Discovering DMD – Identification & Distinction

Figure from Duchenne’s 1881 Treatise on the diseases of the nervous system, showing a child with what would become known as DMD. (Ref 23)

            The evolution of our knowledge and characterization of Duchenne Muscular Dystrophy (DMD) goes back nearly two centuries. Duchenne Muscular Dystrophy is named after Dr. Guillame Benjamin Duchenne of France, also known as Duchenne de Boulogne, a prominent physician in the late nineteenth century who published extensive works on this disease. Dr. Duchenne first reported DMD in an 1861 book cataloging a variety of illnesses, describing a patient suffering from muscle wasting. He termed this condition “Pseudo-hypertrophic paralysis”, based on the characteristic pseudohypertrophy of the calves he observed.1 He described the disease more extensively in a later publication in 1868.2,3 The disease was named after him for supposedly being the first to describe it. There is controversy, however, regarding who was truly responsible for first observing the disease. A British physician, Edward Meryon, published his own detailed clinical account of the illness over a decade earlier in 1851, identifying the same basic symptoms of muscle weakness, reduced ability to walk at a young age, and the conversion of some muscle tissue to fatty tissue and calling it the granular degradation of voluntary muscles.2 Impressively, Meryon also correctly observed a defect in the sarcolemma, which would not be well-understood for another century.4 Both would later claim that the other had described something else and that the credit for uncovering the disease should go to themselves. Scholars generally agree, however, that DMD or any of the related muscular dystrophy was first described in 1836 by Dr. Gaetano Conte of Italy, who described two brothers with enlarged calves and progressive muscular wasting.2

            From the time of Duchenne through the first half of the twentieth century, not much more was discovered about DMD. The relatively scarce literature during this period was, for the most part, limited to the recording of new cases and clinical descriptions of their symptoms. In the 1950s, the field began to move forward with early distinctions being made between different forms of muscular dystrophy as well as a slightly more advanced understanding of the genetic basis of DMD. In 1954, Walton and Natrass identified a form of muscular dystrophy termed Limb Girdle Muscular Dystrophy (LGMD), distinct from what would be known as Duchenne type.5 The following year, German doctor Peter Becker characterized another distinctive manifestation of muscular dystrophy, a form with much milder symptoms called Becker Muscular Dystrophy (BMD).6  By this period, the search for the gene responsible for DMD dominated the non-clinical literature. Scholars were aware that DMD was an X-linked recessive disorder from observations of numerous families and pedigrees in which it was prevalent.7 Decades of linkage analyses were unable to locate the gene responsible for it however, until nearly the 1980s.

            The 1980s saw the beginning of a massive leap in the knowledge of the genetic, cellular, and biomolecular basis of the disease. A number of studies in the late 1970s and early 1980s uncovered the locus of the DMD gene at Xp21.8,9 Two methods independently came to this conclusion, one through . In 1983, Davies et al. mapped the DMD gene locus on the short arm of the X chromosome using linkages to two RFLP markers flanking the locus.10 Other researchers found X-autosomal translocations in DMD females.11,12 In these cases, the breakage point of the X-chromosome was at the p21 band. Additionally, these translocations occurred on the active X chromosome, with the normal X chromosome remaining inactivated (hence the expression of DMD). With the location of the DMD gene found, the search then began to characterize the gene. In 1986, Monaco et al. isolated a cDNA clone containing segments of the human DMD transcript conserved between mice and humans using a deletion detecting locus, pERT87, as a base for sequencing by genome walking.13 In 1987, this sequence was used to isolate the full coding sequence of DMD.14

Locus of the DMD gene on short arm of the X chromosome, from NIH. (Ref 25)

… . … …..The most revolutionary discovery, however, would come in December of 1987, when researchers finally uncovered the biochemical defect responsible for the condition. In December of that year, Dr. Eric Hoffman et al. discovered and characterized the protein product of the DMD gene, dystrophin.15 This was a major advancement in the understanding of the pathology of muscular dystrophies – which now had a tangible underlying cause in dystrophin deficiency – as well as the cytology and histology of muscles in general. As described on the previous page, dystrophin links the cytoskeleton of muscle fibers to the extracellular matrix, via a transmembrane complex in the sarcolemma that provides stability and a scaffolding function. This system is critical to the strength of the muscle fibers and the correct localization and regulation of numerous signaling molecules. Dystrophin was localized to the sarcolemma in 1988.16

Domain map of the dystrophin protein, taken from Yilmaz and Sechtem (2012). In red, N-terminal actin-binding domain. Green: Spectrin-like repeats of rod domain. Yellow: Cys-rich domain. Purple: C-terminal domain. Described on first page. (Ref 24)

… . . … . …The dystrophin associated protein complex (DAPC) was discovered soon after the discovery of dystrophin. The DAPC was first observed in 1989 by showing that dystrophin is tightly bound to a membrane bound glycoprotein that interacts with wheat germ agglutinin (WGA)-Sepharose.17 It was then described in depth by Ervasti et al. in a series of papers in the early 1990s. In the first of these, Ervasti et al. identified four novel components of the DAPC, and showed that at least one of these is lost in the absence of dystrophin.18 In 1991, they identified six proteins constituting the complex by size: 156, 59, 50, 43, 35, and 25 kd. They  proposed that dystrophin associated with the 59 kd protein, linking it to the 50, 43, 35, and 25 kd transmembrane proteins, and finally to the extracellular 156 kd protein.19 Numerous studies took place throughout the rest of the 1990s and early 2000s further characterizing the DAPC, whose components are given on page 1.20

.. . . ………New knowledge of dystrophin and the DAPC enabled researchers to distinguish between the various known forms of muscular dystrophies for the first time. Monaco et al. showed that in DMD patients, the reading frame of the DMD gene is disrupted, while in BMD, it is maintained. Thus, he established the “Reading Frame Rule”, as a means of distinguishing the two phenotypes.21 In DMD, the gene undergoes a frameshift mutation, which completely prevents transcription of functional dystrophin. In BMD, which occurs less commonly, the reading frame is maintained after the mutation, which allows for the expression of a truncated, but still partially functional dystrophin protein, hence the far milder symptoms present in BMD. LGMD is now known to be a sizeable family of muscular dystrophies each caused by an autosomal mutation affecting various muscle proteins, including the dystroglycans and sarcoglycans of the DAPC.22

… . . ……..There is much more we can still learn about dystrophin and DMD in spite of the many advancements of recent decades. In the last two decades, we have come to further understand the pathology of DMD and have been narrowing down on promising genetic treatments.


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