American Journal of Medical Genetics 32:407-410 (1989)
Estimate of the Proportion of Duchenne Muscular Dystrophy with Autosomal Recessive Inheritance
Mayana Zatz, Maria Rita Passos-Bueno, and Debora Rapaport Departamento de Biologia, Instituto de BiociGncias, Uniuersidade de Stio Paulo, Stio Paulo, Brazil
The aim of the present report was to estimate the proportion of autosomal recessive (AR) in- heritance among families with affected males diagnosed as Duchenne muscular dystrophy (DMD) in which X-linked inheritance could not be confirmed. A total of 470 families was studied: 20 with at least one affected girl with Duchenne-like phenotype and 450 with only affected boys. Based on the number of families with at least one affected girl and the number of patients per sibship among these pedigrees, the proportion of families with DMD inherited as an AR trait was estimated at 6.8%. It is also estimated that 2.5-4% of male isolated pa- tients diagnosed as DMD may have the AR form, which could be one possible explanation for the inconsistent results between clinical diagnosis and dystrophin assessment in one case recently reported.
KEY WORDS: Duchenne phenotype, types of inheritance, dystrophin
INTRODUCTION Duchenne muscular dystrophy (DMD) is a lethal wast-
ing disease of childhood characterized by a progressive degeneration of skeletal muscles. Most cases are caused by an abnormal gene located on the short arm of the X chromosome.
According to several earlier reports, some cases are clue to autosomal recessive (AR) inheritance [Dubowitz, 1980; Gardner-Medwin and Johnston, 1984; Somer et al., 1985; Yoshioka et al., 19861. Affected girls with a severe form of childhood muscular dystrophy are easily identified, but when the patient is a boy, the clinical picture may be indistinguishable from typical X-linked DMD. The dis- tinction between these two forms is important for genetic counseling, in particular for sisters of affected boys.
The gene product of the DMD gene, which was named dystrophin, has been identified and is apparently absent
Received for publication August 3, 1988; revision received Sep- tember 21, 1988.
Address reprint requests to Mayana Zatz, Departamento de Biologia, Universidade de Sio Paulo, Caixa Postal 11461,05499 Sio Paulo, SP, Brazil.
0 1989 Alan R. Liss. Inc.
or reduced in muscle from DMD patients [Hoffman et al., 19871. However, in a recent publication, one of the 38 patients diagnosed as DMD had apparently normal dystrophin [Hoffman et al., 19881. This patient had no family history of DMD, and his mother and sister had normal creatine-kinase (CK) levels.
This finding raised the possibility that this patient might have, in fact, the AR form of DMD. In order to test this hypothesis, we have estimated, what is the proportion of possible AR Duchenne-like affected pa- tients in a large sample of Brazilian families with boys diagnosed with DMD.
MATERIALS AND METHODS
A total of 470 families with patients affected with muscular dystrophy ascertained in our center during the last 19 years are included in the present study.
In all of them, the diagnosis was established through clinical examination, family history, elevated serum crea- tine-kinase (CK) and, in most, also, serum pyruvate- kinase (PK), and typical electromyography and/or mus- cle biopsy.
In 450 families, only affected boys were found. In 294 families, the propositus was an isolated case; in 74, there were two or more affected boys in the same sibship; and in the remaining 82, typical X-linked inheritance was found. Therefore, in a total of 368 families (294 + 74), X-linked inheritance could not be confirmed.
Girls affected with a clinical picture and laboratory findings very similar to X-linked DMD were found in 20 additional families. All of them had serum CK in the same range as X-linked DMD and electromyography and muscle biopsy with a myopathic pattern. In 13 families, the propositus was an isolated case; in one family, three affected sisters were found; and in the remaining six, boys and girls were equally affected.
RESULTS Families With at Least One Affected Girl
Chromsome analysis was performed in all girls (iso- lated patients as well as in the three affected sisters). An X/autosome translocation was found in three DMD girls who were isolated cases: X/6 [Zatz et al., 19811 and X/2 and X/22 [Vianna-Morgante et al., 19861. The remaining ten showed no visible chromosome abnormality.
CK in 23 patients (aged 16 months to 18 years) from
408 Zatz et al.
the 15 families with at least one affected girl ranged from 48.5 to 1680.0 Sigma units (S.U.) (normal up to 10.0 S.U.). The lowest value was found in a 14-year-old girl who had been in a wheelchair for 4 years, whereas the highest value was measured in a 7-year-old ambulatory girl. In the probands from the 450 families with boys diagnosed as DMD and of comparable age, the CK levels ranged from 45.0 to 2930.0 S.U., showing, therefore, a great overlap in both distributions.
In two families, the propositus had affected older brothers who were able to walk beyond age 15 and were therefore excluded from the analysis. Hence, there were 15 families in which the patients had a clinical picture or laboratory findings indistinguishable from X-linked DMD (Table I).
Among them, ten were isolated cases, one had three affected sisters, and in the remaining four, patients of both sexes were observed. A total of 23 patients (19 girls and four boys) were found in these 15 families.
In order to check if the proportion of affected patients was in accordance with AR inheritance, a correction for a bias when there is multiple incomplete ascertainment was performed as proposed by Davie . According to the methodology described in Emery , if
P = theoretical proportion, i.e., .25
R = (number of affected in all sibships) = 23 T = (total number of individuals in all sib- = 60
J = (number of pedigrees with one affected = 10
Q = (number of sibships with 2 propositus) = 3
Then P = (R - J)/(T - J) and the variance is (R - J)(T - R) + 2Q(T - R)2
(T - J)3 (T - J)4 or P = .26 k .lo, which is in accordance with AR inher- itance.
Proportion of Consanguineous Marriages Among parents of sibships with af-
fected girls, at least seven out of 15 were consanguineous: five first-degree, one first-degree once-removed, and one second-degree once-removed cousins. In one sibship, the parents came from an Indian inbred community in the Amazonas, and one girl was an adopted child.
Consanguinity was found in eight of 294 families with isolated cases (two first-degree, five first-degree once-removed, and one second-degree once- removed cousins), in two of 74 families with multiple
TABLE I. Genealogies With a t Least One Girl Affected With Duchenne-like Phenotype No. in sibship
Family Propositus 1 0 1
2 . 0 0 0 0 0 0 0 1
3 0 0 1
4 0 0 . 0 0 0 1
7 0 0 1
9 0 0 0 1
100.0. 1 / 11 oo.oooo0m 2
12 00 0 0 0 1
1 3 0 0 1
14 0 0 1 / 1500 1 / TOTAL 16
f 6 0 0 ON0 0 0 0 0 0 0 0
Consanguinity Adopted child
First cousins once removed
Second cousins once removed
(W O), Affected patients.
Proportion of DMD With Autosomal Recessive Inheritance 409
affected boys in the same sibship (two first-degree cousins), and in only one of 82 families with X-linked DMD (first-degree once-removed cousin).
Est imate of t he Proport ion of Sibships With Possible Autosomal Recessive DMD
Among All Families In 294 of 450 families with affected boys, the propositus
was an isolated case, and in 74, there were two or more affected boys in the same sibship; that is, in 368 families, X-linked inheritance could not be confirmed. Therefore, the total number of families with Duchenne-like phe- notype patients was 368 + 15 = 383.
If affected girls have the same chance as boys to be ascertained, the proportion of pedigrees with AR inher- itance among all cases may be roughly estimated, based on the following observations (Table 11): 1) number of families with at least one affected girl, whose clinical picture and laboratory findings (serum enzymes, electro- myography, and muscle biopsy) are indistinguishable from the one found in boys with X-linked DMD; and 2) probability of ascertainment according to number of pa- tients per sibship with at least one affected girl.
Accordingly, the proportion of families with AR DMD would be 26.21/(368 + 15) or 6.8%. If this is true, a boy diagnosed as DMD, with no family history, would have a 3-4% probability of having the AR form, as illustrated in Table 111.
DISCUSSION Apparent AR DMD has been reported by several au-
thors as reviewed by Emery  and Walton and Gardner-Medwin [ 19881. According to the first author, in Britain and North America, the X-linked form would seem to be at least 20 times more common than is AR DMD. However, he points out that this situation may be
TABLE 11. Estimate of Proportion of AR DMD Among the 383 Families in Which X-Linked Inheritance Could Not Be
Confirmed Probability of ascertainment No. families Proportion expected
n 11 - (1/2)1 observed after correction .~ ~
1 112 10 20.00 2 314 3 4.00 3 718 1 1.14 4 15/16 1 1.07 Total 15 26.21 n = no. of affected patients per family with at least one affected girl.
TABLE 111. Likelihood That an Isolated Boy Diagnosed as DMD has the AR From Probability of inheritance
according to estimated population frequency AR X-linked
Prior probability .07 .93 Conditional probability 5 0 1.00
Joint probability .0375 .93 for an affected male
Posterior probability .0375/(.0375 + .93) .93/(.0375 + .93) -3.9% ~ 9 6 . 1 %
different in other countries. For instance, the purportedly AR type of muscular dystrophy seems to be more common in Switzerland and in certain Arabic communities in Tunisia and Sudan, and in these latter two populations, it is apparently also more severe.
In a study of 12 affected girls (and two affected broth- ers), Gardner-Medwin and Johnston [ 19841 concluded that most if not all had an AR form of muscular dystro- phy. They observed that toe-walking was a prominent early manifestation and ability to walk was retained until 11-15 years or later [Walton and Gardner-Medwin, 19881.
The early toe-walking reported by these authors was observed in only one of the affected girls and none of the boys from the present sample. In the three affected girls with DMD and X/A translocation reported earlier by our group [Zatz e t al., 1981; Vianna-Morgante et al., 19861, loss of ambulation occurred from age 10 to 11 years. Among 23 affected patients from the 15 families reported here, six were confined to a wheelchair from ages 10 to 12 years and one boy was able to walk until age 14 years. The remaining, younger than age 10, were still ambula- tory. Three girls showed no calf hypertrophy, including a preclinical 16-month-old who had two older affected sisters. She was also the only patient with a negative Gower sign. Although this sample is very small, loss of ambulation occurred apparently slightly later than in our previous report of boys with X-linked DMD [Bortolini and Zatz, 19861. However, in individual cases, this differ- ence is not enough to allow a distinction between the two forms of DMD.
An interesting observation is that the proportion of consanguineous parents among pedigrees in which X- linked inheritance could not be confirmed (10/368) was 2.7%, whereas in X-linked families, it was only 1.2% (1 in 82). This gives further support to the hypothesis that among the former families a proportion are AR.
In their recent study of 103 patients with various neuromuscular disorders, Hoffman et al. [ 19881 con- cluded that quantitative abnormalities of dystrophin re- sult in a more severe clinical phenotype than do quali- tative abnormalities. According to their report, most patients with typically severe DMD (92%) had little or no dystrophin; most patients with intermediate cases (57%) also had reduced amounts of dystrophin, although higher than in the first group, whereas most of the patients with clinically mild Becker dystrophy (67%) had qualitative rather than quantitative abnormalities in dys- trophin. However, in 7.7% of their patients, the clinical phenotype did not correlate with the results of dystrophin assessment. Among 38 boys diagnosed as typical DMD, one boy with no family history of DMD had apparently normal dystrophin.
As seen in Table 111, we have estimated that at least in our population 3-4% of isolated affected boys might, in fact, have the AR form of the disease. If in North America and Britain about 5% of the pedigrees are in- herited as AR [as suggested by Emery, 19871, this pro- portion would be approximately 2.6%, which is close to one in 38 found by Hoffman et al. . Such patients, erroneously classified as having X-linked DMD, could be one possible explanation for some of the inconsistent
410 Zatz et al.
findings between clinical phenotype and results of dys- trophin reported above.
Investigations of dystrophin in muscle from girls with a clinical picture indistinguishable from X-linked DMD would be of great interest, and as pointed out by Hoffman et al. , should prove helpful in delineating myopa- thies that overlap clinically with Duchenne and Becker dystrophy.
The collaboration of the following persons is gratefully acknowledged Dr. Paulo A. Otto, Mrs. Mariz Vainzof, Jane M.L. Rocha, Dr. Rita C.M. Pavanello, Mr. Roberto Schreiber, Mrs. Marta Canovas, and Marcia Luiza das Neves. This work was supported with grants from Fun- daciio de Amparo a Pesquisa do Estado de Sho Paulo (FAPESP), Conselho Nacional de Desenvolvimento Cientifico e Tecnolhgico (CNPq), Financiadora de Estu- dos e Projetos (FINEP), and AssociaGiio Brasileira de Distrofia Muscular (ABDIM).
Bortolini ER, Zatz M (1986): Investigation on genetic heterogeneity in
Dubowitz V (1960): Progressive muscular dystrophy of the Duchenne
Duchenne muscular dystrophy. Am J Med Genet 24:lll-117.
type in females and its mode of inheritance. Brain 83:432-439.
Davie AM (1979): The singles method for segregation analysis under incomplete ascertainment. Ann Hum Genet 42507-512.
Emery AEH (1986): Methodology in Medical Genetics, 2nd Ed. Edin- burgh, London, Melbourne, and New York Churchill Livingstone.
Emery AEH (1987): Duchenne Muscular Dystrophy. Oxford, New York, Tokyo: Oxford University Press.
Gardner-Medwin D, Johnston HM (1984): Severe muscular dystrophy in girls. J Neurol Sci 64:70-87.
Hoffman EP, Brown RH Jr , Kunkel LM (1987): Dystrophin: The protein product of the Duchenne muscular dystrophy locus. Cell 51:919-928....