Page 38 - Hematology_ Basic Principles and Practice ( PDFDrive )

10 Part I Molecular and Cellular Basis of Hematology

thus a means by which information regarding chromatin structure is regulation at the mRNA level are regulated alternative mRNA splic-
passed to daughter cells after DNA replication occurs. ing, control of mRNA stability, and control of translational efficiency.
As discussed elsewhere (see Chapter 5), additional regulation at the
protein level occurs by mechanisms modulating localization, stability,
ENHANCERS, PROMOTERS, AND SILENCERS activation, or export of the protein.
A cell can regulate the relative amounts of different protein iso-
Several types of cis-active DNA sequence elements have been defined forms arising from a given gene by altering the relative amounts of
according to the presumed consequences of their interaction with an mRNA precursor that are spliced along one pathway or another
nuclear proteins (see Fig. 1.5). Promoters are found just upstream (to (alternative mRNA splicing). Many striking examples of this type of
the 5′ side) of the start of mRNA transcription (the CAP). mRNA regulation are known—for example, the ability of B lymphocytes to
polymerases appear to bind first to the promoter region and thereby make both IgM and IgD at the same developmental stage, changes
gain access to the structural gene sequences downstream. Promoters in the particular isoforms of cytoskeletal proteins produced during
thus serve a dual function of being binding sites for mRNA poly- red blood cell differentiation, and a switch from one isoform of the
merase and marking for the polymerase the downstream point at c-myb proto-oncogene product to another during red blood cell dif-
which transcription should start. ferentiation. Abnormalities in mRNA splicing due to mutations at
Enhancers are more complicated DNA sequence elements. the splice sites can lead to defective protein synthesis, as can occur in
Enhancers can lie on either side of a gene or even within the gene. β-globin, leading to a form of β-thalassemia. The effect of controlling
Enhancers bind transcription factors and thereby stimulate expression the pathway of mRNA processing used in a cell is to include or
of genes nearby. The domain of influence of enhancers (i.e., the exclude portions of the mRNA sequence. These portions encode
number of genes to either side whose expression is stimulated) varies. peptide sequences that influence the ultimate physiologic behavior
Some enhancers influence only the adjacent gene; others seem to of the protein, or the RNA sequences that alter stability or
mark the boundaries of large multigene clusters (gene domains) translatability.
whose coordinated expression is appropriate to a particular tissue type The importance of the control of mRNA stability for gene regula-
or a particular time. For example, the very high levels of globin gene tion is being increasingly appreciated. The steady-state level of any
expression in erythroid cells depend on the function of an enhancer given mRNA species ultimately depends on the balance between the
that seems to activate the entire gene cluster and is thus called a rate of its production (transcription and mRNA processing) and its
locus-activating region (see Fig. 1.5). The nuclear factors interacting destruction. One means by which stability is regulated is the inherent
with enhancers are probably induced into synthesis or activation as structure of the mRNA sequence, especially the 3′ and 5′ UTRs. As
part of the process of differentiation. Chromosomal rearrangements already noted, these sequences appear to affect mRNA secondary
that place a gene that is usually tightly regulated under the control structure, recognition by nucleases, or both. Different mRNAs thus
of a highly active enhancer can lead to overexpression of that gene. have inherently longer or shorter half-lives, almost regardless of the
This commonly occurs in Burkitt lymphoma, for example, in which cell type in which they are expressed. Some mRNAs tend to be highly
the MYC proto-oncogene is juxtaposed and dysregulated by an unstable. In response to appropriate physiologic needs, they can thus
immunoglobulin enhancer. be produced quickly and removed from the cell quickly when a need
Silencer sequences serve a function that is the obverse of enhancers. for them no longer exists. Globin mRNA, on the other hand, is
When bound by the appropriate nuclear proteins, silencer sequences inherently quite stable, with a half-life measured in the range of
cause repression of gene expression. Some evidence indicates that the 15 to 50 hours. This is appropriate for the need of reticulocytes to
same sequence elements can act as enhancers or silencers under dif- continue to synthesize globin for 24 to 48 hours after the ability to
ferent conditions, presumably by being bound by different sets of synthesize new mRNA has been lost by the terminally mature
proteins having opposite effects on transcription. Insulators are erythroblasts.
sequence domains that mark the “boundaries” of multigene clusters, The stability of mRNA can also be altered in response to changes
thereby preventing activation of one set of genes from “leaking” into in the intracellular milieu. This phenomenon usually involves nucle-
nearby genes. ases capable of destroying one or more broad classes of mRNA
defined on the basis of their 3′ or 5′ UTR sequences. Thus, for
example, histone mRNAs are destabilized after the S-phase of the cell
TRANSCRIPTION FACTORS cycle is complete. Presumably this occurs because histone synthesis is
no longer needed. Induction of cell activation, mitogenesis, or termi-
Transcription factors are nuclear proteins that exhibit gene-specific nal differentiation events often results in the induction of nucleases
DNA binding. Considerable information is now available about these that destabilize specific subsets of mRNAs. Selective stabilization of
nuclear proteins and their biochemical properties, but their physio- mRNAs probably also occurs; α-globin mRNA, for example, is sta-
logic behavior remains incompletely understood. Common structural bilized by the protective binding of a specific stabilizing protein to a
features have become apparent. Most transcription factors have nuclease target sequence in its 3′ UTR.
DNA-binding domains sharing homologous structural motifs The amount of a given protein accumulating in a cell depends on
(cytosine-rich regions called zinc fingers, leucine-rich regions called the amount of the mRNA present, the rate at which it is translated
leucine zippers, and so on), but other regions appear to be unique. into the protein, and the stability of the protein. Translational effi-
Many factors implicated in the regulation of growth, differentiation, ciency depends on a number of variables, including polyadenylation
and development (e.g., homeobox genes, proto-oncogenes, antionco- and presence of the 5′ cap. The amounts and state of activation of
genes) appear to be DNA-binding proteins and may be involved in protein factors needed for translation are also crucial. The secondary
the steps needed for activation of a gene within chromatin. Others structure of the mRNA, particularly in the 5′ UTR, greatly influences
bind to or modify DNA-binding proteins. These factors are discussed the intrinsic translatability of an mRNA molecule by constraining
in more detail in several other chapters. the access of translation factors and ribosomes to the translation
initiation signal in the mRNA. Secondary structures along the coding
REGULATION OF mRNA SPLICING, STABILITY, AND sequence of the mRNA may also have some impact on the rate of
elongation of the peptide.
TRANSLATION (POSTTRANSCRIPTIONAL REGULATION) Changes in capping, polyadenylation, and translation factor effi-
ciency affect the overall rate of protein synthesis within each cell.
It has become increasingly apparent that posttranscriptional and These effects tend to be global rather than specific to a particular
translational mechanisms are important strategies used by cells to gene product. However, these effects influence the relative amounts
govern the amounts of mRNA and protein accumulating when a of different proteins made. mRNAs whose structures inherently lend
particular gene is expressed. The major modes of posttranscriptional themselves to more efficient translation tend to compete better for