The tyrosine kinase 
activity of the chimeric BCR/ABL gene product 
is evoked and strongly correlates with the trans- 
formation 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 P210 BCR/ABL is produced, and in the 
case of Phi -positive acute lymphocytic leuke- 
mia, a PI 85 BCR/ABL protein product is com- 
monly found. We have undertaken to determine 
the precise contribution of BCR sequences to the 
tyrosine kinase activity of the ABL segment and to 
the malignant potential of the gene product. 
A variety of studies have now documented the 
specific role of BCR in the activation of the ABL 
tyrosine kinase. Site-directed mutagenesis has es- 
tablished that the BCR segment is essential for 
transformation by the chimeric oncogene and 
acts through the tyrosine kinase domain of ABL. 
We have recently described a new class of pro- 
tein-protein interactions that regulate this activa- 
tion. Within the first exon segment of BCR, there 
are two strong SH2 (SRC homologous domain 2) 
binding regions that can tightly bind to ABL. All 
previous SH2 interactions have been mediated by 
proteins containing phosphotyrosine. Interest- 
ingly, the BCR protein requires phosphoserine 
and phosphothreonine for strong binding, but 
not phosphotyrosine. BCR represents a new class 
of protein-protein interaction domains important 
in intracellular signaling. 
Further analysis of BCR has shown that it also 
represents a new class of protein kinases. The first 
exon region of BCR contains a serine/threonine 
kinase activity that can be distinguished from the 
traditional protein kinase family by several crite- 
ria, including its containment of catalytic activity 
within a single exon, its reactivity with nucleo- 
tide analogues, and the role of essential cysteines 
within the nucleotide-binding cleft. Work from 
other laboratories shows that BCR also has a 
GTPase-activating protein domain at its carboxyl 
terminus that regulates the action of small gua- 
nine nucleotide-binding proteins. BCR appears 
to be a multifunctional protein. Its normal func- 
tion remains to be determined. 
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, be- 
cause the normal gene is toxic to most cell types 
when highly expressed. The precise mechanism 
of toxicity is not established, but probably relates 
to a cell cycle-blocking effect. 
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 pro- 
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