582

21. Navani, S. V. Postgrad. med. J. 1966, 42, 659.22. Moore, H. D. Br. med. J. 1968, ii, 536.23. Northfield, T. C. ibid. 1971, iv, 86.24. Fine, J., Hermanson, L., Frehling, S. Am. J. Surg. 1938, 107, 125. Henry, F. M., Lawrence, J. H., Bridge, E. V., Williams, O. L

War Med., Chicago, 1944, 6, 395.

AddendumTwo further cases have been successfully treated

by oxygen breathing since this paper was submittedfor publication.

Hypothesis

DOPA AS AN ANTIMETABOLITE IN

HUNTINGTON’S DISEASE

FRED BASKIN ROGER N. ROSENBERG

Department of Neurosciences, School of Medicine,University of California, San Diego, La Jolla,

California 92037, U.S.A.

Summary A point mutation affecting the primarystructure of an enzyme which usually

distinguishes between dopa or a dopa metabolite andtyrosine could explain the neuronal cell death which ischaracteristic of Huntington’s disease. The dopametabolite would then act as an antimetabolite,inhibiting the tyrosine t-R.N.A. acylation step, blockingprotein synthesis, and eventually causing cell death.

INTRODUCTION

Huntington’s disease is transmitted as an auto-

somal dominant, in that one affected allele is sufficientfor eventual expression of symptoms. The possibilitythat this is only partial dominance, whereby the pre-sence of two affected alleles is prenatally fatal, cannotbe ruled out. The disease is characterised principallyand most consistently by the death of the smallneurons in the putamen and caudate nucleus. This

degeneration apparently begins before an average ageof 35 and continues with increased chorea for an

average of 14 years until death. 1

ROLE OF DOPA METABOLITES

Huntington’s disease has usefully if imperfectlybeen described as the mirror image of Parkinson’sdisease 2 because it is generally associated with choreaand is exacerbated and, in some at-risk children ofknown carriers, possibly induced by L-dopa,3 whereasParkinson’s disease is associated with a rigidity whichis ameliorated by L-dopa. However, dopamine levels inthe basal ganglia of Parkinson’s patients are very low,whereas they seem to be grossly normal post mortem inthe basal ganglia of patients with Huntington’s disease. 4These apparently normal dopamine levels, coupledwith the facts that dopa seems to exacerbate whilea-methyltyrosine, an inhibitor of dopa synthesis,seems to ameliorate choretic symptoms,5 suggest a

hypersensitivity to dopa or its metabolites in patientsmanifesting the disease and possibly even in youngat-risk patients in whom symptoms have not yetdeveloped.We suggest that this sensitivity is due to an alteration

in an enzyme which normally discriminates betweendopa (or a dopa metabolite) and some structurallysimilar molecule such as tyrosine. The normallypresent dopa metabolite would then act as a potentantimetabolite, specifically inhibiting the tyrosinet-R.N.A. acylation step, blocking protein synthesis,and eventually causing cell death.

Indeed, dopa and dopamine are found in high concen-trations only in the basal ganglia neuropil, specifically in thenigrostriatal neuronal terminals of the caudate nucleus andputamen,6 in agreement with the specificity of the primarylesion. Again, dopa and dopamine a block protein syn-thesis in normal brain tissue at concentrations 10-50 timesnormal, apparently blocking the acylation of tyrosine andphenylalanine t-R.N.A. species. We postulate that in

Huntington’s disease a point mutation has caused an amino-acid substitution at the active site of the tyrosine ligaseenzyme, causing it to lose its evolutionarily gained, nor-mally adequate ability to discriminate between tyrosineand dopa or dopamine. The very low mutation-rate for

Huntington’s disease lends support to the existence ofthis type of very specific change in the primary structureof the affected enzyme. Dopa, acting as an antimetabolite,would then block an appreciable fraction of the proteinsynthesis where present, and, with increasing age and-thenormally declining efficiency of neuronal protein synthesis,this synthesis would eventually fall below some criticallevel necessary for cell viability, causing, in effect, prematureneuronal death. The levels of dopamine in caudate andputamen tissue as a whole (6 and 9 g. per g. wet weight,respectively) are at least 10 times greater than in any otherdissectible brain part. Lesser, and perhaps later, lesionsin certain cerebral cortical areas might then be the secon-dary effect of the loss of innervation from basal ganglionneurons.

There are analogous situations where a natural meta-bolite in one species serves as a potent antimetabolite in asecond, because the relevant enzyme in the second specieslacks the necessary specificity to discriminate between itsnormal substrate and the novel one. For example, several

r species of the Lilacea group of plants synthesise a prolineI analogue, azetidine-2-carboxylic acid, which the host

prolyl-t-R.N.A. synthetase ignores, but which the analogousenzyme of all other species sees as an antimetabolite,effectively blocking protein synthesis and eventually

’ causing cell death.9 Again, mutants resistant to an anti-metabolite have been found, where the resistance lies in amutated aminoacyl t-R.N.A. synthetase able to discriminatebetween the natural and modified aminoacid.lo

’ As a possible corollary of this dopa-antimetabolite hypo-; thesis, one can propose a vicious circle whereby a drop inL protein synthesis in caudate and putamen in postsynapticf terminals might raise free tyrosine levels which would

> then induce higher levels of tyrosine hydroxylase, increasing

i dopa levels. Loh et a1.12 have suggested that carriers of thel disease could be detected by their putatively increased

l tyrosinase activity, which by increasing dopamine levels

, might cause the structural lesions discussed above. How-l ever, raised dopamine levels predicted by this hypothesisi

are not found. 4 It could be argued that the low-normalL dopamine per g. wet weight tissue levels observed were

in fact elevated because most of the small caudate neuronshave been lost in the course of the disease, but since mostof the dopamine resides in presynaptic nigrostriatal fibres

s from the substantia nigra,6 this does not seem to be justified.

T

9 TESTING THE HYPOTHESIS

Why it is the small neurons of the caudate and1 putamen which die when it is the presynaptic fibres

583

from the substantia nigra which are believed to havethe highest concentrations of dopa metabolites 6 is less

clear. Nothing is known about the subcellular locationof dopamine in the basal ganglia in Huntington’sdisease. Probably dopamine is packaged presynapti-cally where it can do no harm and has its effects post-synaptically, or perhaps the presynaptic terminals aredestroyed as in the case of 6-hydroxy dopamine,with the postsynaptic small neurons dying because ofdeinnervation and further inhibition of protein syn-thesis.

Caudate-putamen necropsy material should beexamined because of the possibility that the affectedenzyme in Huntington’s disease patients is an iso-

enzyme found only in brain or, possibly, only in basalganglia.The enzyme activities still present post mortem

should be sufficient to run a comparative challenge ofvarious enzymatic activities responsible for the in-

corporation of tyrosine into protein by normallyendogenous levels of dopa or dopamine. If our

hypothesis is correct, this incorporation should beblocked 50% after correcting for the relative amount

of neurons as opposed to glia present in the necropsymaterial.Our hypothesis is supported by an impressive amount

of circumstantial evidence, and if verified by the lineof experiments suggested above would constitute anovel type of autoinhibition mechanism of geneticdisease and one of the first molecular explanationsof a dominant genetic disease.

REFERENCES

1. Bruyn, G. W. in Handbook of Clinical Neurology (edited byP. J. Vinken and G. W. Bruyn); vol. VI, p. 298. Amsterdam, 1968.

2. Klawans, H. C., Ilahi, M. M., Ringel, S. P. Confinia Neurol. 1971,33, 297.

3. Klawans, H. C., Paulson, G. W., Barbeau, A. Lancet, 1970, ii, 1185.4. Ehringer, H., Hornykiewicz, O. Klin. Wschr. 1960, 38, 1236.5. Birkmayer, W. Wien. klin. Wschr. 1969, 81, 10.6. Sano, I., Gamo, T., Kakimoto, Y., Taniguchi, K. O., Takesada, M.,

Nishinuma, K. Biochim. biophys. Acta, 1959, 32, 586.7. Bosman, H. B., Shea, M. B., Case, K. R., Pike, G. Z. Physiol. Chem.

Phys. 1971, 3, 226.8. Weiss, B. F., Munro, H. N., Ordonez, L. A., Wurtman, R. J.

Science, 1972, 177, 613.9. Peterson, P. J., Fowden, L. Biochem. J. 1965, 97, 112.

10. Hirshfield, I. N., Tamford, J. W., Zamecnik, P. C. Biochim.biophys. Acta, 1971, 259, 344.

11. Shokeir, M. H. K. Am. J. hum. Genet. 1972, 24, 29a.12. Loh, H. H., Stolman, S., Lee, C. Y. Life Sci. 1971, 10, 1171.

Reviews of Books

Nutrition: A Priority in African DevelopmentEdited by Bo VAHLQUIST. Stockholm: Almqvist & Wiksell.1972. Pp. 228. Sw. kr. 40.

THE 1970s may be remembered by nutritionists as theperiod during which the key words were " planning "," national policies ", and " development ". In contrastto the 1960s, during which the emphasis was on particularstrategies and applied nutrition projects, it is now widelybelieved that only carefully planned and concerted pro-grammes have any chance of substantially improvingnutrition in developing countries. In practice this impliesthe complete integration of nutrition programmes with allaspects of health and social development. There havebeen several international seminars and symposia devotedto nutrition planning and to national nutrition policies asan aspect of development. This book is an account of oneorganised by the Dag Hammerskjold Foundation in 1971.Several areas of concern might be expected to give rise tofruitful dialogues between nutritionists, economists, andplanners. Are nutrition workers in a position to providethe planners with a sufficiently detailed analysis of thecauses and nature of malnutrition, and to identify associatedsocial, economic, and ecological factors ? How can

nutritionists convince Governments and administratorsthat improved nutrition should be accepted as a centralsocial objective of development ? Clearly, once nutritionis so accepted, its incorporation into a development plandemands the establishment of budgetary priority in relationto other social programmes, but what must nutritionists doto ensure an adequate priority rating for nutrition projects ?Sadly, this symposium contributes little to any of theseareas. Most of the nutritionists fail to make a sufficientlycritical appraisal of past and present views of the nature ofnutrition problems. The fundamental connection betweenmalnutrition and poverty is largely ignored in favour ofthe repeated but none the less largely discredited idea thatmost malnutrition is associated with dietary proteindeficiency (i.e., more with eating the wrong kind of foodsthan with simply not having enough to eat). An incorrectdiagnosis by the nutritionists leads to incorrect assessmentsby planners. Thus Alan Berg writing about nutrition

planning in India suggests that just as life expectancy hasbeen greatly increased by " effective, easy-to-administerand very low cost means of controlling traditional epidemics;the same concepts may be applied to improving diets.Food fortification (e.g., of bread and of tea with lysine)may some day be the equivalent of a smallpox vaccination".The apparent cheapness and simplicity of the food-fortification approach to malnutrition problems has fortoo long blinded its advocates to the lack of either a soundscientific basis or any proven efficacy of such measures.What if the Indians are not short of lysine but simplyneed more food ? The central problem is how to convinceGovernments that average energy intakes of the order of2000 kcal. (8-4 MJ) per head per day are an indication ofwidespread material and social deprivation and that a

nutritional target for social development policies shouldbe a level of demand between 2500 and 3000 kcal. By farthe most incisive contribution is by an economist, KurtSavosnick, who disclaims any previous connection withnutrition. His thesis is simply that if budgetary priority fornutrition programmes are to be decided by cost-benefitanalysis, this can only be based upon measures of thebenefits to be derived-e.g., it is not sufficient for nutri-tionists simply to say that intellectual and physical per-formance can be improved by eliminating malnutrition, orthat infant-mortality rates can be reduced by nutritionintervention programmes, they must be able to say by howmuch. But perhaps, as Savosnick says, the most importantquestions will arise when even the most careful analysisreveals little or no economic advantage for such pro-grammes, because, for example, these represent a formof " present consumption " rather than investment of

resources, and will hence result in a reduction of futurebenefits for society as a whole, although they may improvethe lot of some individuals. What has then to be decidedis the value of " kindness " in relation to economic growth,or perhaps of humanity now rather than affluence tomorrow.

Antenatal Diagnosis of Genetic DiseaseEdited by A. E. H. EMERY, department of human genetics,University of Edinburgh. Edinburgh: Churchill Living-stone. 1973. Pp. 169. E2.50.

PRENATAL diagnosis offers parents at high risk of havingchildren with chromosomal disorders or metabolic errors