923 A diagnosis of sickle-cell anxmia was based on standard criteria.’ Ophthalmological examination showed a best corrected distance visual acuity of 20/70 (right, R) and 20/400 (left, L); best corrected near vision of J-7 (R) and J-10 (L). Applanation tonometry revealed pressures of 12.0 mm Hg (R) and 10 mm Hg (L). Fundoscopic examination was only possible on the right eye and was normal. Slit-lamp examination was normal except for lenticular examination showing 2+ posterior subcapsular cataract (L) and 3+ posterior subcapsular cataract (R). Neither cells nor flare were observed in the anterior chamber. Intracapsular cataract extraction by cryoextraction technique was done under local anxsthesia with 2% ’Carbocaine’ (mepivacaine) given as a retrobulbar and Van Lint injection. Cataract extraction from the left eye was done without complications but the aqueous humor was noted to be clear, thick, tenacious, and mucoid in character, unlike anything in the surgeon’s previous experience. 5 days later the cataract in the right eye was extracted, by a different surgeon who also noted the same peculiar character of the aqueous humor. The aqueous was not analysed further. The postoperative course of the patient was uneventful, and 3 months after the operation the best corrected visual acuity was 20/40 (right) and 20/40 vears.
(left). The significance of this finding and its relation to sickle-cell anxmia are not known. Perhaps the physical change in the aqueous humor resulted from leakage of protein due to vascular damage to the circulation of the anterior segment secondarv to chronic intravascular sickling. KRAIG LORENZEN Departments of Medicine and Surgery, KODURI P. RAO Divisions of Hematology and Ophthalmology, ASHOK R. PATEL Cook County Hospital, Chicago, Illinois 60612, U.S.A JAMES DESNICK
EVOLUTION OF SICKLE VARIANT GENE
SIR,-The 13 kilobase (kb) Hpa 1 restriction enzyme site
polymorphism, close to the sickle variant of j3-globin, raises interesting evolutionary questions, as discussed by Dr Kurnit (Jan. 13, p. 104). He calculates that about 2785 generations would be needed to generate the 13% of sickle genes not associated with the 13 kb Hpa 1 polymorphic variant if the sickle mutation arose in a chromosome carrying this variant. This is inconsistent with the assumption that the sickle mutation occurred at the time agriculture and malaria were introduced into Africa, only 60-80 generations ago.4 Kurnit therefore argues for a multicentric origin of the sickle gene. We should like to suggest an alternative interpretation of the data, assuming a much older origin of the sickle gene. Though the increase in frequency of the sickle-cell gene may be traced to the introduction of malaria to Africa only 60-80 generations ago, this by no means implies that the mutation arose then. Indeed, it is more likely that the sickle mutation arose much earlier, in a founder with the 13 kb site, and that it achieved and was maintained at a low frequency due to random drift, until it was favoured by selection at the time of the introduction of malaria. At this time it would have already existed in coupling with both the 13 kb and the 7.6kb sites, and both of these combinations would then have expanded due to selective pressure and so relatively quickly given rise to the situation today. Using the standard population genetic random mating model for two linked loci and assuming, for simplicity, no change in the frequencies of either the sickle gene or the Hpa 1 sue before the onset of malaria, we can calculate the number
of generations (n) taken to generate the 13% recombinant chromosomes from the formula (1-r)"=(0 87-P)/( 1-P), where P is the gene frequency of the Hpa 1 site, and r the recombination fraction between the sickle site and the Hpa 1 site. When P=O, implying that the Hpa 1 site is very rare and when r=1/20 000, n=2785 generations, as calculated by Kurnit. When P=0-03 present observed frequency of the 13 kb site in normal individuals, as given by Kan and Dozy2) n=2878. This number increases as the frequency of the Hpa 1 site increases. Rather than using the data as evidence of a multicentric origin we can use them to estimate the age for the sickle gene. This ranges from 2785 generations (or about 69 625 years) if the Hpa 1 site was very rare, to values as high as 6022 generations (150 550 years) if the 13 kb site was at one time more frequent than it is now. This analysis suggests that the origin of the sickle mutation might well predate the origin of the major human racial groups, although its striking increase in frequency was much more recent. As mentioned by Kurnit, polymorphisms for restriction enzyme sites are likely to provide powerful new tools for evolutionary and genetic studies. These polymorphisms, which identify variation at the level of the gene itself, may provide a quantum jump in the range of available genetic markers for study. Kan and Dozy2 have emphasised their application to the general problem of prenatal diagnosis of hereditary diseases, even where the specific biochemical defect is not known. If the level of polymorphism for these sites is as high as the intial studies suggest, and given the range of available restriction enzymes, one can envisage finding enough markers to cover systematically the whole human genome. Thus, only 200-300 suitably selected probes might be needed to provide a genetic marker for, say, every 10% recombination. Such a set of genetic markers could revolutionise our ability to study the genetic determination of complex attributes and to follow the inheritance of traits that are so far difficult or impossible to study at the cellular level. Association within families between a defined genetic marker and a trait whose inheritance is not clearly defined, provides the best evidence, through genetic linkage, for genetic determination. This powerful approach has so far been limited by the range of genetic markers available, but restriction-enzyme polymorphisms may soon solve this nroblem.
(the
Genetics
Laboratory, Department of Biochemistry, University of Oxford, Oxford OX1 3QU
UNEMPLOYMENT, MORBIDITY, AND MORTALITY SiR,—The independent examination sought by Dr Draper and his colleagues (Feb. 17, p. 373) and by you in your editorial of March 31 has been made.I.2 In Australia, associations were detected between annual changes in unemployment and changes in ischxmic-heart-disease mortality, infant mortality, analgesic prescribing, suicide, and all-cause mortality. These Australian findings generally confirm those by Brenner in the U.S.A.3 Lags of three to six years were detected in the mortality series. The research at Macquarie University went on to examine three questions raised in Lancet correspondence: What is the mechanism of the association between unemployand mortality? (2) Given an annual association between the business cycle and mortality, what association, if any, exists between perturbations in the business cycle (e.g., the "great depression") and mortality? (3) What are the relative contributions of medical therapeutics and business-cycle factors to mortality-rates?
(1)
ment
In the Australian work 1
E. SOLOMON W. F. BODMER
an
indicator
senes
of "national
Schneider, R. G., Schmidt, R. M. in Abnormal Hemoglobins and Thalassemia
edited by R. M. Schmidt); p. 33. New York, 1975.
2 Kan, Y W., Dozy, A. M. Lancet, 1978, ii, 910. Kan, Y. W., Dozy, A. M. Proc. natn. Acad. Sci. U.S.A., 1978, 75, 5631. 4. Weisenfeld, SL. Science, 1967, 157, 1134.
1. Bunn, A. R., Drane, N T New Doctor, 1977, 5, 53. 2. Bunn, A. R. Am. J. publ. Hlth in the press 3. Brenner, M. H. Estimating the Social Costs of National Economic U.S. Government Printing Office. 1976.
Policy
(left). The significance of this finding and its relation to sickle-cell anxmia are not known. Perhaps the physical change in the aqueous humor resulted from leakage of protein due to vascular damage to the circulation of the anterior segment secondarv to chronic intravascular sickling. KRAIG LORENZEN Departments of Medicine and Surgery, KODURI P. RAO Divisions of Hematology and Ophthalmology, ASHOK R. PATEL Cook County Hospital, Chicago, Illinois 60612, U.S.A JAMES DESNICK
EVOLUTION OF SICKLE VARIANT GENE
SIR,-The 13 kilobase (kb) Hpa 1 restriction enzyme site
polymorphism, close to the sickle variant of j3-globin, raises interesting evolutionary questions, as discussed by Dr Kurnit (Jan. 13, p. 104). He calculates that about 2785 generations would be needed to generate the 13% of sickle genes not associated with the 13 kb Hpa 1 polymorphic variant if the sickle mutation arose in a chromosome carrying this variant. This is inconsistent with the assumption that the sickle mutation occurred at the time agriculture and malaria were introduced into Africa, only 60-80 generations ago.4 Kurnit therefore argues for a multicentric origin of the sickle gene. We should like to suggest an alternative interpretation of the data, assuming a much older origin of the sickle gene. Though the increase in frequency of the sickle-cell gene may be traced to the introduction of malaria to Africa only 60-80 generations ago, this by no means implies that the mutation arose then. Indeed, it is more likely that the sickle mutation arose much earlier, in a founder with the 13 kb site, and that it achieved and was maintained at a low frequency due to random drift, until it was favoured by selection at the time of the introduction of malaria. At this time it would have already existed in coupling with both the 13 kb and the 7.6kb sites, and both of these combinations would then have expanded due to selective pressure and so relatively quickly given rise to the situation today. Using the standard population genetic random mating model for two linked loci and assuming, for simplicity, no change in the frequencies of either the sickle gene or the Hpa 1 sue before the onset of malaria, we can calculate the number
of generations (n) taken to generate the 13% recombinant chromosomes from the formula (1-r)"=(0 87-P)/( 1-P), where P is the gene frequency of the Hpa 1 site, and r the recombination fraction between the sickle site and the Hpa 1 site. When P=O, implying that the Hpa 1 site is very rare and when r=1/20 000, n=2785 generations, as calculated by Kurnit. When P=0-03 present observed frequency of the 13 kb site in normal individuals, as given by Kan and Dozy2) n=2878. This number increases as the frequency of the Hpa 1 site increases. Rather than using the data as evidence of a multicentric origin we can use them to estimate the age for the sickle gene. This ranges from 2785 generations (or about 69 625 years) if the Hpa 1 site was very rare, to values as high as 6022 generations (150 550 years) if the 13 kb site was at one time more frequent than it is now. This analysis suggests that the origin of the sickle mutation might well predate the origin of the major human racial groups, although its striking increase in frequency was much more recent. As mentioned by Kurnit, polymorphisms for restriction enzyme sites are likely to provide powerful new tools for evolutionary and genetic studies. These polymorphisms, which identify variation at the level of the gene itself, may provide a quantum jump in the range of available genetic markers for study. Kan and Dozy2 have emphasised their application to the general problem of prenatal diagnosis of hereditary diseases, even where the specific biochemical defect is not known. If the level of polymorphism for these sites is as high as the intial studies suggest, and given the range of available restriction enzymes, one can envisage finding enough markers to cover systematically the whole human genome. Thus, only 200-300 suitably selected probes might be needed to provide a genetic marker for, say, every 10% recombination. Such a set of genetic markers could revolutionise our ability to study the genetic determination of complex attributes and to follow the inheritance of traits that are so far difficult or impossible to study at the cellular level. Association within families between a defined genetic marker and a trait whose inheritance is not clearly defined, provides the best evidence, through genetic linkage, for genetic determination. This powerful approach has so far been limited by the range of genetic markers available, but restriction-enzyme polymorphisms may soon solve this nroblem.
(the
Genetics
Laboratory, Department of Biochemistry, University of Oxford, Oxford OX1 3QU
UNEMPLOYMENT, MORBIDITY, AND MORTALITY SiR,—The independent examination sought by Dr Draper and his colleagues (Feb. 17, p. 373) and by you in your editorial of March 31 has been made.I.2 In Australia, associations were detected between annual changes in unemployment and changes in ischxmic-heart-disease mortality, infant mortality, analgesic prescribing, suicide, and all-cause mortality. These Australian findings generally confirm those by Brenner in the U.S.A.3 Lags of three to six years were detected in the mortality series. The research at Macquarie University went on to examine three questions raised in Lancet correspondence: What is the mechanism of the association between unemployand mortality? (2) Given an annual association between the business cycle and mortality, what association, if any, exists between perturbations in the business cycle (e.g., the "great depression") and mortality? (3) What are the relative contributions of medical therapeutics and business-cycle factors to mortality-rates?
(1)
ment
In the Australian work 1
E. SOLOMON W. F. BODMER
an
indicator
senes
of "national
Schneider, R. G., Schmidt, R. M. in Abnormal Hemoglobins and Thalassemia
edited by R. M. Schmidt); p. 33. New York, 1975.
2 Kan, Y W., Dozy, A. M. Lancet, 1978, ii, 910. Kan, Y. W., Dozy, A. M. Proc. natn. Acad. Sci. U.S.A., 1978, 75, 5631. 4. Weisenfeld, SL. Science, 1967, 157, 1134.
1. Bunn, A. R., Drane, N T New Doctor, 1977, 5, 53. 2. Bunn, A. R. Am. J. publ. Hlth in the press 3. Brenner, M. H. Estimating the Social Costs of National Economic U.S. Government Printing Office. 1976.
Policy
