THE
FATE OF ANTISENSE OLIGOS IN VIVO:
Gene therapy seeks to treat disease with a nucleic acid that changes the
level of expression of a specific gene product, ie, a protein. The central
dogma of molecular biology holds that each protein is synthesized (translated)
using information contained in a messenger RNA (mRNA) molecule copied (transcribed)
from a specific DNA sequence (gene) on a chromosome. The nucleic acid sequence
of the chromosomal DNA determines the nucleic acid sequence of the mRNA,
which is used to determine the amino acid sequence of the protein. RNA,
like DNA, can form a double-stranded structure by base-pairing with another
nucleic acid whose sequence is the complement of the original strand. Indeed,
each mRNA molecule, which is called a ``sense'' strand, encodes a specific
nucleic acid sequence because it has been synthesized using the complementary
``antisense'' strand of the DNA double helix as a template. Mammalian cells
spontaneously take up short pieces of single-stranded DNA (called oligonucleotides
or oligos for short). When these oligos contain the antisense sequence of
a gene, they can pair (hybridize) with an mRNA encoding the sense sequence.
Upon hybridization, antisense oligos thus reduce synthesis of the encoded
protein, either through steric inhibition of translation or by making the
mRNA molecule sensitive to cellular nucleases--or both. Minor changes in
the structure of the single-stranded DNA backbone (eg, using phosphorothioate
linkages instead of naturally occurring phosphate linkages) can assist antisense
oligos in resisting nucleases, making them more effective. This kind of
gene therapy is already used in clinical trials. It is important to appreciate
that there are large differences between treating cultured cells with antisense
oligos and administering antisense oligos to animals or patients. In this
issue of Laboratory Investigation, Butler and colleagues provide new information
about where oligos go when they are administered to rodents by the intravenous
route used in clinical trials. By three separate pproaches, using either
antibodies specific for a particular oligo, fluorescently tagged oligos,
or radioisotope-tagged oligos, the investigators find similar answers to
this question, namely that oligos are concentrated in the epithelium of
renal proximal tubules, presumably by reabsorption from urine; in the macrophages
of liver and probably spleen; and more surprisingly in sinusoidal endothelium
of liver and in connective tissue cells of skin and viscera. This study
thus provides important clues about which cells will likely be most sensitive
to antisense therapy in vivo, and also highlights the continuing utility
of the classic tools of the anatomic pathologist in the era of molecular
medicine.
MHC AND OPPORTUNISTIC INFECTION
IN SIMIAN AIDS:
Genes located in the major histocompatibility complex (MHC) control the
vigor of the specific immune response to simple antigens. It is now well
understood how several of these immune response genes, namely those encoding
class I and II MHC molecules, actually work. MHC molecules bind peptides
derived from partial proteolysis of proteins. The antigen receptors on mature
lymphocytes recognize peptides derived from foreign (microbial) proteins
only when they are a part of a complex with an MHC molecule. MHC molecules
are highly polymorphic, with hundreds of common allelic forms expressed
in the population. The structural differences among these allelic forms
of MHC molecules determine which foreign peptides can bind and thus which
peptides can be recognized by T cells and activate an immune response. In
this manner, the set of MHC gene alleles inherited by each individual determines
whether the T cells of that individual can react to a specific peptide,
and, by extension, whether that individual can make an effective protective
immune response to a specific pathogen. This relationship is well established
for inbred mouse strains. Normally, in humans, there are enough different
T-cell clones, MHC molecules expressed by the various class I and II gene
loci, and peptides that can be derived from the various proteins of a particular
microbe that the MHC-encoded immune response gene effects are not significant.
However, when the number of protective T cell clones is reduced--eg, as
in AIDS--the importance of MHC genes in determining susceptibility to infection
might be predicted to increase. In the present issue of the journal, Baskin
and coworkers test this prediction in simian AIDS. Strikingly, analysis
of opportunistic infections in SIV-infected rhesus monkeys shows two strong
associations of MHC genes with particular infections, namely a class I-linked
susceptibility to cytomegalovirus and a class II-linked susceptibility to
Cryptosporidium. These findings might have been expected because class I
genes play a mor important role in resistance to intracellular pathogens
(such as cytomegalovirus), and class II genes have a stronger function in
resistance to extracellular pathogens (such as Cryptosporidium). It would
be premature to begin MHC (ie, HLA)-typing human patients to predict their
susceptibility to different infections. Nevertheless, the clear association
of MHC allotype with susceptibility to infection in a primate population
is a significant confirmation of our current theory of immunity and its
relationship to infection immunity.
MAST CELLS CONTROL HAIR
FOLLICLE REGRESSION:
Mast cells (MC) have long been appreciated as ever-present sentinels, clustered
in connective tissue adjacent to blood vessels and armed to release an awesome
and diverse array of chemotactic, vasoactive, and spasmogenic compounds
at the slightest perturbation. Recent evidence from several investigators
now suggests that MC may also play an important role in controlling physiologic
tissue remodeling independent of their participation in IgE-mediated inflammatory
and allergic reactions. Armed for such purpose by a rich complement of growth
modulatory factors--such as IL-1, IL-6, TNF-[alpha], and transforming growth
factor-[beta]1--MC appear to facilitate normal remodeling processes including
those involved in skeletal homeostasis and dentition, cyclic menstruation,
mucosal nerve density and organization; and cyclic hair follicle (HF) growth
and regression. It has been difficult, however, to establish unequivocally
a mechanistic relationship between MC degranulation and physiologic remodeling,
an issue compounded by the observation that even in animals in which the
Kit gene has been knocked out, and which are consequently deficient in MC,
bone remodeling and HF cycling continues. In this month's Laboratory Investigation,
Maurer and colleagues re-examine the relationship of MC activation to the
process of HF cycling by examining in detail the relationship of perifollicular
MC activation during the process of anagen-catagen-telogen transformation
of back skin HF in mice. Spontaneous catagen induction (regression) of the
HF correlates with a dramatic MC degranulation, followed by a decrease in
MC numbers. This apparent relationship was experimentally verified as causal
by demonstrating that in vivo activation of dermal MC induced premature
catagen development, whereas inhibition of MC degranulation retarded normal
catagen development. Catagen development was also found to be retarded in
the Kit k/o mice. Collectively, these observations provide some of the strongest
evidence yet that MC play heretofore unappreciate roles in physiologic tissue
remodeling, and also reveal novel control pathways that may prove relevant
to the control of normal hair growth (or no-growth).
CD22 AND CIRCULATING MYELOMA
CELLS:
Studies of teratocarcinomas in mice and humans have been pivotal in establishing
that the malignant cells that accumulate in tumors have often differentiated
from a less mature precursor. The concept that anaplasia underlies carcinogenesis
has thus been replaced by the notion that tumors result from interruption
of the differentiation pathway of a given cell lineage. A corollary is that
the most primitive cells of a tumor represent the repository from which
much of the tumor mass is derived, and that the trafficking of these ``tumor
stem cells'' determines the metastatic manifestations of the disease. Many
studies of multiple myeloma have suggested that circulating early myeloma
cells feed the clonal end-stage plasma cell population in the bone marrow,
and are thus responsible for the dissemination of the disease. The precise
nature of these ``myeloma stem cells,'' however, has remained uncertain.
The paper by Perfetti et al in this issue of the journal demonstrates that,
in some instances, the circulating myeloma-related cells are mature or late
B lymphocytes defined by the expression of membrane CD22, a marker that
is lost on more mature plasma cells. Furthermore, these authors provide
functional evidence that the early myeloma cells are precursors of the tumoral
plasma cells, and show that anti-CD22 antibodies can be used to deliver
a toxin that can, in vitro, eliminate these early-myeloma B cells, thereby
preventing development of malignant plasma cells. The clinical implications
are twofold: first, the existence of early-myeloma B cells might explain
instances of chemoresistance; and second, CD22 may well be a suitable therapeutic
target to eradicate these cells.
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