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GENETIC CONSIDERATIONS OF GOUT J. EDWIN SEEGMILLER One of the greatest needs in medicine at the present time is the identification of the precise genetic factors that are responsible for some of the more common diseases. I t is known that a familial aggregation exists for myocardial infarction, hypertension, diabetes mellitus, manic depressive psychoses, schizophrenia, chondrocalcinosis, and osteoarthritis as well as for gout. Only within the past decade has a beginning been made in identifying abnormal gene products responsible for gouty arthritis ( 1 4 ) and hypercholesterolemia (5). T h e approach being made in these two disorders promises to provide examples for the types of investigations that may prove fruitful for other of the more common disorders. T h e familial incidence found for gouty arthritis within a given family depends to a considerable extent on the perseverance of the physician, as well as on his ability to isolate environmental factors such as lead poisoning which could be contributing to the disease. A familial incidence has been reported by some physicians in as many as 75y0 of families (2,3). T h e clarity with which the inheritance of any given disorder in a given pedigree can be recognized depends to a considerable extent on how close the abnormality being studied is to the abnormal gene product. For example the inheritance of gouty arJ. Edwin Seegmiller, M.D.: Professor of Medicine, Department of Medicine, University of California, San Diego, La Jolla, California 92093. Address reprint requests to Dr. Seegmiller.

thritis is much less clear in a pedigree than is the inheritance of hyperuricemia, because only 25y0 of hyperuricemic relatives will have gouty arthritis (2,3). In addition environmental factors can alter the degree to which genetic factors are expressed. For example liyperuricemia is a late manifestation of chronic lead toxicity (6) and both Kelley and Klinenberg have reported a large number of gouty patients in certain areas of the United States who showed evidence of lead poisoning with a special EDTA infusion test originated by Emmerson (6). I n some areas of the country lead poisoning occurs from the high lead content of “moonshine” whiskey. T h e magnitude of knowledge of the human genome in comparison to the amount yet to be learned is very small, and knowledge of the genetic factors operating in gout reflects this fact. T h e DNA content of each human cell is sufficient to code lo7 polypeptide units, each containing an average of 150 amino acids (7). Estimates of the actual number of genes present in the human species is only about 1 % of this amount. A portion of the 99% of nongenetic DNA is undoubtedly composed of reiterated sequences. T h e actual number of genes so far identified as genetic factors in the human, as compiled by McKusick, is 1883, of which around half are firmly established (7). Therefore only about lo3 human genes have been identified by their expression in humans. Of these known genetic factors the abnormal gene

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product has been identified i n only 150 to 200, and most of these disorders are recessively inherited enzyme defects requiring homozygosity for clinical expression (8). Obviously we have just begun to understand some of tlie functions present in the human genome. T h e precise way in which a defective gene gives rise to a clinically expressed hereditary disease has been well worked out for only a relatively few abnormalities, most of which are recessively inherited disorders. T h e greatest insight into the molecular mechanism by which a mutation produces its effect was provided by the detailed studies of sickle-cell hemoglobin showing the substitution of just one of the 146 amino acids i n the beta chain of hemoglobin (9-1 1). T h e amino acid valine replaces the normal glutamic acid to form sickle-cell hemoglobin (1 1). As a result of the elucidation of tlie genetic code by Nirenberg (12), the mutational event responsible for the amino acid substitution can be traced to the hypothetical substitution of a single base in tlie DNA molecule resulting in uracil’s replacing the base adenine in messenger RNA. Of course we d o not yet have the technical capability to analyze the messenger RNA or tlie DNA to prove this presumed base substitution. A base substitution in DNA can give rise to a great many different types of defects in addition to simple amino acid substitutions. Tlie chain terminating sequences UAG, UAA, or UGA can be formed and thereby cause the ribosomes to stop translation of tlie messenger RNA and thus interrupt the synthesis before completion of the polypeptide chain. Another mechanism is tlie deletion or insertion of a base that can form what is referred to as a “frameshift” mutation i n which the reading of three adjacent bases by the ribosome is thrown off register by the addition or deletion. Consequently the code for all amino acids beyond tlie “frame-shift” mutation is altered to produce incorporation of a n entirely different series of amino acids into the mutant protein. “Frame-shift” mutations have so far been found only in bacterial systems. Likewise mutations in an operator gene that cause enzyme deficiency or over-activity have been found only in bacteria. By far the majority of identified enzyme deficiencies show a recessive mode of inheritance. Therefore clinical expression is found only in the liomozygous state i n which the abnormal gene is contributed from each heterozygous but clinically unaffected parent. T h u s each offspring has a 25% chance of

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obtaining the two defective genes required for clinical expression of the disorder. Far less is known of the abnormal gene products responsible for dominantly inherited diseases. I n these diseases clinical expression occurs in tlie heterozygous state; offspring of an affected parent therefore have a 50% chance of being affected. An abnormality of a structural protein is thought to underlie most dominantly inherited disorders. T h e mutation described by Becker (pp 687-694) shows a dominantly inherited increase in specific activity and is therefore of special interest. Abnormalities of receptor sites or cell transport mechanisms are also excellent candidates. Demonstration of a reduced number of receptor sites for low-density lipoproteins on tlie surface of fibroblasts cultured from patients with a dominantly inherited hypercliolesterolemia has provided an excellent example of the mechanism by which a dominantly inherited disorder could be produced (5). X-linked disorders are much more easily recognized from the pedigree because of their lack of maleto-male transmission. This recognition is based on tlie presence of only one X chromosome in the male so that all defects on that X chromosome can be expressed in the male. As a consequence 50% of the male offspring of a female heterozygous for the disorder will be affected. Tests for heterozygosity are based o n the presence of both phenotypically normal and mutant cells i n hair roots or in fibroblasts cultured from skin biopsies. T h e two cell types confirm tlie random inactivation of the X chromosome at an early stage of fetal development (13,14). T h e influence of the mutant cells is undoubtedly attenuated in the female by partial correction of tlie defect in tlie mutant cells by metabolic cooperation (15). A large gap in our knowledge exists between those molecular mechanisms so far worked out in just a few examples and tlie practical clinical problems involving evaluation of a patient with gouty arthritis with its hereditary tendency. T h e pattern of inheritance can be especially helpful in providing clues for detecting tlie rare gouty patient with known aberrations of metabolism. For example any gout patient whose family history shows a maternal inheritance of the gout with no male-tomale transmission should be considered a possible candidate for tlie variant of X-linked uric aciduria (tlie Lesch-Nyhan syndrome), which is a n X-linked disorder discussed in greater detail elsewhere (pp 673680). Another disorder with the same pattern of in-

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heri tance is gout associated with vasopressin-unresponsive diabetes insipidus (16). T h e author and coworkers have identified a few recessively inherited examples of enzyme defects responsible for gout, but if there is n o history of gout in the family and particularly if the patient is a premenopausal woman with early onset of gout, the possibility of the rare recessively inherited disorder, glycogen storage disease Type I, should be considered. This disease is especially likely if the patient gives a history of frequent epistaxis and retardation of growth and sexual development, and shows a liepatomegaly and a subnormal fasting blood sugar level. If the patient proves to be intellectually retarded instead of unusually intelligent, screening of the urine for ketoacids should be made to look for branched-chain ketoacids characteristic of maple syrup urine disease. These are both very rare disorders, but unless they are kept i n mind these specific types of gout will certainly not be diagnosed. If family history suggests a dominantly inherited disorder, the type of gout discussed by Becker ( p p 687-694) should be considered. It provides another example of the mechanism by which a single gene in heterozygous individuals can produce, in this case, a metabolic overproduction of a normal end product. T h e past decade has brought recognition of the ubiquity of genetic heterogeneity. I n gouty arthritis this genetic heterogeneity occurs at many different levels of organization. A wide variety of underlying types of metabolic disorders are now known to produce the hyperuricemia required for development of gouty arthritis. However genetic heterogeneity also exists even within a single gene defect. All patients with X-linked uric aciduria (the Lesch-Nyhan syndrome and its variants) show deficiency of the enzyme hypoxantliine-guanine phosphoribosyltransferase (HPRT). A variety of degrees of severity of clinical expression relate in part at least to the severity of the impairment of functional enzyme activity produced by the mutation. T h e most severe deficiency and clinical expression is of course the Lesch-Nyhan syndrome. However even these patients show variations i n the clinical expression of self-mutilation which may reflect, in part, genetic as well as environmental factors (1-4,17). Less severe degrees of enzyme deficiency are found in patients who develop only gouty arthritis, whereas at values of enzyme activity between these extremes an attenuated neurologic involvement is found. Even in patients with severe enzyme deficiency a heterogeneity is found at the molecular level i n the

cause of the deficiency. A mutation affecting the affinity of the enzyme for its substrate is found in some patients, whereas in different gouty families an increased or decreased thermal lability of the H P R T enzyme has been found (I-4,17). As a consequence, within the same family each patient generally tends to show a similar degree of clinical expression. Additional genetic factors conceivably can modify the clinical expression even within the same family. Amelioration or exacerbation of clinical presentation within a given family could reflect the presence of additional genetic mutations impinging upon the mechanism involved in production of clinical expression of the disease.

Influence of Genetic Factors on Therapy Long-term therapy of gouty arthritis is directed toward correcting the hyperuricemia that is responsible for the development of the clinical disease. Because such therapy requires a lifelong commitment to treatment with the drug, it is worth taking the extra time and effort needed to prescribe the very best drug for a particular patient’s form of disease. To this end, the author has routinely determined the amount of uric acid excreted in sequential 24-hour collections of urine obtained during the last 3 days of a 6-day period o n a purine-free diet. Immediately after their presentation to the clinic with a n acute attack of gout, most patients are well motivated to undertake the extra effort required. Furthermore the institution of long-term therapy with either a uricosuric drug o r allopurinol must await a defervescence of the acute attack; this delay can be well used for collecting data upon which the rationale of drug selection can be based. Most patients require very detailed instructions on how to collect the 24-hour urine. Placing 3 ml of toluene in the bottle and storing the urine at room temperature will prevent development of microbial contamination. Before a n aliquot is removed for measurement of both uric acid and creatinine, any urinary sediment must be brought into solution by gentle warming in warm water with frequent agitation. T h e upper limit of normal for adult males is 600 mg of uric acid per day. Individuals excreting less than this amount who have normal renal function are obviously not flagrant overproducers of uric acid. Presumably their primary cause of hyperuricemia is a less efficient renal mechanism for excretion of uric acid than is found in normal subjects. T h e uricosuric

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drug probenecid provides the most appropriate rational treatment because it enhances the effectiveness of renal clearance of uric acid by the kidney. Excretion of quantities of uric acid greater than 600 mg per day is evidence of substantial overproduction of uric acid. This group of patients should be given allopurinol for treatment because it not only blocks uric acid formation but decreases the total amount of purines being formed and thereby has a special corrective therapeutic effect. Exceptions to this generalization are patients with X-linked uric aciduria who merely substitute the oxypurine precursors of hypoxanthine and xanthine for the deficit in uric acid production. Evidently HPRT enzyme is required for this additional effect of allopurinol. Nevertheless these patients d o benefit from the capability of allopurinol to divide the load of a sparingly soluble metabolite, uric acid, into three different molecular species, each with its own independent solubility. Furthermore hypoxanthine and xanthine are excreted much more efficiently by the kidney than is uric acid.

SUMMARY A great many genetic factors and genetic meclianisms are involved in the production of gouty arthritis. Genetic heterogeneity is to be expected in this disorder at all levels of expression. T h e genetic factors responsible for hyperuricemia, whether from gross overproduction of uric acid or from decreased renal excretion, can influence such practical matters as the selection of the proper therapy for control of the disease.

REFERENCES 1. Seegmiller JE: Biochemical and genetic studies of an

X-linked neurological disease. Harvey Lectures, Series 65, New York, Academic Press, 1971, pp 175-192 2. Seegmiller JE: Diseases of purine and pyrimidine metabolism, Duncan’s Diseases of h4etabolism. Seventh Edition. Edited by PK Bondy, LE Rosenberg. Philadelphia, W. B. Saunders Company, 1974, pp 655-774 3. Wyngaarden JB, Kelley WN: Gout, T h e Metabolic Basis of Inherited Disease. Third Edition. Edited by JB Stanbury, JB Wyngaarden, DS Fredrickson. New York, hlcGraw-Hill Book Company, 1972, pp 889-968

4. Kelley IYN, Wyngaarden JB: T h e Lesch-Nyhan syndrome, T h e hletabolic Basis of Inherited Disease. Third Edition. Edited by JB Stanbury, JB M7yngaarden, DS Fredrickson. New York, hIcGraw-Hill Book Company, 1972, pp 969-991 5. Golditein JL, Brown hIS: Binding and degradation of low density lipoproteins by cultured human fibroblasts. Comparison of cells from a normal subject and from a patient with homozygous familial hypercholesterolemia. J Biol Chem 249:5153-5162, 1973 6. Emmerson BT: Chronic lead nephropathy: the diagnostic use of calcium EDTA and the association with gout. Aust Ann hIed 12:310, 1963 7. McKusick V: Mendelian Inheritance in hlan. Catalogs of Autosomal, Dominant, Autosomal Recessive, and XLinked Phenotypes. Third Edition. Baltimore, T h e Johns Hopkins Press, 1971 8. Raivio KO, Seegmiller JE: Genetic diseases of metabolism. Annu Rev Biochem 41:543-576, 1972 9. Pauling L, Itano HA, Singer SJ, et al: Sickle cell anemia: a molecular disease. Science 110:543, 1949 10. Pauling L: Abnormality of Hemoglobin Molecules in Hereditary Hemolytic Anemias. Harvey Lectures, New York, Academic Press, 1954 1 I . Ingram VM: Hemoglobin and Its Abnormalities. Spring field, Charles C. Thomas, 1961 12. Nirenberg MW, hlatthaei KH: T h e dependence of cell-free protein synthesis in E . colz upon naturally occurring or synthetic polyribonucleotides. Proc Natl Acad Sci USA 47: 1588, 1961 13. Lyon hIF: Gene action in the X-chromosome of the Nature 190:372-373, 1961 mouse (Mus. musculus L), 14. Beutler E: Biochemical abnormalities associated with hemolytic states, Mechanisms of Anemia. Edited by IM Weinstein, E Beutler. New York, McGraw-Hill Book Company, 1962, pp 195-236 15. Friedmann T, Seegmiller JE, Subak-Sharpe JH: Metabolic cooperation between genetically marked human fibroblasts in tissue culture. Nature (Lond) 220:272274, 1968 16. Gorden P, Robertson GL, Seegmiller JE: Hyperuricemia: a concomitant of congenital vasopressinresistant diabetes insipidus in the adult. N Engl J bled 284:1057-1060, 1971 17. Seegmiller JE: Inherited deficiency of hypoxanthineguanine phosphoribosyltransferase in X-linked aciduria. Am Hum Genet 6:75-163, 1976