MOLECULAR GENETICS OF LYMPHOCYTE DEVELOPMENT 
Stanley J. Korsmeyer, M.D., Associate Investigator 
The research effort in Dr. Korsmeyer 's laboratory 
is focused on the molecular genetics of lymphoid 
differentiation and the corresponding lymphoid 
neoplasms. The immunoglobulin (Ig) and T cell re- 
ceptor (TCR) genes undergo programmed rear- 
rangements and deletions during normal develop- 
ment and are the sites of interchromosomal 
translocation introducing new proto-oncogenes in 
lymphoid tumors. 
L t(l4;18) Lymphomas and the Bcl-2 Gene. 
A. Bcl-2 regulation and deregulation. The chromo- 
somal breakpoint of the t(l4;18) that typifies follic- 
ular B cell lymphoma juxtaposes a new putative 
proto-oncogene, Bcl-2, with the Ig heavy-chain 
locus. The t(l4;18) generates chimeric Bcl-2-\% 
RNAs that are excessive in these mature B cells. 
Monoclonal antibodies generated to the Bcl-2 pro- 
tein indicate that levels of this membrane-associ- 
ated intracellular protein are also increased in lym- 
phoma cells. A predominant mechanism for the 
deregulation of the Bcl-2-\% fusion gene proved to 
be novel. Despite the substitution of an Ig 3' end, 
the mRNA stability was not altered. However, the 
substitution of an Ig for the Bcl-2 3' end has a dra- 
matic effect on the processing or transport of this 
message. A series of chimeric gene constructs have 
confirmed that the Bcl-2-lg fusion mRNA is much 
more efficiently handled in B cells than the Bcl-2 
message. 
B. Functional role of Bcl-2. The finding that Bcl-2 
was juxtaposed with the Ig locus in follicular lym- 
phoma but not in normal cells suggested it func- 
tioned as a proto-oncogene. Several approaches 
were undertaken to evaluate the oncogenic activity 
oi Bcl-2. Retroviral expression vectors were used to 
introduce deregulated Bcl-2 into B lymphoblastoid 
lines. Deregulated Bcl-2 consistently produced a 
three- to fourfold increment in clonogenicity. More- 
over, Bcl-2 proved capable of complementing an ex- 
ogenous myc gene present within lymphoblastoid 
lines, augmenting both clonogenicity and tumorige- 
nicity. Retroviral Bcl-2 was introduced into a variety 
of interleukin-dependent lines, to determine if Bcl- 
2 was directly involved in a growth factor pathway. 
Overexpressed Bcl-2 could not eliminate the long- 
term need for any interleukin. Instead, Bcl-2 ex- 
tended the short-term survival of interleukin-de- 
prived cells. Deregulated Bcl-2 did not influence 
cell cycle progression, while its predominant effect 
was to delay the onset of cell death. This effect was 
manifest in interleukin-3 (IL-3) -deprived pro- 
lymphocytes, promyelocytes, and mast cells but not 
IL-2- or IL-6-dependent cells. Thus Bcl-2 interferes 
with cell death, but in a cell-type and/or factor-re- 
stricted fashion. 
C. Bcl-2-Ig transgenic mice. The most comprehen- 
sive and unrestricted test of a gene's transforming 
capacity is to place it transgenically. Dr. Korsmeyer 
and his colleagues designed Bcl-2-\g minigenes re- 
creating the fusion gene found at the breakpoint. 
These minigenes were placed into the germline of 
mice to assess the prospective effects of the 
t(l4;18) during development. The transgene dem- 
onstrates a lymphoid pattern of expression and uni- 
formly results in an expanded follicular center cell 
population. Hyperplastic splenic follicles coalesce 
to form massive regions of white pulp resembling 
early splenic follicular lymphoma. Mice also de- 
velop regional lymphadenopathy with abnormal 
cellular infiltrates. The expanded lymphoid com- 
partment consists of B220, IgM/IgD mature B cells. 
Provocatively, thefic/-2-Ig transgene confers a long- 
term survival advantage to these B cells. Bcl-2-lg 
transgenic mice document a prospective role for 
the t(l4;18) in B cell growth and the pathogenesis 
of follicular lymphoma. 
II. Translocations in T Cell Acute Lymphoblastic 
Leukemia Juxtapose New Putative Proto-oncogenes 
with the 8 TCR Locus. 
A. t(ll;14)(pl5;qll) breakpoint and deregulated 
Ttg-i. The most common translocations in T cell 
neoplasms involve I4qll. Many of these T cell 
acute lymphoblastic leukemias (ALLs) are immature 
in phenotype, lacking ap TCRs and at times CD3. 
Dr. Korsmeyer and his colleagues reasoned that 
such ALLs would have translocations involving the 
8 TCR. This made it possible to obtain chromo- 
somal breakpoints for the other partners of llpl3, 
llpl5, and 10q24 involved in ALL. For example, 
the t(ll;14)(pl5;qll) breakpoint results from an 
inadvertent TCR recombinase-mediated break at 
lip 15 that recombines with the 8 TCR during its 
assemblage at a pre-T cell stage. The der (11) 
breakpoint resembled a coding joint with lip 15 in- 
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