Pergamon Neuromusc. Disord., Vol. 4, No. 4, pp. 301-303, 1994.

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INTRODUCTION

UNANSWERED QUESTIONS IN DUCHENNE MUSCULAR DYSTROPHY

A L A N E. H . EMERY

R e s e a r c h D i r ec to r , E N M C , B a a r n , The N e t h e r l a n d s

Abstract--Though the molecular and biochemical bases of Duchenne muscular dystrophy are known, many questions still remain unanswered. They range from the nature and cause of gene deletions to the relationship between dystrophin defects and the clinical phenotype, both in affected males and female carriers. These questions are discussed in the light of recent developments.

Key words: Duchenne muscular dystrophy, Becker muscular dystrophy, dystrophin, brain involvement, muscle involvement, cardiac involvement, clinical features, manifesting carriers.

INTRODUCTION

Duchenne muscular dystrophy is a well-defined clinical disorder in which the primary defect is a deficiency of muscle dystrophin. However, despite a considerable amount of research, there still remain a number of unanswered questions. Here some of the perhaps more obvious ones are discussed, which range from the molecular basis of the disease to its clinical expression.

DISCUSSION

The dystrophin gene

In around 70% of cases, mutations of this enormous gene, residing at Xp21, result in detectable deletions and duplications, the distri- bution of which is bimodal. Interestingly there is also a bimodal distribution of hot spots of recombination within the gene [1] which coincide with the distribution of deletions. This suggests that these two events (recombination and deletion) could be in some way related. One possibility is that micro-inversions within the large gene itself might be responsible [2]. Furthermore, the occurrence of different muta- tions within the same family, which is now well documented [3] could perhaps also result from such a mechanism. Of relevance is that gene inversions have recently been reported in many cases of haemophilia A(4).

The dystrophin protein

Rare exceptions to the Monaco rule, that out-

of-flame deletions which disrupt the reading frame result in Duchenne muscular dystrophy whereas in-flame deletions result in Becker muscular dystrophy, have been explained in terms of various molecular mechanisms. But the situation is still not entirely clear. For example, why do out-of-frame deletions involving exons 3-7 produce a variable phenotype [5]? And as yet there is no satisfactory explanation for the very wide variation in clinical features associated with in-frame deletions involving the central rod region of dystrophin. Is it possible that such mutations could also be the cause of various muscle problems in later life and might conceiv- ably affect cardiac function in otherwise healthy individuals?

Clinical phenotype in Duehenne muscular dystrophy

Why calf pseudohypertrophy occurs in Duchenne (and Becker) muscular dystrophy but is uncommon in other forms of dystrophy, and in some cases of Duchenne muscular dystrophy may involve many different muscle groups as well, is not at all clear. Nor is there yet an explanation for certain other well-documented features of the disease, such as macroglossia, thymus hyper- plasia and hyperoestrogenaemia. Also, among the most fundamental questions which remain unanswered is why muscle groups become severely affected early on (e.g. quadriceps), others only later (e.g. soleus) and yet others never become clinically affected (e.g. extraocular). This might be related to fibre size, because as fibre size increases so the surface-to-volume ratio

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decreases, leading to a higher level of membrane stress during contraction [6]. Furthermore, though the disease is well defined, there is nevertheless variation in severity both within, but particularly between, families. It has been suggested that this variability might be related to the small amounts of muscle dystrophin found in some individuals and that these low concen- trations may actually have some functional significance [7]. This cannot be the entire story because even in those cases with no detectable muscle dystrophin, loss of ambulation varies considerably from age 6 to age 11 yr.

Brain and cardiac &volvement

Several different promoters associated with different dystrophin transcripts have now been identified [8, 9]. It is tempting to consider that defects of the brain promoter might account for mental impairment which can occur in the disease. But deletions of this promoter have been found in those with normal intellect. In any event, the impairment of intellect seems not to be global but particularly to affect verbal rather than perfor- mance IQ (for a review see [10]). Though molecular studies suggest involvement of the peripheral nerve and retina, there are no obvious clinical manifestations related to these structures.

Cardiac involvement also seems not to be entirely global but particularly affects the postero-lateral wall of the left ventricle. Since dystrophin is presumably absent throughout the myocardium this is difficult to explain. Perhaps this localized involvement may be a reflection of the longitudinal distribution of cardiac muscle fibres in the posterior wall of the left ventricle [11] and, thus, the forces are directed along this axis.

The relationship between heart involvement in Duchenne (and Becker) muscular dystrophy and a cardiac muscle dystrophin promoter is not yet entirely clear [12]. For example, Becker muscular dystrophy with dilated cardiomyopathy as the only feature of the disease has been reported with the same type of mutation in the same region (exons 45-53) of the dystrophin gene that occurs in typical Becker muscular dystrophy without major cardiac involvement. On the other hand, deletions of the muscle promoter and first exon of the dystrophin gene seem likely to be the cause of X-linked dilated cardiomyopathy where skeletal muscle dystrophin is normal [13, 14].

Progressive muscle weakness

Despite the fact that muscle dystrophin is defective even in the fetus, muscle weakness does not develop until childhood (or much later in

Becker muscular dystrophy) and is thereafter progressive. The reason for the delay in onset might be partly related to the increase in muscle fibre size which occurs during growth. But why weakness then becomes progressive is not so easily explained. There are a number of possibili- ties which are not necessarily mutually exclusive (for example, loss of muscle protein and enzymes, increased intracellular calcium, 'exhaustion' of the regenerative powers of satellite cells, and perhaps even increasing fibrosis). Answers to these fundamental questions may perhaps prove valuable in design- ing an effective therapeutic agent.

Manifesting carriers

Between 5 and 10% of female carriers of Duchenne muscular dystrophy have some degree of muscle weakness. This varies from very mild to quite marked and is often asymmetric. This is due to individual variations in random inactiva- tion of the X chromosome in carriers. The variation in muscle histology and muscle dystro- phin staining in these females is a reflection of the expression and 'domain' of the dystrophin gene in myonuclei.

It is puzzling that there should be no apparent correlation between muscle dystrophin and weakness [15] and, oddly, no apparent improve- ment in muscle power with age. In fact, most manifesting carriers become weaker over the years. Finally, no satisfactory explanation has yet been found for the fact that manifesting carriers are not infrequently familial (for a review see [16]).

CONCLUSIONS

Though the molecular and biochemical bases of Duchenne muscular dystrophy are known, many questions still remain unanswered. They range from the nature and cause of deletions of the gene to the relationship between dystrophin defects and the clinical phenotype, both in affected males as well as female carriers. No doubt answers to these fundamental questions may not only throw more light on the pathogene- sis of muscular dystrophy in general, but could perhaps also suggest novel approaches to treatment.

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