Structural Determinants of a-Globin 
Gene Expression 
i 
Stephen A. Liebhaber, M.D. — Associate Investigator 
Dr. Liebhaber is also Associate Professor of Human Genetics and Medicine (Hematology ) at the University 
of Pennsylvania School of Medicine. He received his B.A. degree in chemistry from Brandeis University 
and his M.D. degree from Yale University. He took clinical training in internal medicine, hematology, and 
molecular biology at four other universities. As a postdoctoral fellow with David Schlessinger at Wash- 
ington University, Dr. Liebhaber examined ribosomal RNA processing, and with Y. W. Kan at the Univer- 
sity of California, San Francisco, he studied human globin gene expression and genetic defects in a-thal- 
assemia. Before moving to Philadelphia, he was a faculty member of the Department of Medicine at UCSP. 
EXPRESSION of a eukaryotic gene is a multi- 
step process. Specific regions of chromatin 
must be activated and transcribed, transcripts pro- 
cessed, and mature mRNA exported to the cyto- 
plasm for translation into protein. Gene expres- 
sion can be controlled at any or all of these steps. 
Our laboratory has largely concentrated on study- 
ing the expression of the human globin genes. 
These genes encode hemoglobin, the major red 
cell protein responsible for oxygen transport 
from the lungs to peripheral tissues. Since the 
hemoglobin molecule, a2/^2' is composed of an 
equal number of a- and |8-globin chains, normal 
synthesis demands balanced expression of both 
sets of genes. Defects in either set result in an 
imbalance of expression and consequent anemia: 
a- or i8-thalassemia. Thalassemias result from al- 
most 100 different mutations in the globin genes, 
affecting the health of millions worldwide. 
Certain characteristics of globin gene expres- 
sion make it particularly interesting for study. 
The extremely high level of globin mRNA in the 
differentiating red cell (over 95 percent of total 
cellular mRNA) has no equal in any other cell 
type. This abundance reflects both high levels of 
synthesis (transcription) and an unusual stability 
of the mature globin mRNA. 
Another interesting characteristic is that the a- 
and /3-globin gene clusters follow an orderly se- 
quence of expression during embryologic devel- 
opment. This results in a well-defined switch 
from embryonic to adult globin gene expression 
during development of the fetus. The switching 
results in the synthesis of successive hemoglo- 
bins with oxygen afiinities that match changes in 
the uterine environment. The active transcrip- 
tion, unusual mRNA stability, and clearly defined 
pattern of developmental switching are areas of 
special focus in our laboratory. 
The loss of a-globin gene expression observed 
in a-thalassemia usually results from deletion or 
abnormal structure of one or more of the a-globin 
genes. We have recently studied a series of pa- 
tients with a rather unusual form of a-thalassemia 
in that their silenced a-globin genes have entirely 
normal structures. When these genes are isolated 
and reintroduced into cells, they function as well 
as normal ones. The cause of the abnormality in 
these patients appears to lie in a region of DNA 
outside the genes themselves. 
Through extensive DNA mapping, we have de- 
tected large deletions outside the a-globin clus- 
ter in three a-thalassemic individuals. One of 
these ectopic deletions is separated from the a- 
globin gene by at least 30,000 bases. By compar- 
ing the maps of each deletion, a region of com- 
mon overlap is noted. Since a similar set of signals 
critical for transcriptional activation has been 
identified adjacent to the |S-globin gene cluster, 
one can speculate that such signals serve coordi- 
nately to activate the expression of both the a- 
and |S-globin clusters in the red cell. 
The human a-globin gene cluster contains a f- 
globin gene expressed specifically in the embryo 
and two a-globin genes, al and a2, expressed in 
the fetus and the adult. The switch from embry- 
onic ^globin to adult a-globin occurs at the end 
of embryonic development (7-8 weeks of gesta- 
tion). This critical developmental switch, which 
occurs widely in mammals, presents a well- 
defined model system for studying developmen- 
tal control of gene expression. 
To establish a system in which to study switch- 
ing within the human a-globin gene cluster, we 
have introduced the human and a-globin genes 
into fertilized mouse eggs to generate transgenic 
mice. The red cells of these mice appropriately 
express the human transgenes during develop- 
ment. In the embryonic period, there is parallel 
expression of the human and mouse f-globin 
genes, and by day 12 of development, parallel 
expression of the a-globin genes. These data sug- 
gest that 1 ) the human transgenes contain the nec- 
essary information for appropriate developmen- 
tal control and 2) the factors responsible for 
developmental switching in the red cell have 
been sufficiently conserved during evolution to 
substitute in the control over the human trans- 
genes. By generating transgenic mice that carry 
human and a-globin genes with specific alter- 
ations, and by studying their pattern of develop- 
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