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| The Human Genome Project and Drug Discovery in Psychiatry: Identifying Novel Targets |
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SHELDON H. PRESKORN, MD |
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Journal of Psychiatric Practice, March 2001, 133-137 |
| On the other hand, this seems to be a distinct advantage of this psychology. One might be able to learn from the specific effect of a given drug something about the true nature of the symptom. |
| Emil Kraepelin, MD, 1921 father of modern psychiatry1 |
This column is the third in a series on the human genome project and its implications for drug dis-covery for brain diseases, particularly those involving disturbed higher brain functions: cog-nition, perceptual organization, and personality. The sec-ond column in this series 2 covered the following points:
(Equation 1)
Effect = potency for X drug X biological site of action concentration variance
This column will extend the discussion without repeat-ing that material. For this reason, some readers may wish to refer to the previous column in the November 2000 issue of this journal for definitions of terms and a sum-mary of the stages of drug development.
DRUG DISCOVERY IN PSYCHIATRY AND NEUROTRANSMITTERS
The first stage of drug development is drug discovery. The goal of drug discovery is to develop a new chemical enti-ty (NCE) ideally for a new site of action (the first variable in Equation 1). Regulatory proteins are the sites of action of most drugs. For psychiatric drugs, those regulatory proteins fall into three broad classes:
These mechanisms of neurotransmission are critical to the organization and function of the brain. By specifically altering such sites of action, drugs can influence specific brain function, as foreseen by Kraepelin in 1921. One of the goals of the human genome project is to identify new neurotransmitters and their related regulatory proteins.
Between the 1920s (when Kraepelin was writing his textbooks of psychiatry) and the late 1960s, only a few amino acids and amines were recognized as neurotrans-mitters. Following the logic laid out by Kraepelin, psychi-atric researchers in the 1960s developed the biogenic amine theories of affective illnesses and the dopamine theories of schizophrenia.3 Those theories were deduced from studying the effects of serendipitously discovered psychiatric drugs. These theories have dominated biolog-ical research in psychiatry and psychiatric drug develop-ment for most of the second half of the 20th century; yet these neurotransmitters are found in only a few hundred thousand out of the billions of neurons in the human brain. During the 1970s, the number of neurotransmit-ters identified in the mammalian brain expanded from a handful to over 70 (Table 1). This increase was primarily the result of the identification of a large number of neu-ropeptide neurotransmitters (Table 1). Nevertheless, cur-rent discussions of clinical psychopharmacology generally focus on only four or five neurotransmitters.
| Table 1. Transmitters in the mammalian brain | ||
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NEUROPEPTIDES
Neuropeptides are short sequences of amino acids. They were identified as neurotransmitters in the brain as a result of the development of radioimmunoassays and immunohistochemical techniques. Using radioimmunoas-says, researchers could quantify the levels of neuropep-tides in specific regions of the brain. Using immunohistochemistry, they could map the anatomy of the neuropeptide neurotransmitters. Based on these maps, scientists could postulate what functions a specific neuropeptide system might serve in the mammalian brain based on the adage that function generally follows structure: If a neurotransmitter is found in a region of the brain that has been implicated in a specific process (e.g., feeding), then perhaps that neurotransmitter mediates or influences that process. Moreover, researchers could test these theories by injecting the specific neuropeptide of interest into brain regions containing terminal fields for the neuropeptide and then observe the resultant changes in brain output (e.g., changes in feeding).
While this column is not intended to provide a defini-tive review of neuropeptides, a few comments may help put the rest of the discussion in perspective. These neu-rotransmitters are found at both synaptic and nonsynap-tic sites in the brain. Their primary role appears to involve slow signaling. However, they also exert trophic actions.4,5 For example, vasoactive intestinal peptide can dramatically affect fetal growth in vitro6 and calcitonin gene-related peptide (CGRP), after release from the motor end-plate, regulates the expression of acetylcholine receptors.7 Thus, these neurotransmitters may have developmental implications as well as longer term adap-tive functions that may be more important and funda-mental to the pathophysiology of some psychiatric diseases, such as schizophrenia or bipolar illness, than amino acid or amine neurotransmitters. These factors explain why neuropeptides are enticing targets for a drug discovery program.
The fact that neuropeptides have trophic functions raises the possibility of developing somatic therapies that influence biological processes far more complex than synaptic transmission. The implications are intriguing because the human brain, even in the adult, is by its very nature metastable. Its structure changes in response to the environment. That fact is fundamental to the process called learning (and by extension to psychotherapy). The degree and rapidity of the plasticity of the human brain is what has conferred such adaptability on the species. Thus, the human brain is constantly being reshaped by the growth and withdrawal of cell processes and contacts, by gene induction and repression.
Neuropeptides are only one class of neuronal trophic and differentiation factors involved in the maturation and reshaping of the brain. Table 2 lists other proteins (i.e., gene products) that have been identified in the human brain. These proteins are potent and specific mechanisms that promote or mitigate neuronal death or synaptic reshaping and thus permit changes in the bal-ance and interconnectedness of neurotransmitter sys-tems, which in turn result in the modification of, or the capacity to modify, behavior. These regulatory proteins are therefore also potential targets for specific drug dis-covery programs.
Parenthetically, the development of such somatic inter-ventions might dramatically increase the effectiveness of a variety of kinds of psychosocial interventions-and might, in fact, require such interventions to be effective. Consider that a revolutionary new orthopedic device (e.g., a new joint replacement) may be of minimal to no value without conjoint physical therapy. In other words, addressing "hardware" issues is likely to make "software" (i.e., psychosocial) interventions more effective and more critical.
| Table 2. Neuronal differentiation and growth factors in mammalian brain | |
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GENETIC APPROACHES TO TRANSMITTER-RECEPTOR IDENTIFICATION
Although radioimmunoassays and immunohistochem-istry were highly successful during the 1970s in facilitat-ing the identification of new neuropeptides, the pace of discovery had slowed down considerably by the mid-1980s. At that time, the potential of genetic approaches became apparent. In 1987, Masu et al. reported the isola-tion of a cDNA for a bovine substance K receptor.8 This receptor was found to be coupled to a G-protein and to have seven transmembrane spanning segments. Subsequently, the neuronal substance P, neurotensin, and several opioid receptors were also cloned and found to share the same properties (i.e., being coupled to a G-pro-tein and having seven transmembrane spanning seg-ments).9-12
Based on these findings, the concept arose that these various receptors for different neuropeptides all belonged to a supergene family of receptors. In other words, they were a genetic variation on a theme in much the same manner as the supergene family of cytochrome P450 (CYP) enzymes (which I have discussed in a number of previous columns). In a fashion analogous to the CYP enzymes, the supergene family of neuropeptide receptors can be subdivided into smaller families based on the degree of shared sequence homology and function or pharmacological profile.
Parenthetically, all of the concepts I have explored with regard to CYP enzymes, including the existence and functional importance of polymorphisms and their rele-vance to understanding biological variance (variable 3 in Equation 1), have direct parallels in other regulatory supergene families, such as those for receptors. Thus, the debate that occurred in the 1990s about the functional significance of CYP enzyme induction and inhibition13,14 has relevance to future discoveries concerning receptors. Hopefully, lessons learned from those debates will help to shorten the lag time between new discoveries and the application of the resultant knowledge to the practice of medicine.
The discovery that neuropeptide receptors share a common structure (i.e., G-protein coupled receptors with seven transmembrane spanning segments) raised the possibility that the genome could be scanned for DNA sequences that had a high probability of coding for pre-viously unidentified neuropeptide receptors. Once such DNA sequences were identified, molecular biological approaches could be applied to determine whether the sequence was in fact a gene that coded for a receptor. These approaches include taking the sequence (cDNA) and transfecting it into a cell line or a single cell organ-ism that does not normally contain the sequence and then studying the consequence of its expression-includ-ing whether its expressed product is incorporated into the cell membrane in a fashion consistent with being a receptor and whether it is linked to a G-protein.
ORPHAN RECEPTORS AND REVERSE PHARMACOLOGY
Such a protein can be accepted as a putative receptor on the basis of these characteristics. Nevertheless, it is an "orphan" in the sense that it has no identified natural ligand (i.e., neurotransmitter) or function. However, the identification of such an "orphan" permits researchers to use "reverse pharmacology" to fill in the gaps in our knowledge.15
Classically, pharmacology has moved from observing a function to the identification of the neurotransmitter and receptor involved. For example, gut peristalsis is observable physiology. Herbs and other natural products have been found that can influence gut peristalsis pre-sumably by affecting the normal processes (i.e., the structures) that mediate this physiology. Moreover, the anatomy of the nervous system within the gut from the nerve fibers to the smooth muscle could be seen under the light microscope. Ex vivo experiments could therefore be performed with biological preparations such as guinea pig ileum to determine the specific constituents in the herbs or natural products that could either stimulate or block gut peristalsis. From these observations, the neu-rotransmitters and receptors mediating the physiology could be identified.
This process can now be reversed using structure to determine physiology.16-18 The identification of orphan receptors thus holds the promise of uncovering biological processes that were previously either unknown or at least poorly understood. In essence, the orphan receptor can be used as "bait" to troll for and thus identify the nat-ural ligand or neurotransmitter. Using techniques anal-ogous to radioimmunoassay and immunohistochemistry from the 1970s, researchers can map the distribution of the neurotransmitters and receptors in the brain. Using these maps, researchers can then identify the functions subsumed by the transmitter and its receptor. That infor-mation can in turn be used to determine whether this physiology mediates a process that would be promising with regard to launching a drug discovery program aimed at one or more of the regulatory proteins involved.
Because of advances made possible by the human genome project in the identification of genes coding for previously unknown receptors, the number of identified receptors (Table 1) may double in the next 10 years. This development in turn holds the promise that these recep-tors and reverse pharmacology will make possible signif-icant strides in the understanding of normal and abnormal physiological mechanisms in the brain rele-vant to psychiatric disease. The human genome project thus has tremendous potential for capitalizing on the observation made by Kraepelin concerning the use of drugs to understand the physiology underlying psychi-atric disease.
Parenthetically, it is important for prescribers to real-ize that a "med check" is essentially a form of therapeutic drug monitoring. Prescribers use their skills and knowl-edge to determine the nature and the magnitude of the effect (equation 1) that their treatment has produced in their patients. In other words, the prescriber is assaying whether his or her medication engaged the right target (variable 1) to the right degree (variable 2) in the right patient (variable 3). However, further discussion of "med checks" as therapeutic drug monitoring is a topic for another column.
The next column in this series will illustrate that the developments I have described here are not science fiction but are already underway. From the drug discovery stand-point, a dilemma is posed by the sheer number of poten-tial targets that the human genome project is uncovering. The critical issue will be to select targets with a high like-lihood of being clinically important and commercially viable. How this can be done will be the topic of the next column.