DNA repair and VDJ recombination share certain 
components. 
Attempts are now being made (in collaboration 
with Drs. Penny Jeggo and Tom Stamato) to isolate 
genes encoding activities that complement the CHO 
mutations. Elucidation of such activities should 
yield insights into VDJ recombination and, perhaps, 
into certain human diseases that affect both DNA 
repair and lymphocyte development. 
Control of VDJ Recombination 
Regulation of which Ig or TCR gene segments are 
assembled in particular cells and stages within lym- 
phoid lineages is effected by modulating accessibil- 
ity of substrate gene segments to the common VDJ 
recombinase. Dr. Alt's group has developed a novel 
approach in which test substrates of VDJ recombina- 
tion are introduced into ES cells, which are subse- 
quently differentiated into normal lymphocytes in 
somatic chimeric mice. 
Assays for specific rearrangement of the substrates 
in such ES cell-derived lymphocytes demonstrated 
that several different Ig and TCR transcriptional en- 
hancer elements can also serve to target associated 
variable-region gene segments for VDJ recombina- 
tion. To test directly the role of enhancer elements 
in targeting the recombination of endogenous gene 
segments. Dr. Alt's group replaced one copy of the 
Ig heavy-chain intronic enhancer element in ES cells 
with an expressed neo" gene without modifying any 
sequences directly involved in VDJ recombination. 
These ES cells were used to generate somatic chi- 
meric mice, and ES cell-derived pre-B cell lines 
were established. Analyses of these cell lines demon- 
strated that this enhancer replacement resulted in a 
cis-acting block in the assembly of endogenous Ig 
heavy-chain variable-region genes. 
Dr. Alt's group is currently employing "hit-and- 
run" gene-targeted mutational approaches to elimi- 
nate, mutate, or replace more precisely endogenous 
transcriptional regulatory elements to study their 
role in control of VDJ recombination. They are also 
continuing to employ VDJ recombinase-inducible 
cell lines that they have recently generated for use 
in the context of transfection approaches aimed at 
elucidating in detail how specific motifs within 
particular enhancer or promoter elements confer 
accessibility to linked V, D, and J segments for VDJ 
recombination. (The studies described in this para- 
graph were also supported by grants from the Na- 
tional Institutes of Health.) 
Novel Animal Models to Study Development 
of the Immune System 
Dr. Alt's group has developed a novel ES cell- 
based approach for rapidly assessing the role of par- 
ticular genes or DNA segments in the development 
or function of lymphocytes. Blastocysts derived 
from Rag-2-deficient mice are injected with either 
normal or mutant ES cells and used to generate so- 
matic chimeric mice. Somatic chimeras derived 
from Rag- 2 -deficient blastocysts injected with nor- 
mal ES cells develop mature lymphocytes, all of 
which are ES cell derived. Therefore it is possible to 
mutate both copies of any gene that does not affect 
ES cell viability and test for its role in lymphocyte 
development by this method. 
For example, the group has assayed blastocysts in- 
jected with ES cells in which both copies of the Ig 
heavy-chain locus were disrupted in such a way that 
it was impossible to form functional heavy-chain 
genes. The resulting somatic chimeric mice form 
normal T cells but not B cells. Dr. Alt's group is now 
employing this approach to test the function of a 
variety of different lymphocyte-specific and more 
generally expressed genes (e.g., myc family genes) 
in the context of lymphoid development. 
Dr. Alt is also Professor of Genetics and Pediat- 
rics at the Children 's Hospital, Boston, and Har- 
vard University Medical School. 
Books and Chapters of Books 
Alt, F.W., editor. 1992. Immunology in the 21st 
Century. St. Louis, MO: Sigma Chemical Co. 
Articles 
Alt, F.'W., Oltz, E.M., Young, F., Gorman, J., Tac- 
cioli, G., and Chen, J. 1992. VDJ recombination. 
Immunol Today 13:306-3l4. 
Alt, F.'W., Rathbun, G., Oltz, E., Taccioli, G., and 
Shinkai, Y. 1992. Function and control of recom- 
bination-activating gene activity. Ann NY Acad 
Sci 651:211-294. 
Li, S C., Rothman, P., Boothby, M., Ferrier, P., 
Glimcher, L., and Alt, F.'W. 1991 • Control of im- 
munoglobulin heavy chain constant region gene 
expression. Adv Exp Med Biol 292:245-251. 
Ma, A., Fisher, P., Dildrop, R., Oltz, E., Rathbun, 
G., Achacoso, P., Stall, A., and Alt, F.W. 1992. 
Surface IgM mediated regulation of RAG gene ex- 
pression in En-N-myc B cell lines. EMBO f 
11:2727-2734. 
Moroy, T., Fisher, P.E., Lee, G., Achacoso, P., 
Wiener, F., and Alt, F.'W. 1992. High frequency 
of myelomonocytic tumors in aging E/ix L-myc 
transgenic mice, f Exp Med 175:313-322. 
Moroy, T., Verbeek, S., Ma, A., Achacoso, P., 
Berns, A., and Alt, F. 1991. EyuN- and E^L-myc 
cooperate with E/xpim- 1 to generate lymphoid tu- 
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