692

CARRIER DETECTION IN DUCHENNE MUSCULAR DYSTROPHY: ASSESSMENT OF THE EFFECT OF AGE ON DETECTION-RATE WITH SERUM-CREATINE-KINASE-ACTIVITY G. A. NICHOLSON R. J. T. PENNINGTON

D. GARDNER-MEDWIN J. N. WALTON

Muscular Dystrophy Research Laboratories, Newcastle General Hospital, Newcastle upon Tyne, and

Department of Neurology, University of Newcastle upon Tyne

Summary

In 40 known carriers of the gene for

Duchenne muscular dystrophy (mean years) the detection-rate with a standard assay of serum-creatine-kinase was 53%. In 52 daughters of known carriers (mean age 16 years) the detection-rate was 45% after correction for age. This implies a much higher detection-rate (about 90%) in young carriers and age 38

i

Mm

Fig.

4- Time

tory response

to

of

acid-secretory and pepsin-secrepentagastrin after administration of raniti-

course

mean

dine. Each point represents mean of 4 patients, expressed as percentage of secretory response immediately preceding intraduodenal administration (I.D. infusion) of ranitidine. Vertical columns represent absolute values of acid and pepsin outputs during the 100 min after administration of saline (control) or ranitidine. C=control test; R=ranitidine. Values are post-ranitidine secretion compared with control value (/o).

pepsin output

was

and 52% after 80 mg

32% after administration of 40 mg

of the drug (fig. 4). Discussion

This study has confirmed4that ranitidine hydrochloride, a new histamine-H2-receptor antagonist, produces profound and sustained gastric secretory inhibition in man. While we did not compare the inhibitory effects of cimetidine and ranitidine in the same individuals, we had previously reported that 400 mg cimetidine inhibited nocturnal acid secretion of patients with duodenal ulcer by an average of 66%.5 By the same technique, in duodenal-ulcer patients secreting approximately the same quantity of acid at night, 80 mg of ranitidine inhibited nocturnal acid secretion by an average of 95%. Our finding therefore accords with the recent report4 that ranitidine is more powerful than cimetidine in inhibiting pentagastrin-stimulated secretion. Since ranitidine is a very effective gastric secretory inhibitor after absorption from the alimentary tract of patients with duodenal ulcer, clinical trials of the drug in the "peptic" diseases are warranted. We thank Mrs H. Wood and Dr B. Kilgallon for valuable technical assistance. Dr D. A. Richards (Glaxo-Allenburys Research, Ware, Herts, England) kindly provided the ranitidine. K. G. W. gratefully acknowledges a research grant from the Scottish Hospital Endowments Research Trust. Requests for reprints should be addressed to K. G. W. REFERENCES

1.

Black, J. W., Duncan, M. E. Nature,

W. A.

M., Durant, G. J., Ganellin, C. R., Parsons,

1972, 236, 385.

2. Ganellin, C. R., Durant, G. J., Emmett, J. C. Fedn Proc 1976, 35, 1924. 3. Berstad, A. Scand. J. Gastroent. 1970, 5, 343. 4. Domschke, W., Lux, G., Domschke, S. Lancet, 1979, i, 320. 5. Saunders, J. H. B., Cargill, J. M., Wormsley, K. G. Digestion, 1977, 15, 452.

suggests that discrimination between carriers and noncarriers may best be achieved by testing in childhood. Introduction X-LINKED Duchenne muscular dystrophy (D.M.D.) is of a small group of genetic diseases for which a carrier-detection test exists. Serum-creatine-kinase (S.C.K.) assay has been shown to be the best single test for carrier detection; however, s.C.K. levels are abnormal in only 50-80% of known carriers, depending in part on the laboratory in which the test is done. Failure to detect all known carriers with this method has been attributed largely to the non-specific nature of the test, but attempts to produce tests of greater accuracy and specificity have been unsuccessful. Failure to develop a more accurate test for the carrier state has been attributed by some to lyonisation of the defective X chromosomes in some carriers.2,3 Detection-rates of up to 90% have been obtained in some X-linked diseases with known biochemical defects-for example, h2emophilial-I and Lesch-Nyhan disease.’ Serum-creatine-kinase-activity falls with age in boys with D.M.D., and since a carrier is a mosaic of defective and normal X chromosomes, s.c.K. levels in carriers might also be expected to fall with age. This has been demonstrated in carriers of the gene for the more benign Becker form of muscular dystrophy,* and some authors have suggested that this occurs in D.M.D. carriers,9-12 although others have found no clearcut age effect in carriers.13-15 We have investigated the effect of age on carrierdetection rates with s.c.K., using data from the Newcastle Muscular Dystrophy Clinic. The results suggest that the optimum age for carrier testing with s.c.K. may be lower than was previously thought. one

Methods Serum-creatine-kinase activity was estimated by the method of Pearce and others.14 The upper limit for normal women aged 16 or over (60 i.u./l) was determined from the results of testing 80 women volunteers with no history of recent illness and no high alcohol intake or unusual exercise in the preceding 48 h. The upper limit was defined as the 95% confidence limit of the log creatine-kinase (c.K.) values. The log C.K. transfor-

693 TABLE I-SERUM-CREATINE-KINASE DETECTION-RATE IN KNOWN

values were linear on a frequency-distribution plot and because the results were equally distributed about the median s.c.K. value. This new range of normal activity replaces that of Gardner-Medwin et al.11 The normal upper limit for girls under 16 (73 i.u./l) was similarly established for a separate group of 58 healthy volunteers aged 5 to 15 years with no history of recent illness or unusual exercise in the preceding 48 h. Known carriers included probable and definite carriers as defined by Pearce et al. 16 Where more than one S.C.K. estimation was recorded for any subject, the figure used in this analysis was the first result recorded. For carrier testing in practice we attach great importance to using the mean of three or more separate s.c.K. estimations. To avoid selection of either carrier or normal females in the daughter group only data from daughters, without offspring, of known carriers were included for analysis. mation

was

used because the

log

C.K.

D.M.D. CARRIERS AND DAUGHTERS OF KNOWN CARRIERS

Results The C.K. results, plotted against age, are shown for the two series of normal females, for 40 known carriers, and for 52 daughters of known carriers in figs. 1-3. The S.C.K. activities for the normal control subjects decline slightly with age (fig. 1). 53% of the known carriers had S.C.K. levels above 60 i.u./l, and 45% of the daughters of known carriers had raised levels (above 73 i.u./l for those under 16 years of age, above 60 i.u./l for older women). For daughters under 16 the detection-rate was higher still (table i). Since each daughter of a known carrier has a 0.55 risk of being

a carrier herself, the detection-rate in the daughters would be expected to be half that in known carriers. The rates found in this series suggest that 90-100% of the true carriers may have been detected. The mean age of the known carriers was 38 years and that of the daughters of known carriers 16 years. The simplest explanation of the apparent discrepancy in the detection-rates is that the S.C.K. level falls with age much more steeply in carriers than in normal females. The s.c.K. levels in the known carriers over an age range of six decades showed no clear fall with age (fig. 2), whereas the levels in the daughters of known carriers seemed to fall over the first two decades. Repeat S.C.K. estimations on 11 known carriers over periods of several years (fig. 3) showed some tendency to fall, although levels in several cases remained static, especially when these were within or close to the normal limits. Over the same period there was no significant change in the mean value for normal women, indicating that the S.C.K. assay had not altered in sensitivity. There was no significant fall in s.c.K. activity in 5 normal females after 11 years.

Discussion Age (yr)

suspected for some years that S.C.K. levels carriers may fall with age, but since a known D.M.D. carrier is first defined when she has an affected son, results for known carriers before the reproductive It has been

Fig. 1-Serum-creatine-kinase activities for 80 normal adult females and 58 normal girls under 16, plotted against the age of each subject. The regression line and 95% confidence limits method of least squares.



were

fitted

by

the

Age (yr) Fig. 2-Results for S.C.K. testing in 40 known D. M. D. shown against the age of the subject. The regression line was fitted by the method of least squares.

in

D.M.D.

3-Results of serial S.C.K. carriers.

Fig. carriers

Results obtained least squares.

for 11 known D.M.D.

7-to-10 year period for each individual are each individual were fitted by the method of

over a

plotted. Regressions for

testing

694 age can be obtained only retrospectively. In definite carrier daughters of patients with Becker muscular dystrophy a progressive fall in S.C.K. levels with age has been shown.8 The lack of data from D.M.D. carriers aged under 16 to include in the regression analysis may explain why no fall of s.c.K. levels with age was found for the group of known carriers in this study. Thomson et a1.1o assumed a 100% carrier-detection rate in young D.M.D. carriers and hence regarded all normal s.c.K. results as representing non-carriers. They obtained, over a full age range, a regression showing a fall of s.c.K. levels with age. We consider this approach inadmissible as it begs the question of c.K. levels in young carrier

girls. Some authors have reported a fall in s.c.K. activities with age in individual carriers.’,", 12 Our results (fig. 3) are similar and suggest that any fall with age may be obscured by individual variations when results for a number of individuals are plotted over a small age range. It was apparent in the Newcastle data (fig. 3) that over a short period (3-5 years) individual variation was often greater than any fall with age. An estimate of the general tendency of s.c.K. to fall with age in a population of carriers can be obtained by TABLE II-S.C.K. CARRIER-DETECTION RATES AMONG FEMALE

RELATIVES OF D.M.D.

PATIENTS*

lations differed. The Newcastle results for young subjects are similar to those of the two other clinics and suggest that the relatively low Newcastle adult-carrier detection-rate noted in this study and in previous reports could be due to an older adult carrier population. If it is confirmed in further large series that S.C.K. testing gives very high degrees of discrimination between carriers and normal girls in childhood this will have two important implications for genetic counselling in D.M.D. Firstly, it would obviously become important to determine the age at which discrimination was greatest and to endeavour to do s.c.K. testing at that age in all girls known to be at risk. Secondly, if carrier or non-carrier status could be ascribed with substantially more confidence to the daughters of mothers who have one son with D.M.D. but no other affected male relatives, the estimated risk in the mothers and their other female rela-, tives would in many cases be significantly modified. It is often suggested that any test which gives a very high degree of discrimination between carriers and noncarriers could provide a method of screening the whole population. Without such high discrimination any method of population screening for carriers could give many false-positive and false-negative results, and there would be no means of verifying these. The incidence of the carrier state is approximately 1 in 2000 female births; and therefore to identify carriers with even 50% probability any test would have to discriminate them from 99.9% of the population. We think it is unlikely that a single estimation of serum-creatine-kinase activity will provide a degree of discrimination of this order. However, repeated testing in young girls or ppssibly a combination of tests might ultimately be shown to resolve the majority of false-positive results obtained by

preliminary screening. *Comparison

of results of s.c.K.

testing in three

D.M.D.

clinics. Carrier-detection

expressed as the ratio of carriers detected by having an elevated S.C.K. level compared with the total number of females tested. The ratio was obtained by comparing the carrier-detection rate found in the daughters of known carriers to that predicted as half that of the relevant known-carrier group. Results for

rates are

Poland are obtained from the data of Hausmanowa-Petrusewicz 18 those for North Carolina from the data of Roses et al.11

et

al.11 and

S.C.K. detection-rates in known carriers and the estimated carrier-detection rate in daughters of known carriers. Data from two D.M.D. clinics in Polandl7 and North Carolina18 show a presumptive detection-rate in daughters of known carriers of about 1.5times that of adult carriers. This increase in the presumptive detection-rate in younger age groups from other clinics is similar to the increase which is apparent from our data (table n). The results from these three clinics when analysed in this way indicate a detection-rate in young carriers of 90-100%, which agrees with Moser’s suggestionll of a 90% detection-rate in young carriers. Our results for daughters of known carriers (of which half were adult daughters) suggest at least a 90% detection-

We thank the mothers and daughters, contacted through Newcastle Central High School, who gave blood-samples. This work was supported by grants from the Medical Research Council and the Muscular Dystrophy Group of Great Britain and a Bushell Travelling Fellowship to G. A. from the Royal Australasian College of Physicians.

Requests for reprints should be addressed to J. N. W., Regional Neurological Centre, General Hospital, Westgate Road, Newcastle upon Tyne NE4 6BE.

comparing

The results for daughters under 16 indicate a predetection-rate of closer to 100%, but further results are required for an accurate figure to be obtained. Our results, showing striking changes in S.C.K. detection-rates in carriers of different ages, might explain variations in s.C.K. carrier-detection rates reported from different clinics, if the age structures of the carrier popurate.

sumptive

REFERENCES 1.

Walton, J. N., Gardner-Medwin, D. in Disorders of Voluntary Muscle (edited by J. N. Walton); p. 574. Edinburgh and London, 1974. 2. Sugita, H., Tyler, F. H. Trans. Ass. Am. Physns, 1963, 76, 231. 3. Pearson, C. M., Fowler, W. M., Wright, S. W. Proc. natn. Acad. Sci. U.S.A. 1963, 50, 24. 4. Biggs, R., Rizza, C. R. Lancet, 1976, ii, 431. 5. Ratnoff, O., Jones, P. K. Ann. intern. Med 1977, 86, 521. 6. Klein, H. G., Aledort, L. M., Bouma, B. N., Hoyer, L. W., Zimmerman, T. S., DeMets, D. L. New Engl. J. Med. 1977, 296, 959. 7. Franke, U., Felsenstein, J., Gartler, M., Migeon, B. R., Dancis, J., Seegmiller, J. E., Bakay, F., Nyhan, W. L. Am. J. hum. Genet. 1976, 28, 123 8. Skinner, R., Emery, A. E. H. Lancet, 1974, ii, 1023. 9. Moser, H., Vogt, J. ibid. p. 661. 10. Thomson, W. H. S., Sweetin, J. C., Elton, R. A. Nature, 1974, 249, 151. 11. Moser, H. Praxis, 1977, 66, 814. 12. Munsat, T. L., Baloh, R., Pearson, C. M., Fowler, W. J. Am. med. Ass. 1973, 226, 1563. 13. Emery, A. E. H. J. neurol. Sci. 1969, 8, 579. 14. Pearce, J. M. S., Pennington, R. J. T., Walton, J. N. J. Neurol. Neurosurg Psychiat. 1964, 27, 1. 15. Gardner-Medwin, D., Pennington, R. J. T., Walton, J. N. J. neurol. Sci. 1971, 13, 459. 16. Pearce, J. M. S., Pennington, R. J. T., Walton, J. N. J. Neurol. Neurosurg. Psychiat. 1964, 27, 181. 17. Hausmanowa-Petrusewicz, I., Niebroj-Dobosz, I., Borkowska, J., Lukasik, E., Liszewska-Pfejer, D. in Pathogenesis of Human Muscular Dystrophies (edited by L. P. Rowlend); p. 32. Amsterdam, 1977. 18. Roses, A. D., Roses, M. D., Metcalf, B. S., Hull, K. L., Nicholson, G. A., Hartwig, G. B., Roe, C. R. Ann. Neurol. 1977, 2, 271.

Carrier detection in Duchenne muscular dystrophy: Assessment of the effect of age on detection-rate with serum-creatine-kinase-activity.

692 CARRIER DETECTION IN DUCHENNE MUSCULAR DYSTROPHY: ASSESSMENT OF THE EFFECT OF AGE ON DETECTION-RATE WITH SERUM-CREATINE-KINASE-ACTIVITY G. A. NIC...
381KB Sizes 0 Downloads 0 Views