Inborn Errors of Signal Transduction: Mutations in G Proteins and G Protein-coupled Receptors as a Cause of Disease
| Title: | Inborn Errors of Signal Transduction: Mutations in G Proteins and G Protein-coupled Receptors as a Cause of Disease |
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| Author: | Spiegel, M.D., Allen M. |
| Publisher: | Journal of Inherited Metabolic Disease |
| Date Published: | January 01, 1997 |
| Reference Number: | 152 |
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Just as Garrod is generally credited as the 'father' of studies of inborn errors of metabolism (Garrod 1923), so can Albright be credited as the 'father' of studies of inborn errors of signal transduction. In his 1942 report on pseudohypoparathyroidism (PHP), also termed Albright hereditary osteodystrophy (McKusick 103580), he astutely recognized (before the advent of sensitive hormone assays) that the typical features of hypoparathyroidism observed in patients with PHP are due not to deficiency of parathyroid hormone but rather to end-organ resistance to the actions of the hormone (Albright et al 1942). Thus was born the concept of 'hormone resistance', now recognized to be caused by loss-of-function mutations in hormone receptors or more distal components in a signal transduction pathway.
Inborn errors of metabolism are generally caused by loss-of-function mutations of enzymes, with phenotypic features reflecting the consequences of excessive substrate, deficient product, or both. For inborn errors of signal transduction caused by loss-of-function mutations of signalling components, phenotypic features are caused by loss of the normal physiological actions of hormones or other extracellular 'first messengers' that activate receptors.
Unlike most inborn errors of metabolism, disorders of signal transduction may also be caused by gain-of-function mutations of signalling components. Such mutations cause constitutive activation that is independent of normal physiological stimulation by 'upstream' components of the signal transduction cascade. activating mutations may occur in the germline in inherited disorders or as somatic mutations in sporadic disorders. Indeed, another disorder described by Albright, the McCune-Albright syndrome (McKusick 174800), is a sporadic disease with pleiotropic features caused by an activating somatic mutation of a critical signalling component termed a G protein (Weinstein et al 1991).
In the past few years, loss- and gain-of-function mutations in G proteins and G protein-coupled receptors (GPCR) have been identified as the cause of several diseases. Understanding the molecular basis of these diseases provides valuable insights into G protein and GPCR structure and function, and may be important for diagnosis and treatment. In this paper, I briefly describe G protein-coupled signal transduction, provide a general framework for understanding how G protein and GPCR mutations cause diseases, and describe several examples of disorders caused by G protein and GPCR mutations.
G PROTEIN-COUPLED SIGNAL TRANSDUCTION -- A BRIEF OVERVIEW
For detailed coverage of the enormous body of data on the structure and function of G proteins and GPCR, the interested reader is referred to recent monographs and reviews (Spiegel et al 1994; Iismaa et al 1995; Neer 1995). Owing to limitations of space, here I will give only a brief overview, emphasizing some features relevant to the pathophysiology of disorders caused by defective G protein-coupled signal transduction. G proteins couple receptors for diverse extracellular signals to effectors such as enzymes and ion channels. The majority of polypeptide hormones, all monoamine neurotransmitters, prostaglandins and even extracellular Ca2+ signal their target cells through GPCR. GPCR comprise a superfamily sharing a common structural and functional motif, a single polypeptide with seven membrane-spanning domains. All GPCR act by promoting release of tightly bound GDP from G protein a subunits, thereby enabling GTP to bind and activate the G protein (Figure 1). Within the GPCR superfamily, differences in sequence and structure presumably contribute to differences in ligand recognition and in G protein coupling. For small ligands such as catecholamines, the binding site is within the membrane bilayer in a pocket formed by several of the membrane-spanning domains. For larger polypeptide hormones, the extracellular amino-terminus and one or more extracellular loops may be involved in ligand binding. Different classes of GPCR couple exclusively or preferentially to specific G proteins. G protein coupling involves the intracellular loops and intracellular carboxy-terminus of the receptor. A plausible model of GPCR function suggests that GPCR are dynamic proteins, moving spontaneously between conformations favouring and not favouring G protein coupling. According to this model, binding of an agonist would stabilize the conformation favouring G protein coupling, thus 'activating' the receptor.
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G proteins consist of three distinct polypeptide gene products. The a subunit binds guanine nucleotides with high affinity and specificity. The b and g polypeptides are tightly but noncovalently associated in a functional dimer subunit. The heterotrimer, associated with the inner surface of the plasma membrane (Figure 1), is required for high-affinity coupling to GPCR. Upon a subunit binding of GTP and dissociation from the bg dimer, each subunit can independently modulate the activity of one or more effectors such as adenylyl cyclase, the enzyme that generates the second messenger cAMP. There are 16 mammalian a subunit genes. They vary in range of expression and in specificity of receptor--effector coupling. Some such as Gs-a, responsible for stimulation of cAMP formation, are expressed ubiquitously. Based on degree of amino acid identity they have been divided into four subfamilies: Gs, Gq, Gi and G12.
MUTATIONS IN G PROTEINS AND GPCR: GENERAL CONSIDERATIONS
There are three principal determinants of the phenotypic expression of mutations in G proteins and GPCR. (1) The range of expression of the mutated gene: mutations in a ubiquitously expressed gene such as Gs-a in general will cause more generalized manifestations than those caused by mutations in a gene such as a receptor that is more restricted in expression (but see next). (2) Germline (inherited) versus somatic (postzygotic) mutations: the former potentially cause manifestations in every cell in which the gene is expressed. Somatic mutation of even a ubiquitously expressed gene, in contrast, would still lead to manifestations that are localized to the cells derived from the progenitor in which the original somatic mutation occurred. (3) Nature of the mutation: mutations can be broadly divided into those causing gain versus those causing loss of function.
Loss-of-function mutations block normal mRNA and/or protein synthesis, prevent the synthesized protein from reaching its normal subcellular location (the plasma membrane in the case of GPCR and G proteins (see 1 in Figure 1)), or impair function despite synthesis and normal targeting of the protein. Many GPCR mutations, including missense mutations, will cause abnormal folding of the protein with retention in the endoplasmic reticulum. GPCR mutations compatible with normal protein synthesis and trafficking may nonetheless cause loss of function by impairing agonist binding to or activation of receptor (2 in Figure 1) or by impairing receptor coupling to and activation of G protein (3 in Figure 1). G protein loss-of-function mutations, in addition to impairing normal protein synthesis or trafficking, may block receptor coupling or activation by GTP (3 in Figure 1) or impair effector interaction (4 in Figure 1).
Gain-of-function mutations cause inappropriate or constitutive activation. A constitutively activated GPCR presumptively assumes the conformation that leads to G protein activation, even without binding of the hormone that normally activates the receptor. Likewise, a constitutively activated G protein signals its effector despite the lack of normal 'upstream' signals from a hormone-activated GPCR. Such mutations could either accelerate release of GDP and lead to receptor-independent G protein activation (3 in Figure 1) or block the GTPase reaction that terminates G protein activation (5 in Figure 1). Gain-of-function mutations are by definition dominant, and thus heterozygotes for germ-line mutations will be clinically affected. For loss-of-function mutations, the situation is more complex. Pure loss-of-function mutations may cause no overt clinical dysfunction in heterozygotes. For many GPCR, there is sufficient signal sensitivity and amplification that a 50% reduction in receptor number does not lead to clinically apparent disease.
Loss-of-function mutations are generally associated with inherited disorders, while gain-of-function mutations may occur in the germline in inherited disorders or as somatic events in sporadic disorders. In the latter case, a gain-of-function mutation confers a proliferative advantage on the cell in which the somatic event occurs, leading to a clonal neoplasm and eventually clinically evident disease. Germline mutations of certain GPCR and G proteins may never be detected simply because they would be incompatible with life. This could be true of heterozygous gain-of-function mutations or of homozygous null mutations in which inappropriate signal activation or total lack of signalling, respectively, would be lethal. When such germline mutations are compatible with life, the timing of onset of clinical disease may be quite variable, even though the mutation is already present at birth. For gain-of-function mutations, the timing of disease onset may reflect several factors including the degree of constitutive activation of the particular mutation, critical developmental events such as cell proliferation necessary for a response to signal activation, and the eventual failure of mechanisms attempting to compensate for inappropriate signal activation.
Mere identification of sequence variation in a GPCR or G protein gene does not constitute proof of a disease-causing mutation. Some sequence differences may simply be polymorphisms of no pathophysiological consequence (but see later discussion for possible role of polymorphisms in predisposing to disease).
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Identification of mutations only in affected members of a kindred and not in unaffected members or in 'normal control' subjects is suggestive of pathophysiological relevance. Also, certain mutations may be predicted to be functionally significant, e.g. those causing truncation of a GPCR, based on available information on GPCR and G protein structure and function. More rigor-ous [sic] proof that a mutation identified is responsible for the disease being studied requires mutagenesis of the normal gene and appropriate functional studies of the expressed gene product.
DISEASES CAUSED BY GPCR MUTATIONS
Loss-of-function mutations: Germline loss-of-function mutations in a large number of GPCR have been identified as causes of disease (Table 1; see Spiegel (1995) for detailed review and listing of mutations). The diseases caused by loss-of-function mutations of the ACTH, FSH, LH, TSH, and GHRH receptors are transmitted as autosomal recessives; they require homozygous or compound heterozygous mutations for clinical expression. Certain rhodopsin loss-of-function mutations act as dominant negatives; they lead to misfolding and retention in the endoplasmic reticulum, causing photoreceptor cell degeneration and autosomal dominant retinitis pigmentosa (McKusick 180100). Heterozygous loss-of-function mutations of the calcium receptor (CaR) cause generalized insensitivity to extracellular calcium, familial hypocalciuric hypercalcaemia (McKusick 145980). Homozygous mutations of the CaR cause a more severe clinical form of calcium insensitivity, neonatal hyperparathyroidism (McKusick 239200).
The clinical manifestations of each of the diseases caused by these receptor mutations reflect both the expression and known functions of the involved receptors. Nephrogenic diabetes insipidus (NDI) provides an excellent example. NDI is characterized by failure to concentrate the urine despite adequate, indeed elevated, secretion of vasopressin. There are at least two familial forms inherited in X-linked (McKusick 304800) and autosomal recessive (McKusick 222000) fashion, respectively (Fujiwara et al 1995). The V2 vasopressin receptor, one of at least three vasopressin receptor subtypes, is expressed primarily in the kidney and mediates the antidiuretic action of the hormone. The clotting factor response to vasopressin is also a V2 response and is also defective in males with X-linked NDI. Other actions of vasopressin such as the hypertensive response and ACTH release are mediated by V1a and V1b receptor subtypes, respectively, and are normal in subjects with NDI. Thus the V2 vasopressin receptor gene was already an excellent 'candidate' gene for X-linked NDI. Localization of the V2 vasopressin receptor gene to the same chromosomal site, Xq28, as the X-linked form of NDI immediately prompted study of this receptor in subjects affected with this form of the disorder.
To date, more than 70 distinct loss-of-function mutations have been identified in many different kindreds (Fujiwara et al 1995; Spiegel 1995). These include nonsense, frameshift and missense mutations involving all regions of the receptor. A single loss-of-function mutation causes clinically evident disease in males, not because of a dominant-negative effect but because males are hemizygous for the receptor gene. Most females heterozygous for the mutant receptor gene are clinically unaffected carriers, since random X-inactivation results on average in only a 50% reduction in normal receptor number. Rarely, female heterozygotes show clinically apparent diabetes insipidus, presumably due to unfavourable X-inactivation (Moses et al 1995). The plethora of mutations identified in X-linked NDI reflects in part the ease of transmission of the disease by carrier females, the ease of disrupting receptor function with mutations almost anywhere in the receptor, and a clinically obvious phenotype that brings affected males to medical attention. Autosomal recessive NDI phenotypically resembles the X-linked disease but is caused by loss-of-function mutations in the aquaporin-2 gene (Fujiwara et al 1995). This gene encodes a renal tubular membrane water transporter that is the distal target of vasopressin-stimulated cAMP action.
Gain-of-function mutations: After site-directed mutagenesis of adrenergic receptors showed that missense mutation of critical residues in the third intracellular loop caused constitutive activation (Lefkowitz et al 1994), similar naturally occurring mutations were identified in the receptors for LH (Shenker et al 1993a) and TSH (Parma et al 1993). Activating mutations of the LH receptor cause familial male precocious puberty (FMPP), an autosomal dominant disorder (McKusick 176410), in which affected males show signs of virilization often by age 4 years or even earlier. The disease may also occur sporadically (Yano et al 1994) either as a new germline mutation or as a somatic event. Gonadotropins are suppressed indicative of autonomous gonadal hyperfunction. Aspartate-578 (in the sixth transmembrane domain) to glycine is a common missense mutation found in FMPP, but a number of other missense mutations have been identified (Shenker 1995). An aspartate-578 to tyrosine mutation was identified in a subject with onset of precocious puberty by 1 year of age (Laue et al 1995). This mutant causes even greater cAMP stimulation than other constitutively activated LH receptor mutants tested in transfected cells, suggesting a correlation between degree of constitutive activation and timing of disease onset. Interestingly, there are no apparent clinical manifestations in obligate female carriers of the mutant receptor gene. Evidently, LH receptor activation of cAMP formation in testicular Leydig cells is sufficient to stimulate testosterone production and even spermatogenesis, but is ineffective in triggering puberty in females without concomitant FSH action. Other examples of diseases caused by gain-of-function mutations of GPCR are listed in Table 1.
DISEASES CAUSED BY G PROTEIN a SUBUNIT MUTATIONS (Table 2)
Loss-of-function mutations: Pseudohypoparathyroidism (PHP) was initially described by Albright as a disorder caused by resistance to parathyroid hormone (PTH). Subsequent studies indicated that subjects with PHP show a subnormal urinary cAMP response to PTH, suggesting a defect in signal transduction proximal to cAMP formation. In the form of the disease termed PHP type Ia (McKusick 103580), resistance not only to PTH but to several other hormones that act by stimulating cAMP formation is present, and is associated with phenotypic features including obesity, short stature, mild mental
| Table 2 Diseases caused by G protein a subunit mutations | ||
| G protein a subunit | Disease | Mutation type |
| as | ||
| as | PHP Ia with precocious puberty | |
| as | Acromegaly, hyperfunctional thyroid nodules, | |
| at | Congenital night blindness | |
| ai2 | Ovarian and adrenocortical tumours | |
retardation- and certain bony anomalies, collectively termed Albright hereditary osteodystrophy. Heterozygous, germline loss-of-function mutations of the Gs-a gene mapped to the distal long arm of chromosome 20 have been identified in PHP Ia (see Spiegel et al (1994) and references therein for details). Most abolish normal protein expression. The pleiotropic manifestations of the disease are consistent with the ubiquitous expression of this key gene regulating stimulation of cAMP formation. Many different mutations of the gene have been identified in different families, but two observations remain unexplained: (a) the variability in clinically evident hormone resistance among hormones acting via cAMP stimulation; and (b) the occurrence of the identical GS-a gene mutation within a kindred in subjects with PHP and pseudoPHP. The latter individuals show some of the phenotypic features of Albright hereditary osteodystrophy but lack hormone resistance altogether (note that a mutant Gs-a gene is not found in completely unaffected individuals within such kindreds). It appears that heterozygous loss-of-function mutation of the Gs-a gene is necessary but not sufficient for full expression of the PHP Ia phenotype.
Gain-of-function mutations: Heterozygous, somatic mutations of the Gs-a gene have been identified in sporadic GH-secreting pituitary somatotroph tumours and hyperfunctioning thyroid adenomas (Lyons et al 1990). These mutations have involved two residues (arginine-201 and glutamine-227) known to be critical for GTPase activity. Missense mutations at these positions reduce GTPase activity (5 in Figure 1), causing constitutive Gs activation with resultant inappropriate cAMP stimulation. In certain target organs such as endocrine glands and melanocytes, increased cAMP may cause both hyper-function [sic] and proliferation. Activating mutations at arginine-201 of Gs-a have also been found in the McCune-Albright syndrome, characterized by autonomous endocrine hyperfunction, patchy skin hyperpigmentation, and polyostotic fibrous dysplasia (Weinstein et al 1991). This is a sporadic disorder and the Gs-a mutations are somatic mutations found in a mosaic distribution in affected individuals. In theory, germline acti-vating [sic] mutations of Gs-a might be lethal; somatic mutations occurring early in embryo-genesis might be compatible with survival, but still cause multiorgan involvement in the observed mosaic distribution. In particularly severe cases, the mutation has been identified in 'nonclassic' organs such as heart, liver and the GI tract where it may lead to early death (Shenker et al 1993b).
Comparable activating mutations of the Gi2-a gene have been identified in ovarian and adrenal cortical neoplasms but their aetiological significance is unclear (Lyons et al 1990). Recently, a germline heterozygous missense mutation, glycine-38 to aspartate, in the transducin a subunit (at), a G protein expressed primarily in retinal rod photoreceptors, was identified in an autosomal dominantly inherited form (McKusick 163500) of congenital stationary night blindness (Dryja et al 1996). This mutation is believed to cause constitutive activation of the photoreceptor signalling pathway, with resultant rod dysfunction and night blindness.
CONCLUSIONS
Identification of mutations in GPCR and in G protein a subunits has provide[d] major insights into the structure and function of these key signaling components. In many cases study of disease mutations has offered clues to key structural elements that would have required considerably greater effort using artificial mutagenesis techniques. Disease phenotypes caused by mutations, moreover, have often provided compelling evidence that supports the suspected physiological function of receptor or G protein or, as in the case of Hirschsprung disease (aganglionic megacolon; McKusick 600155) caused by endothelin B receptor loss-of-function mutations (Chakravarti 1996), suggests entirely unsuspected functions for the receptor (in that case receptor function is required for normal development of neural crest-derived cells). Mutation identification has already had an important impact on disease diagnosis and should in the future lead to novel and more effective treatments. For diseases caused by activating, gain-of-function receptor mutations, it may be possible to treat with so-called inverse receptor agonists that would deactivate mutated receptors. As prospects for gene therapy improve, gene replacement for diseases caused by loss of function, and treatment with dominant negative genes to block effects of activating, gain-of-function mutations may become feasible.
It also seems likely that further study will uncover additional disorders caused by defects in G protein-coupled signal transduction, not only involving GPCR and G protein a subunits, which have been most intensively studied to date, but also other components of the pathway such as the bg dimer. In addition, more subtle forms of dysfunction may be discovered such as polymorphisms of receptors or G proteins that predispose to disease rather than overt disease-causing mutations.
Study of animal models of disease, particularly naturally occurring mouse mutants as well as artificially created mouse gene knockouts, should provide further insights in identifying disease candidates. For example, knockout of the mouse Gi2-a gene leads to an ulcerative colitis-like disease complete with occasional progression to adenocarcinoma of the colon (Rudolph et al 1995), raising questions about the importance of this G protein in human forms of the disorder. Additional examples are sure to emerge as more knockouts are created and more disease genes in mice are identified.
REFERENCES
- Albright F, Burnett CH, Smith PH, Parson W (1942) Pseudohypoparathyrodism: an example of 'Seabright -- Bantam syndrome'. Endocrinology 30: 922-932.
- Chakravarti A (1996) Endothelin receptor-mediated signaling in Hirschsprung disease. Hum Mol Genet 5: 303-307.
- Dryja TP, Hahn LB, Reboul T, Arnaud B (1996) Missense mutation in the gene encoding the a subunit of rod transducin in the Nougaret form of congenital stationary night blindness. Nature Genet 13: 358-360.
- Fujiwara TM, Morgan K, Bichet DG (1995) Molecular biology of diabetes insipidus. Annu Rev Med 46: 331-343.
- Garrod AE (1923) Inborn Errors of Metabolism, 2nd edn. London: Oxford University Press.
- Iismaa TP, Biden TJ, Shine J (1995) G Protein-Coupled Receptors. Austin, TX: RG Landes.
- Laue L, Chan WY, Hsueh AJW, et al (1995) Genetic heterogeneity of constitutively activating mutations of the human luteinizing hormone receptor in familial male-limited precocious puberty. Proc Natl Acad Sci USA 92: 1906-1910.
- Lefkowitz RJ, Cotecchia S, Samama P, Costa T (1994) Constitutive activity of receptors coupled to guanine nucleotide regulatory proteins. Trends Pharmacol Sci 14: 303-307.
- Lyons J, Landis CA, Harsh G, et al (1990) Two G protein oncogenes in human endocrine tumors. Science 249: 655-659.
- Moses AM, Sangani G, Miller JL (1995) Proposed cause of marked vasopressin resistance in a female with an X-linked recessive V2 receptor abnormality. J Clin Endocrinol Metab 80: 1184-1186.
- Neer EJ (1995) Heterotrimeric G proteins: organizers of transmembrane signals. Cell 80: 249-257.
- Parma J, Duprez L, Van Sande J, et al (1993) Somatic mutations in the thyrotropin receptor gene cause hyperfunctioning thyroid adenomas. Nature 365: 649-651.
- Rudolph U, Finegold MJ, Rich SS, et al (1995) Ulcerative colitis and adenocarcinoma of the colon in Galpha-i2-deficient mice. Nature Genet 10: 143-150.
- Shenker A (1995) G protein-coupled receptor structure and function: the impact of disease-causing mutations. Baillière's Clinical Endocrinology and Metabolism 9: 427-451.
- Shenker A, Laue L, Kosugi S, Merendino JJ Jr, Minegishi T, Cutler GB Jr (1993a) A constitutively activating mutation of the luteinizing hormone receptor in familial male precocious puberty. Nature 365: 652-654.
- Shenker A, Weinstein LS, Moran A, et al (1993b) Severe endocrine and non-endocrine manisfestations of the McCune-Albright syndrome associated with activating mutations of the stimulatory G protein Gs. J Pediatr 123: 509-518.
- Spiegel AM (1995) Defects in G protein-coupled signal transduction in human disease. Annu Rev Physiol 58: 143-170.
- Spiegel AM, Jones TLZ, Simonds WF, Weinstein LS (1994) G Proteins. Austin, TX: RG Landes.
- Weinstein LS, Shenker A, Gejman PV, Merino MJ, Friedman E, Spiegel AM (1991) Activating mutations of the stimulatory G protein in the McCune-Albright syndrome. N Engl J Med 325: 1688-1695.
- Yano K, Hidaka A, Saji M, et al (1994) A sporadic case of male-limited precocious puberty has the same constitutively activating point mutation in luteinizing hormone/choriogonadotropin receptor gene as familial cases. J Clin Endocrinol Metab 79: 1818-1823.
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Metabolic Diseases Branch, National Institute of Diabetes and Digestive and Kidney Disease, National Institutes of Health, Bethesda, MD 20892, USA (A. M. Spiegel) 1997 Address photocopy requests to Kluwer Academic Publishers, PO Box 55, Lancaster, LA1 1PE, UK. |
This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)
GPCRs live within the cell membrane, that thin layer that separates the inside of the cell from the outside. If you think of the GPCR as a string, the majority of it sits inside the cell membrane in seven clumps called transmembrane helices. Part of the string snakes outside the cell forming three curves called extracellular loops. Part of the string snakes inside the cell, forming three curves called intracellular loops. The head of the string sits outside the cell and the tail sits inside. (Look at diagram of GPCR.)
Different classes of GPCR couple exclusively or preferentially to specific G proteins, using their intracellular loops and tail end to do so. It is thought that GPCRs can change their shape, moving between a shape that will allow it to couple with a G protein and one that will not allow coupling to occur.
G proteins have three parts: alpha, beta and gamma. When in contact with a GPCR, the G protein temporarily splits apart, the alpha part ends up on one side of the effector, and the beta and gamma parts, still bound together, end up on the other side of the effector. This is how the message of the hormone gets from the receptor to the effector. After the G protein does this, its three parts bind back together. This is a complicated process requiring all the participating parts to be functional. However, sometimes mutations in either the G protein or GPCR render a part dysfunctional. This can lead to physical disorders.
The way mutations of the G protein or GPCR will show themselves depends on three things:
- The range of expression of the mutated gene. If the gene is widely distributed throughout the body, it will cause a generalized manifestation. If it is more narrowly distributed, its manifestations can be more localized.
- Whether the mutation is inherited (germline) or acquired (somatic). Germline mutations may cause manifestations in every cell in which they exist. Somatic mutations would have more localized expression.
- Whether the mutation causes a gain-of-function or a loss-of-function.
Loss-of-function mutations prevent the protein which the mutated gene synthesizes from reaching its job site. Or it can impair the protein's functioning even if it does not prevent the protein from reaching the area where it is supposed to perform its functions. Many GPCR mutations result in either the receptor being improperly shaped so it can't get to its work site, or it is unable to bind with its intended hormone or neurotransmitter. In both cases the message from the extracellular signaler (e.g., hormone or neurotransmitter) does not get transmitted to the effector. Loss-of-function mutations can also impair the receptor's ability to couple with the G protein, so it can get the message from the hormone, but it can't pass it on the effector via the G protein.
Gain-of-function mutations cause inappropriate activation. In the case of GPCR, a mutation could cause it to couple with a G protein even though a hormone hasn't told it to. An inappropriately activated G protein can signal the effector without first being signaled to do so by the GPCR. In both cases a metabolic sequence would be activated even though the body hasn't signaled it to happen. Gain-of-function mutations are associated with both inherited or acquired disorders, whereas loss-of-function mutations are generally associated with inherited disorders.
There are many diseases caused by GPCR and G-protein mutations, and the author describes many of them. However, since readers at this site are primarily concerned with nephrogenic diabetes insipidus (NDI), we will focus on that.
NDI is caused by a loss-of-function mutation. It is characterized by failure to concentrate urine despite adequate amounts of the antidiuretic hormone, vasopressin (VP), being present. In the most common form of inherited NDI, X-linked NDI, mutations in the vasopressin-2 (V2) receptor, a GPCR, prevent it from binding with VP and being able to pass on its hormonal message. More than 70 different loss-of-function mutations have been identified in the V2 receptor among people with X-linked NDI. Not only does it seem that mutations can occur in many different places in the V2 receptor, it also can be affected by different types of mutations, most notably frameshift, nonsense and missense mutations. Autosomal recessive NDI, the much less common form of inherited NDI, is caused by a mutation in a different type of protein, a water-transporting protein called an aquaporin.
Identification of GPCR and G proteins in human diseases such as NDI has provided insight into the way the G protein and GPCR interact to pass along metabolic messages. Knowing the distribution, structure and function of these molecular structures will add to our ability to diagnose and treat disease.