Muscular Dystrophy, Duchenne Type

A number sign (#) is used with this entry because Duchenne muscular dystrophy is caused by mutation in the gene encoding dystrophin (DMD; 300377).

Dystrophin-associated muscular dystrophies range from the severe Duchenne muscular dystrophy (DMD) to the milder Becker muscular dystrophy (BMD; 300376). Mapping and molecular genetic studies indicate that both are the result of mutations in the huge gene that encodes dystrophin, also symbolized DMD. Approximately two-thirds of the mutations in both forms are deletions of one or many exons in the dystrophin gene. Although there is no clear correlation found between the extent of the deletion and the severity of the disorder, DMD deletions usually result in frameshift. Boland et al. (1996) studied a retrospective cohort of 33 male patients born between 1953 and 1983. The mean age at DMD diagnosis was 4.6 years; wheelchair dependency had a median age of 10 years; cardiac muscle failure developed in 15% of patients with a median age of 21.5 years; smooth muscle dysfunction in the digestive or urinary tract occurred in 21% and 6% of the patients, respectively, at a median age of 15 years. In this cohort, death occurred at a median age of 17 years. The authors commented that the diagnosis of DMD is being made at an earlier age but survival has not changed.

Skeletal Muscle

The most distinctive feature of Duchenne muscular dystrophy is a progressive proximal muscular dystrophy with characteristic pseudohypertrophy of the calves. The bulbar (extraocular) muscles are spared but the myocardium is affected. There is massive elevation of creatine kinase levels in the blood, myopathic changes by electromyography, and myofiber degeneration with fibrosis and fatty infiltration on muscle biopsy.The onset of Duchenne muscular dystrophy usually occurs before age 3 years, and the victim is chairridden by age 12 and dead by age 20. The onset of Becker muscular dystrophy is often in the 20s and 30s and survival to a relatively advanced age is frequent.

Moser and Emery (1974) found that some female heterozygotes had myopathy resembling autosomal recessive limb-girdle muscular dystrophy (253600). Serum creatine kinase was particularly elevated in these patients. In most populations, the frequency of manifesting heterozygotes is about the same as that of females with limb-girdle muscular dystrophy.

Soloway and Mudge (1979) remarked that patients with advanced muscular dystrophy may develop hypokalemia from insults (vomiting, diarrhea, diuretics) that would have little effect on normal persons. Reduced intracellular potassium stores are responsible for this perilous situation, which may be the mechanism of death.

In an Italian boy with congenital myopathy, born to nonconsanguineous parents, Prelle et al. (1992) found absence of dystrophin in the patient's muscle by immunohistochemical methods and a deletion of the 5-prime end of the dystrophin gene. Although the patient showed severe mental retardation, there was no cerebral atrophy. Cardiomyopathy was also present.

Frigeri et al. (1998) analyzed AQP4 expression in the skeletal muscle of mdx mice; immunofluorescence experiments showed a marked reduction of aquaporin-4 (AQP4; 600308) expression, suggesting a critical role in the membrane alteration of DMD.

Wakayama et al. (2002) analyzed skeletal muscle samples from 6 patients with DMD and found markedly reduced AQP4 expression by immunohistochemical staining and markedly decreased levels of AQP4 mRNA as measured by RT-PCR, compared to controls. Genomic analysis of the AQP4 gene revealed no abnormalities. The authors concluded that the reduced mRNA was due to either decreased transcription or increased degradation of the message.

Noguchi et al. (2003) performed cDNA microarray analysis on skeletal muscle biopsy specimens from 6 patients with DMD. There was increased expression of genes related to immune response, sarcomere, extracellular matrix proteins, and signaling or cell growth. Upregulation of these genes reflected dystrophic changes, myofiber necrosis, inflammation, and muscle regeneration. Genes related to muscle homeostasis and energy metabolism were downregulated.

Cardiac Muscle

Myocardial involvement appeared in a high percentage of DMD patients by about 6 years of age; it was present in 95% of cases by the last years of life. Severe cardiomyopathy did not develop before age 21 in BMD and few patients showed any cardiac signs before age 13 (Nigro et al., 1983).

Mirabella et al. (1993) noted that electrocardiographic abnormalities had been reported in 6.6 to 16.4% of DMD heterozygous females and that in one carrier female severe cardiomyopathy had been described in association with muscle weakness. They reported 2 carriers with dilated cardiomyopathy and increased serum CK but no symptoms of muscle weakness. Heart biopsies in both patients showed absence of dystrophin in many muscle fibers.

Smooth Muscle

Noting that in DMD functional impairment of smooth muscle in the gastrointestinal tract can cause acute gastric dilatation and intestinal pseudoobstruction that may be fatal, Barohn et al. (1988) studied gastric emptying in 11 patients with DMD. Strikingly delayed gastric emptying times were observed.

Enigmatically, the extraocular muscles (EOMs) remain clinically unaffected during the course of Duchenne muscular dystrophy (Kaminski et al., 1992). Khurana et al. (1995) showed that dystrophin deficiency does not result in myonecrosis or pathologically elevated levels of intracellular calcium in the EOMs. They reported in vitro experiments demonstrating that extraocular muscles are inherently more resistant to necrosis caused by pharmacologically elevated intracellular calcium levels when compared with pectoral musculature. They suggested that the EOMs are spared in DMD because of their intrinsic ability to maintain calcium homeostasis better than other striated muscle groups. This suggested further that modulating levels of intracellular calcium in muscle may be of potential therapeutic use in DMD.

Nervous System

Mental retardation of mild degree is a pleiotropic effect of the Duchenne gene (Zellweger and Niedermeyer, 1965). As indicated later, the finding of dystrophin mRNA in brain may bear a relationship to the mental retardation in DMD patients. Emery et al. (1979) sought heterogeneity in DMD as one explanation for the high birth incidence. Affected boys were categorized according to whether they had severe mental handicap or not. Those with severe mental defect had later age of onset and confinement to wheelchair, less marked fall in creatine kinase with age, and a greater urinary excretion of certain amino acids. In 50 DMD patients with a mean age of 11.1 years (range 3.5 to 20.3), Bresolin et al. (1994) found that 31% had a Wechsler full intelligence quotient (FIQ) lower than 75 and that only 24% had appropriate IQ levels by this index.

Bushby et al. (1995) examined the hypothesis that the nature of the dystrophin mutation may influence the development of mental retardation. Previously, it had been shown that deletions removing the brain-specific promoter were compatible with normal intelligence. Bushby et al. (1995) studied 74 boys with DMD, 18% of which had a full scale IQ of below 70. The authors found no significant IQ difference between the patients with promoter deletions and those without, nor did they find a relationship between the length of the deletion and full scale IQ. They found, however, that boys with distal deletions were more likely to be mentally retarded than were those with proximal deletions.

Retinal Function

Abnormal retinal neurotransmission as measured by electroretinography (ERG) was observed in boys with DMD by Cibis et al. (1993) and Pillers et al. (1993). Electroretinography is a recording of summed electrical signal produced by the retina when stimulated with a flash of light. The dark-adapted ERGs, recorded under scotopic testing conditions, have shown normal a-waves (a response of negative polarity generated by the photoreceptors) but reduced amplitude rod-isolated b-waves (a response of positive polarity originating primarily from the ON-bipolar cells) in DMD patients. This type of ERG abnormality with profound b-wave suppression is commonly associated with night blindness; however, there have been no reports of night blindness or any other visual abnormality in boys with DMD, and dark-adaptometry studies have been normal. Fitzgerald et al. (1994) used long-duration stimuli to separate ON (depolarizing bipolar cell) and OFF (hyperpolarizing bipolar cell) contributions to the cone-dominated ERG to understand better how the retina functions in boys with DMD. In the ERGs of 11 DMD boys, they found abnormal signal transmission at the level of the photoreceptor and ON-bipolar cell in both the rod and cone generated responses. Jensen et al. (1995) examined 16 boys with DMD/BMD of whom 10 had negative ERGs. Eight of the boys had DMD gene deletions downstream from exon 44. Normal dark adaptation thresholds were observed in all patients and there were no anomalous visual functions. Hence, negative ERG in DMD/BMD is not associated with eye disease. Normal ERGs were found in 6 boys with DMD/BMD. Jensen et al. (1995) speculated that a retinal or glial dystrophin may be truncated or absent in the boys with negative ERGs.

The ophthalmic features of DMD include normal ERG a-wave with reduced b-wave, normal visual acuity, and normal retinal morphology. Immunocytochemistry revealed strong AQP4 water channel expression in Muller cells in mouse retina and in fibrous astrocytes in optic nerve. Li et al. (2002) compared ERGs and retinal morphology in wildtype mice and transgenic knockout mice with no Aqp4. Significantly reduced ERG b-wave potentials were recorded in 10-month-old null mice with smaller changes in 1-month-old mice. Morphologic analysis of retina by light and electron microscopy showed no differences in retinal ultrastructure. That retinal function was mildly impaired in Aqp4-null mice suggested a role for Aqp4 in Muller cell fluid balance. The authors suggested that AQP4 expression in supportive cells in the nervous system facilitated neural signal transduction in nearby electrically excitable cells.

Costa et al. (2007) evaluated color vision in 44 patients with Duchenne muscular dystrophy using 4 different color tests. Patients were divided into 2 groups according to the region of deletion in the dystrophin gene: 12 patients had deletion upstream of exon 30, and 32 downstream of exon 30. Of the patients with DMD, 47% (21/44) had a red-green color vision defect. Of the patients with deletion downstream of exon 30, 66% had a red-green color defect. No color defect was found in the patients with a deletion upstream of exon 30. A negative correlation between the color thresholds and age was found for the controls and patients with DMD, suggesting a nonprogressive color defect. The percentage (66%) of patients with red-green defect was significantly higher than the expected value (less than 10%) for the normal male population (P less than 0.001). Costa et al. (2007) suggested that the findings might be partially explained by a retinal impairment related to dystrophin isoform Dp260.

Carrier Females

In a 9-year follow-up of study of 99 Dutch female carriers of DMD or BMD mutations, Schade van Westrum et al. (2011) found that 11 carriers (10%) (10 DMD and 1 BMD) fulfilled the criteria for dilated cardiomyopathy (DCM). Nine of the patients had developed DCM during the follow-up period. These carriers were on average older, were more symptomatic, and more often had hypertension, exertional dyspnea, and chest pain compared to mutation carriers without DCM. The findings suggested that female carriers of a mutation can develop progressive cardiac abnormalities and should undergo routine cardiac evaluation, preferably by echocardiology.

Mercier et al. (2013) reviewed the features of 26 female carriers of pathogenic mutations in the DMD gene who were referred for symptoms related to the disorder before 17 years of age. Five had a Duchenne-like phenotype with loss of ambulation before age 15 years, 13 had a Becker-like phenotype with muscle weakness but persistence of ambulation after age 15 years, and 8 had exercise intolerance. Initial symptoms included significant muscle weakness (88%), mostly affecting the lower limbs, or exercise intolerance (27%). Cardiac dysfunction was present in 19%, and cognitive impairment in 27%. Cognitive impairment was associated with mutations in the distal part of the gene. Muscle biopsy showed dystrophic changes in 83% and mosaic immunostaining for dystrophin in 81%. The X-chromosome inactivation pattern was biased in 62% of cases. Mercier et al. (2013) concluded that carrier females may have significant symptoms of the disorder.

Blau et al. (1983) suggested that the defect in DMD is intrinsic to the undifferentiated myoblast. This was based on the observation that the number of viable myoblasts obtained per gram DMD muscle tissue was greatly reduced and those that grew in culture had a decreased proliferative capacity and aberrant morphology. The hypothesis was tested by determining whether the myoblast defect was X-linked. Webster et al. (1986) obtained muscle cells from 5 females heterozygous for both DMD and G6PD (305900). In a total of 1,355 muscle clones, although the proportion of defective clones was increased, the cellular defect did not consistently segregate with a single G6PD phenotype in the myoblast clones from any individual. The hypothesis that the DMD gene is expressed in skeletal myoblasts and limits proliferation, was further tested by Hurko et al. (1987) established primary muscle culture from a female who was heterozygous for both DMD and G6PD. Both cloned and mass cultures were grown until senescence and the G6PD phenotype was scored. Myoblasts expressing the 2 different alleles at the G6PD locus did not differ in proliferative capacity, suggesting that expression of the Duchenne gene does not result in a decrease in proliferative capacity of the myoblasts. Thus, the hypothesis of Blau et al. (1983) was disproved.

Baricordi et al. (1989) studied the capping phenomenon in lymphoblastoid cell lines and found that they retain an impairment of capping of the type seen in nontransformed lymphocytes (Verrill et al., 1977). This was taken to mean that the capping impairment is an intrinsic cellular defect in DMD and not a phenomenon secondary to progression or activity of the disease. Further, it may indicate that there is a generalized membrane disorder in this condition.

Haslett et al. (2002) used expression microarrays to compare individual gene expression profiles of skeletal muscle biopsies from 12 DMD patients with those of 12 unaffected control patients. They identified 105 genes that differed significantly in expression levels between unaffected and DMD muscle. Many of the differentially expressed genes reflected changes in histologic pathology; e.g., immune response signals and extracellular matrix genes were overexpressed in DMD muscle, indicating the infiltration of inflammatory cells and connective tissue. Significantly more genes were overexpressed than were underexpressed in dystrophic muscle, with dystrophin underexpressed, whereas other genes encoding muscle structure and regeneration processes were overexpressed, reflecting the regenerative nature of the disease.

Straub et al. (2002) found impaired expression of muscle membrane-associated neuronal nitric oxide synthase (NOS1; 163731) in Duchenne patients; mean exhaled nitric oxide was significantly reduced in 13 males with DMD compared to 11 age-matched and 17 adult controls.

In muscle biopsy samples from 13 of 16 DMD patients, Kleopa et al. (2006) observed an age-dependent increase in utrophin (UTRN; 128240) staining, resulting in a mean increase of 11-fold compared to that found in normal adult tissue. In disease tissue, utrophin was present along the whole circumference of the sarcolemma, whereas it was present only along vessels and nerve endings in controls. Expression of utrophin in disease tissue showed a positive correlation with age at wheelchair-dependency in DMD, suggesting that utrophin expression has an ameliorating effect on the severity of DMD.

Patients with DMD have increased blood loss during spinal surgery compared to non-DMD patients. In Duchenne patients, Labarque et al. (2008) found decreased expression of the dystrophin isoforms Dp71 and Dp116 in platelets and skin fibroblasts, respectively, compared to controls. Decreased expression of these isoforms was associated with increased Gs (see, e.g., GNAS; 139320) signaling and activity upon stimulation. Functional studies showed that DMD platelets had slower aggregation in response to collagen with extensive shape changes and reduced platelet adhesion under flow conditions. Platelet membrane receptors were normal. The decreased collagen activation was shown to result from both Gs activation and cytoskeletal disruption. Overall, the findings suggested that DMD platelets have a disorganized cytoskeleton due to dysfunctional dystrophin Dp71, and also manifest Gs hyperactivity with reduced platelet collagen reactivity, which may result in increased bleeding during surgery.

The Haldane rule (Haldane, 1935) predicts that one-third of cases of a genetic lethal X-linked recessive will be the consequence of new mutation. Haldane (1956) further suggested that the mutation rate for Duchenne muscular dystrophy might be higher in males. Such would result in a lower proportion of cases being new mutants. Caskey et al. (1980) concluded that in their series cases resulting from new mutation approached closely the theoretically expected one-third. Ionasescu et al. (1980) concluded that measures of ribosomal protein synthesis, analyzed by discriminant function, identify 95% of proved and presumptive DMD carriers. Bucher et al. (1980) used this measure to test the Haldane rule. They found that only 9 (16.4%) of 55 mothers were noncarriers. When only the mothers of isolated cases were studied, 23.1% (9 of 39) were classified as noncarriers. They felt that a higher male than female mutation rate was the cause of the discrepancy.

In a study of 514 probands who constituted two-thirds of the known cases in Japan, Yasuda and Kondo (1982) could not demonstrate an effect of maternal grandfather's age at birth of the proband's mother. They pointed out that the data relevant to a maternal grandfather age effect in hemophilia A are conflicting, just as the data for DMD are inconsistent with those of Bucher et al. (1980). Examining the frequency of affected boys among the next-born male sibs of 37 initially 'sporadic' cases of DMD, Lane et al. (1983) found that the frequency was significantly greater than predicted by the Haldane theory (p = 0.029). The estimated proportion of new mutant cases in the combined clinic population of 106 families was 0.127 (SE = 0.111). They proposed that the absence of affected males in earlier generations in families of isolated cases may be explained in part by a high ratio of male to female stillbirths and infant deaths which in this study was more than 3 times that in the general population. (Note that there is at least one other 'Haldane's rule' (Haldane, 1922): 'When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous, heterogametic or XY sex.' See discussion of Orr (1993).)

Danieli and Barbujani (1984) concluded that the proportion of sporadic cases was 0.227 +/- 0.048 in an Italian series of 135 families combined with other sets of data. In a segregation analysis of 1,885 DMD families, Barbujani et al. (1990) arrived at an estimate of sporadic cases of 0.229, a significant deviation from the expected 0.333 based on mutation-selection equilibrium. They mentioned the previously discussed possible explanations for the finding, such as sex differences in mutation rate, and added a new one, namely, the occurrence of multiple DMD cases in the same sibship as a consequence of mutational mosaicism of the maternal germ cells, a phenomenon documented in a number of instances.

As might perhaps have been anticipated, a report appeared concerning a man with DMD who had fathered 2 children, a normal son and a carrier daughter (Thompson, 1978).

By analysis of Xp21 DNA markers in a family with 2 affected brothers, Borresen et al. (1987) demonstrated that the mutation had most likely occurred in a grandpaternal sperm. Therefore, barring gonadal mosaicism, it is unlikely that the maternal aunts and their daughters are carriers of the DMD gene.

Miciak et al. (1992) studied 3 boys with DMD, 2 of whom were related as first cousins and the third as a second cousin, all being related through males. They demonstrated that the molecular defect was different in each and speculated about instability of the DMD gene and possible involvement of transposons. They referred to similar observations by Zatz et al. (1991) in 4 Brazilian families. Vitiello et al. (1992) found no instance of mutation in the muscle promoter region of the DMD gene in a series of 115 unrelated DMD and BMD patients from northeast Italy. In 3 cases in which dystrophin of normal size was expressed at low levels, the DNA sequence of the promoter region showed no abnormality.

Gonadal Mosaicism

A possible example of gonadal mosaicism for the DMD locus was discussed by Wood and McGillivray (1988), who described a family in which a female ancestor of an individual with Duchenne muscular dystrophy seemed to have transmitted 3 distinct types of X chromosome to her offspring, as indicated by RFLP analysis. The authors postulated that in this individual the mutation arose as a postzygotic deletion, resulting in germinal mosaicism.

Witkowski (1992) suggested another explanation for those cases in which gonadal mosaicism has been suspected: such a female may represent a chimera that has originated from 2 fertilized eggs, one carrying the mutation. This, of course, has quite different implications regarding the risk that a maternal aunt of the proband is a carrier. Melis et al. (1993) reported a 3-generation family in which 2 sibs were affected with DMD. Immunohistochemical analysis of muscle dystrophin and haplotype analysis of the DMD locus demonstrated that the X chromosome carrying the DMD gene was transmitted from the healthy maternal grandfather to his 3 daughters, including the proband's mother. The definition of carrier status in 2 possible carriers permitted accurate genetic counseling and the prevention of the birth of an affected boy.

Witkowski (1992) presented the pedigree of a family with a balanced autosomal translocation in 3 generations: a son of a carrier exhibited lymphocytes with a normal karyotype as well as lymphocytes with the balanced translocation. She also cited the 47,XXX karyotype as a possible alternative explanation to germline mosaicism; there are known sibships in which boys have received 3 different haplotypes on the X chromosome from the mother. Unexpectedly, Passos-Bueno et al. (1992) observed that among 24 proven germline mosaic cases, 19 (79%) had a proximal mutation, while only 5 (21%) had a distal mutation.

Somatic Mosaicism and Heterozygous Females

Yoshioka (1981) observed unusually severely affected heterozygous females and suggested that factor(s) other than lyonization may be involved. One of the women was the product of a consanguineous mating, suggesting modification of expression by homozygosity at an autosomal locus.

Burn et al. (1986) reported monozygotic twin girls, one of whom had typical clinical features of DMD despite a normal female karyotype and the second of whom was normal. Burn et al. (1986) proposed that differences in lyonization accounted for the findings. Hybridization of fibroblasts from each twin with RAG-mouse cell line deficient in HPRT showed that in the affected twin it was the mother's X chromosome that was predominantly the active one, whereas in the normal twin it was the father's. In female monozygotic twins discordant for muscular dystrophy, Richards et al. (1990) showed that there was a mutation in dystrophin in both twins. Uniparental disomy and chromosome abnormality were excluded, but on the basis of methylation differences of the paternal and maternal X chromosomes, Richards et al. (1990) concluded that uneven lyonization was the underlying mechanism for disease expression in the affected female.

Lupski et al. (1991) pointed out that discordance of the DMD phenotype had never been described in male monozygotic twins. Lupski et al. (1991) described female monozygotic twins who carried the same mutation involving duplication of exons 42 and 43 of the DMD gene. One was a manifesting heterozygote, whereas the other was normal. Unlike the study of Richards et al. (1990) in which the skewed inactivation pattern was symmetrical in opposite directions, one twin being affected with DMD and the other being normal, the skew in this case involved only the affected twin, while the normal twin showed a random X-inactivation pattern. They suggested that the result was consistent with the model of twinning and X-inactivation proposed by Nance (1990) in that these twins probably represented asymmetric splitting of the inner cell mass (ICM): the affected twin probably arose when a small proportion of the ICM split off after lyonization had occurred. In this situation, the original ICM could have given rise to the normal twin with random lyonization, while the newly split cells would experience catch-up growth and lead to the affected twin.

Many DMD patients have rare staining dystrophin-positive fibers. The possibility of somatic mosaicism can be raised, but somatic reversion/suppression is another possibility. Indeed, the dystrophin-positive fibers have been referred to as 'revertants.' The revertants are found in both familial and nonfamilial cases. Klein et al. (1992) found that in patients with deletions, revertants did not stain with antibodies raised to polypeptide sequences within the deletion. These results indicated that positively stained fibers were not the result of somatic mosaicism in deletion patients. Klein et al. (1992) concluded that the most likely mechanism giving rise to positively staining fibers is a second site in-frame deletion. Thanh et al. (1995) used exon-specific monoclonal antibodies to determine which exons are removed in order to correct the reading frame in individual revertant muscle fibers. They showed that 15 revertant fibers in a DMD patient with a frameshift deletion of exon 45 had correction of the frameshift by the additional deletion of exon 44 (or perhaps exon 46 in some fibers) from the dystrophin mRNA, but not by larger deletions. This result was consistent with RT-PCR and sequencing of a minor dystrophin mRNA with an exon 43/46 junction in the biopsy. The results were consistent with somatic mutations in revertant-fiber nuclei, which result in removal of additional exons from dystrophin mRNA. These data did not clearly distinguish between additional somatic deletions and somatic effects on dystrophin mRNA splicing, however, and both mechanisms may be operating.

Pena et al. (1987) reported an extraordinary case of DMD leading to death at age 28 years in a heterozygous monozygotic female twin. Her sister was clinically normal but had an affected son. Eleven affected males in 3 generations and 7 separate sibships of the kindred were known. An undetected monozygotic twinning event was proposed by Glass et al. (1992) to explain a manifesting female for Becker muscular dystrophy. They concluded that females heterozygous for BMD have less likelihood of showing manifestations of muscular dystrophy than do females heterozygous for DMD. Abbadi et al. (1994) reported a pair of female monozygotic twins heterozygous for a deletion in the DMD gene and discordant for the clinical manifestations of the disorder. Results in lymphocytes and skin fibroblast cell lines suggested a partial mirror inactivation with the normal X chromosome preferentially active in the unaffected twin, and the maternally deleted X chromosome preferentially active in the affected twin.

Pegoraro et al. (1994) studied 13 female dystrophinopathy patients--10 isolated cases and 3 with a positive family history for DMD in males. All 13 had skewed X-inactivation patterns in peripheral blood DNA. Of the 9 isolated cases informative in their assay, 8 showed inheritance of the dystrophin gene mutation from the paternal germline. Only a single case showed maternal inheritance. Pegoraro et al. (1994) estimated that the 10-fold higher incidence of paternal transmission of dystrophin gene mutations in these cases is at 30-fold variance with Bayesian predictions and gene mutation rates. Thus they suggested that there is some mechanistic interaction between new dystrophin gene mutations, paternal inheritance, and skewed X inactivation.

Chelly et al. (1986) reported the first observation of a girl with typical DMD and typical 45,XO Turner syndrome. The one X chromosome in the girl was normal by high resolution banding, but DNA analysis by Southern blotting and hybridization with 7 cloned probes mapping in the Xp21 region showed a deletion of 3 of the probes. In this case, the paternal chromosome was lost and the maternal X chromosome suffered a deletion mutation in the Xp21.2 region. Suthers et al. (1989) described a man with Becker muscular dystrophy and the Klinefelter syndrome who was much more mildly affected than his 3 nephews. The mild expression may be due to the fact that he was heterozygous for the muscular dystrophy mutation. The nephews indeed may have had Duchenne muscular dystrophy.

Among 35 children produced by 34 deliveries in 13 women who were mothers of males attending a muscular dystrophy clinic, Geifman-Holtzman et al. (1997) found that 6 (17%) were delivered in the breech position, which is a 5-fold increase above the national standards for term pregnancies. Of the 6 infants with breech presentation, 2 were males affected with DMD, 1 was a female heterozygote, 1 was a male who died perinatally, and the carrier status of the other 2 females was unknown. Most DMD affected males (12/14) were delivered in the vertex position. Thus, the authors concluded that maternal rather than fetal muscle weakness was the significant factor in determining fetal position at term. They suggested that subtle changes in uterine or pelvic girdle muscle tone may contribute to a higher rate of fetal breech presentation in carriers of the DMD gene.

Yoshioka et al. (1998) analyzed X inactivation in 4 manifesting heterozygotes, 5 asymptomatic carriers, and 32 female controls. Ninety-two percent were heterozygous for the CAG repeat in the androgen receptor (AR; 313700) gene. All manifesting carriers showed 70 to 93% skewed inactivation, whereas the asymptomatic carriers showed random inactivation (50-60%). Of the control females, 6% showed greater than 70% skewed inactivation.

Reported genetic mechanisms for female DMD include (1) a skewed pattern of X-chromosome inactivation in female carriers of a DMD mutation (Azofeifa et al., 1995); (2) X;autosome translocations that disrupt the DMD gene (Cantagrel et al., 2004); (3) monosomy X, or Turner syndrome, associated with a DMD mutation in the remaining X chromosome (Chelly et al., 1986); and (4) maternal isodisomy for the X chromosome carrying a DMD mutation (Quan et al., 1997). Katayama et al. (2006) reported a fifth mechanism in a Vietnamese child with DMD confirmed by genetic analysis. Although the child was phenotypically female, the karyotype showed 46,XY, and she was found to have a mutation in the AR gene causing androgen insensitivity syndrome (AIS; 300068). The patient's sister also had the AR mutation and AIS, but did not have the DMD mutation. The unaffected mother was found to be heterozygous for the AR mutation, but did not have the DMD mutation, indicating it was de novo in the proband. Katayama et al. (2006) concluded that the cooccurrence of independent mutations in both the DMD and AR genes constituted a fifth mechanism underlying female DMD.

Rajakulendran et al. (2010) reported 2 unrelated female carriers of DMD mutations who presented in adulthood with marked right-sided hemiatrophy and weakness of the arm and leg muscles. MRI showed muscle atrophy and fatty replacement on the affected side, and histologic studies showed decreased dystrophin staining. Both had increased serum creatine kinase. The older woman had areflexia of the affected side, no family history of muscular dystrophy, and showed skewed ratio of X inactivation in lymphocytes. The younger woman had an affected son and showed normal X inactivation in lymphocytes. Rajakulendran et al. (2010) suggested that a combination of skewed X inactivation in muscle tissue and somatic mosaicism accounted for the marked asymmetric manifestations.

Greenstein et al. (1977) found DMD in a 16-year-old girl with a reciprocal X;11 translocation. The mother was thought not to be a carrier. Possibly the break at Xp21 caused a null mutation; the normal X chromosome was inactivated. Verellen et al. (1978) reported the same situation with X;21 translocation and break at Xp21. Canki et al. (1979) described similar findings in a girl with X;3 translocation with break at Xp21. The mother was thought to be heterozygous.

Zneimer et al. (1993) used a combination of conventional and molecular cytogenetic techniques to investigate the twins first reported by Richards et al. (1990). The twins carried a deletion of approximately 300 kb within the dystrophin gene on one X chromosome. A unique DNA fragment generated from an exon within the deletion was hybridized in situ to metaphase chromosomes of both twins, a probe that would presumably hybridize only to the normal X chromosome and not to the X chromosome carrying the deletion. The chromosomes were identified by reverse-banding (R-banding) and by the addition of 5-bromodeoxyuridine in culture to distinguish early and late replicating X chromosomes, corresponding to active and inactive X chromosomes, respectively. The experiment showed predominant inactivation of the normal X chromosome in the twin with DMD. With an improved method of high resolution R-banding, Werner and Spiegler (1988) showed deletion of Xp21.13 in an 8-year-old boy with normal intelligence and no disorder other than DMD. His healthy mother was heterozygous for the deletion, which was subject to random X inactivation in lymphocytes.

Saito-Ohara et al. (2002) studied a 16-year-old patient with Duchenne muscular dystrophy, profound mental retardation, athetosis, and nystagmus who was shown to have a pericentric inversion of the X chromosome, 46,Y,inv(X)(p21.2q22.2). His mother carried this inversion on one allele. The patient's condition was originally misdiagnosed as cerebral palsy. Because the DMD gene is located at Xp21.2, which is one breakpoint of the inv(X), and because its defects are rarely associated with severe mental retardation, the other clinical features of this patient were deemed likely to be associated with the opposite breakpoint at Xq22. The molecular-cytogenetic characterization of both breakpoints revealed 3 genetic events that probably had disastrous influence on neuromuscular and cognitive development: deletion of part of the DMD gene at Xp21.2, duplication of the proteolipid protein gene (PLP1; 300401) at Xq22.2, and disruption of the RAB40AL gene (300405). Saito-Ohara et al. (2002) speculated that disruption of RAB40AL was responsible for the patient's profound mental retardation.

Tran et al. (2013) reported a 3-year-old Japanese boy with Duchenne muscular dystrophy and moderate mental retardation associated with an intrachromosomal inversion, inv(X)(p21.2;q28), involving both the dystrophin and the KUCG1 (300892) genes. KUCG1 is a long noncoding RNA that shows brain expression. The first exon of KUCG1 was spliced to a dislocated part of the dystrophin gene, producing a chimeric dystrophin transcript. Brain MRI in the patient was normal. Tran et al. (2013) hypothesized that interruption of the KUCG1 gene may have contributed to mental retardation in this patient. However, sequencing of the KUCG1 gene in 10 additional Japanese families with X-linked mental retardation did not identify any mutations.

Duchenne muscular dystrophy is not linked to colorblindness or G6PD (Emery et al., 1969; Zatz et al., 1974). No linkage with Xg has been found; total lod scores were -14.6 and -2.4 for theta of 0.10 and 0.30, respectively (Race and Sanger, 1975).

Lindenbaum et al. (1979) found DMD with X-1 translocation and suggested that the DMD locus is at Xp1106 or Xp2107. A number of females with X-autosome translocations with the breakpoint in the Xp21 band have shown Duchenne muscular dystrophy. One interpretation is that the gene locus is in that region and that the locus on the normal X is inactivated. Murray et al. (1982) found linkage of DMD with a restriction enzyme polymorphism at a distance of about 10 cM. The cloned DNA sequence bearing the polymorphism (lambda RC8) was assigned to Xp22.3-p21 by study of somatic cell hybrids. Spowart et al. (1982) outlined reasons for doubting the location of the DMD gene at Xp21.

Wieacker et al. (1983) studied the linkage between the restriction fragment length polymorphism defined by the cloned DNA sequence RC8 and X-linked ichthyosis. At least 2 crossovers were found among 9 meioses in an informative family, suggesting that RC8 and STS may be about 25 cM apart. Since STS is 15 cM proximal to the Xg locus and since the RC8 and Duchenne muscular dystrophy are closely linked, DMD may be 50 cM or more from Xg. Worton et al. (1984) studied a female with DMD and an X;21 translocation which split the block of genes encoding ribosomal RNA on 21p. Thus, ribosomal RNA gene probes can be used to identify a junction fragment from the translocation site and to clone segments of the X at or near the DMD locus.

Kingston et al. (1983, 1984) found linkage of BMD with the cloned sequence L1.28 (designated DXS7 by the seventh Human Gene Mapping Workshop in Los Angeles; D = DNA, X = X chromosome, S = segment, 7 = sequence of delineation). The interval was estimated to be about 16 cM, which is also the approximate interval between DXS7 and DMD. DXS7 is located between Xp11.0 and Xp11.3. Thus, these 2 forms of X-linked muscular dystrophy appeared to be allelic, a possibility also supported by the finding of both severe and mild disease (Duchenne and Becker, if you will) in females with X-autosome translocations. Contrary to reports of others, Kingston et al. (1984) found no evidence of linkage of BMD to colorblindness; Xg also showed no linkage.

Francke et al. (1985) studied a male patient with 3 X-linked disorders: chronic granulomatous disease with cytochrome b(-245) deficiency and McLeod red cell phenotype, Duchenne muscular dystrophy, and retinitis pigmentosa (see RP3, 300029). A very subtle interstitial deletion of part of Xp21 was demonstrated as the presumed basis of this 'contiguous gene syndrome.' That this was a deletion and not a translocation was demonstrated by the absence of 1 DNA probe from the genome of the patient. Since this probe (called 754) was clearly very close to DMD and recognized a RFLP of high frequency, it proved highly useful for linkage studies of DMD. The close clustering of CGD, DMD, and RP suggested by these findings was inconsistent with separate linkage data, which indicated that McLeod and CGD were close to Xg and that DMD and RP are far away (perhaps at least 55 cM) and as much as 15 cM from each other. At least 4 possible explanations of the discrepancy were proposed by Francke et al. (1985). One suggestion was that the deletion contained a single defect affecting perhaps a cell membrane component with the several disorders following thereon.

Mulley et al. (1988) reported the recombination frequencies between DMD and intragenic markers from 8 informative families containing 30 informative meioses. No recombinants were observed. The authors commented that the average theta between intragenic markers and DMD may be 1 to 2%. Grimm et al.(1989) reported a recombination rate of 4% between 2 subclones of the DNA segment DXS164 within the dystrophin locus, indicating a hotspot for recombination.

Tuffery-Giraud et al. (2009) described a French database for mutations in the DMD gene that includes 2,411 entries consisting of 2,084 independent mutation events identified in 2,046 male patients and 38 expressing females. This corresponds to an estimated frequency of 39 per million with a genetic diagnosis of a 'dystrophinopathy' in France. Mutations in the database include 1,404 large deletions, 215 large duplications, and 465 small rearrangements, of which 39.8% are nonsense mutations. About 24% of the mutations are de novo events. The true frequency of BMD in France was found to be almost half (43%) that of DMD.

Among 624 index cases evaluated for DMD mutations, Oshima et al. (2009) reported that a genomic rearrangement was detected in 238 (38.1%) samples. Deletions were detected in 188 (79.0%), and included 31 cases with single-exon deletions and 157 cases with multi-exonic deletions. Most of the deletions fell between exons 45 and 52 and between exons 8 and 13 of the gene. Duplications were detected in 44 (18.5%) cases, of which 12 involved single exons and 32 multiple exons. Complex rearrangements were detected in 6 (2.5%) cases. The remaining 386 cases showed normal results. Oshima et al. (2009) selected 15 unique rearrangement, of which none shared a common breakpoint, and used array CGH and MLPA analyses to evaluate the mechanism rearrangements. Fourteen of the deletions had microhomology and small insertions at the breakpoints, consistent with a mechanism of nonhomologous end joining (NHEJ) after DNA damage and repair. Analysis of 3 complex intragenic DMD gene rearrangements identified several features that could result in genomic instability, including breakpoints that aligned with repetitive sequences, an inversion/deletion involving a stem-loop structure, replication-dependent fork stalling and template switching (FoSTeS), and duplications causing secondary deletions.

Modifier Genes

Pegoraro et al. (2011) examined 106 DMD patients for variations in 29 genes selected as candidate modifiers of disease severity. Skeletal muscle mRNA profiling identified the G allele of rs28357094 in the promoter of the SPP1 gene (166490), which encodes osteopontin, as having a significant effect on both disease progression and response to glucocorticoids. In an autosomal dominant model, carriers of the G allele (35% of subjects) had more rapid progression and 12 to 19% less grip strength. The association was validated in a second cohort of 156 patients.

Symptomatic Hemizygotes

Clinical diagnosis of males affected with DMD is straightforward. Gait difficulty beginning at age three, progressive myopathic weakness with pseudohypertrophy of calves and massive elevations of serum levels of creatine kinase permit diagnosis. Electromyography and muscle biopsy are confirmatory. Inflammatory changes seen in biopsies taken early in the course of the disorder can erroneously suggest a diagnosis of polymyositis if careful note is not made of the histologic hallmarks of dystrophy.

Heyck et al. (1966) documented a high level of CPK (and other enzymes) in a 9-day-old infant from a family at risk. According to Dubowitz (1976), elevation in cord blood in a proven case had not been documented. Furthermore, many perinatal factors seem to cause elevation of CPK. Mahoney et al. (1977) demonstrated elevated CPK in fetal blood obtained by placental puncture and validated this as a method of prenatal diagnosis by demonstrating histologic changes in the skeletal muscle of the aborted fetus.

Darras et al. (1987) reported experience suggesting that despite the large number of intragenic and flanking DNA polymorphisms then available, uncertainties often remain in the prenatal diagnosis of DMD.

Bartlett et al. (1988) pointed out that mapping of deletions is a more reliable and an easier way to do prenatal diagnosis and carrier detection than by use of RFLPs. They suggested that once the entire gene is available for screening, most DMD boys will show deletions. Katayama et al. (1988) demonstrated the usefulness of RFLPs in prenatal diagnosis and carrier detection of DMD. In some of the examples cited, the authors made use of creatine phosphokinase levels as well. Speer et al. (1989) reviewed the status of prenatal diagnosis and carrier detection using cDNA probes. Clemens et al. (1991) took advantage of the existence of approximately 50,000-100,000 (CA)n loci in the human genome (Tautz and Renz, 1984) for carrier detection and prenatal diagnosis in DMD and BMD. (CA)n loci are a subclass of all short tandem repeat (STR) sequences. Because they are frequently polymorphic, so-called pSTR, they are useful for linkage purposes and are readily studied by PCR.

Bieber et al. (1989) described the use of immunoblotting for dystrophin analysis in the diagnosis of DMD in cases in which a gene deletion