Elsevier

The Lancet Neurology

Volume 2, Issue 12, December 2003, Pages 731-740
The Lancet Neurology

Review
Dystrophin and mutations: one gene, several proteins, multiple phenotypes

https://doi.org/10.1016/S1474-4422(03)00585-4Get rights and content

Summary

A large and complex gene on the X chromosome encodes dystrophin. Many mutations have been described in this gene, most of which affect the expression of the muscle isoform, the best-known protein product of this locus. These mutations result in the Duchenne and Becker muscular dystrophies (DMD and BMD). However, there are several other tissue specific isoforms of dystrophin, some exclusively or predominantly expressed in the brain or the retina. Mutations affecting the correct expression of these tissue-specific isoforms have been associated with the CNS involvement common in DMD. Rare mutations also account for the allelic disorder X-linked dilated cardiomyopathy, in which dystrophin expression or function is affected mostly or exclusively in the heart. Genotype definition of the dystrophin gene in patients with dystrophinopathies has taught us much about functionally important domains of the protein itself and has provided insights into several regulatory mechanisms governing the gene expression profile. Here, we focus on current understanding of the genotype–phenotype relation for mutations in the dystrophin gene and their implications for gene functions.

Section snippets

Long isoforms of dystrophin

Full-length dystrophin is a large rod-shaped protein with a molecular weight of 427 kDa that comprises four domains (figure 1). The amino-terminal domain has homology with α-actinin and contains between 232 and 240 amino-acid residues depending on the isoform. The central—rod—domain is a succession of 25 triple-helical repeats similar to spectrin and contains about 3000 residues. There is a cysteinerich domain of 280 residues. The last—carboxy-terminal—domain comprises 420 residues.15, 17, 18,

Short isoforms of dystrophin

The dystrophin gene also has at least four internal promoters that give rise to shorter dystrophin proteins lacking the actin-binding terminus but retaining the cysteine rich and carboxy-terminus domains that contain the binding sites for dystroglycan, dystrobrevin, and syntrophin. Each of these internal promoters uses a unique first exon that splices into exons 30, 45, 56, and 63 to generate protein products of 260 kDa (Dp260), 140 kDa (Dp140), 116 kDa (Dp116), and 71 kDa (Dp71). Dp260 is

Mutations in the dystrophin gene

The most common changes in dystrophin are intragenic deletions, which account for 65% of dystrophin mutations. Deletions and, more rarely, duplications can happen almost anywhere in the dystrophin gene; however, two deletion hotspots are known—one located towards the central part of the gene and the other towards the 5′ end. The former is the most commonly mutated region and includes exons 45–55 with genomic breakpoints (ie, the endpoint of where the deletions actually occurs) lying within

Disease severity: deletions, duplication and the frame-shift hypothesis

There is no simple relation between the size of the deletion and the resultant clinical disease. For example, the deletion of small exons, such as exon 44, typically results in classic DMD. However, large deletions, which may involve nearly 50% of the gene, have been described in patients with BMD.46, 47, 48 The central and distal rod domains seem to be almost dispensable functionally with some deletions in this region being associated with myalgia and muscle cramps, but not weakness. Some

Exceptions to the reading-frame hypothesis

Exceptions to the reading-frame hypothesis do exist and these include patients with BMD who carry frame-shift deletions or duplications and patients with DMD with inframe deletions or duplications.

Patients with BMD with frame-shift mutations

Patients with BMD with frame-shift mutations have several well-characterised deletions or duplications in the 5′ end of the gene (exons 3–7; 5–7; 3–6) or further downstream (exons 51, 49–50, 47–52, 44 or 45).55, 57, 58, 59, 60 The most common event that allows these patients to produce at least some dystrophin is exon skipping, which occurs via alternative splicing; the carboxy-terminus is always preserved in these patients.61 Diagnosis with a purely molecular genetic approach may be difficult

Exon-skipping events

The mechanisms that lead to exon skipping in patients with out-of-frame mutations are poorly understood and are likely to be caused by several factors. Exon-skipping events limited to a few fibres are routinely found in the dystrophin deficient mdx mice and in about 50% of children with DMD.62, 65, 66 The finding of revertant fibres (in which the mutations are suppressed and functional dystrophin is produced) in a patient is compatible with a diagnosis of DMD. However, several patients with

Patients with DMD with in-frame deletion mutations

Although there is a relation between disease phenotype and the ability to produce dystrophin, there are rare exceptions. This is particularly true for large deletions in the 5′ region that extend into the middle of the rod domain—such as deletions of exons 3–31, 3–25, 4–41, or 4–18.88 This contrasts with the mild BMD phenotype observed for large deletions that do not involve the 5′ principal putative actin binding site of dystrophin but instead remove regions of the rod domain.46, 47, 48, 89

Point mutations

20–35% of patients with DMD and BMD do not have deletions or duplications of the dystrophin gene.94 The identification of mutations in these patients has been hampered by the size of the dystrophin gene. One of the specialist techniques used is the protein truncation test, which can be applied to muscle RNA.95 A few studies have been done with this technique, which is currently offered by one accredited diagnostic laboratory in the UK (Guy's Hospital Clinical Genetic Centre, London).94, 96, 97,

X-linked dilated cardiomyopathy

Although cardiac involvement is invariably associated with DMD and BMD, rare mutations cause an almost exclusive cardiac involvement. The only hint of skeletal muscle involvement in these patients is the high concentrations of creatine kinase in the plasma;112 although one patient with X-linked dilated cardiomyopathy and normal creatine kinase concentrations is on record.113 Although X-linked dilated cardiomyopathy is a rare disorder, the analysis of the mutations that result in this phenotype

CNS involvement in DMD and BMD

Both the brain and the retina are affected by the lack of dystrophin. However, the range of abnormalities found is very significant. Some patients with DMD, for example, may have severe mental retardation and this, when present, is concordant in affected relatives, suggesting a primary role of the mutated dystrophin in that particular family. However, other individuals with DMD have entirely normal intellectual function.

There have been several studies of IQ in affected boys, the results of

Retinal involvement and dystrophin

Although affected boys have normal visual acuity, electroretinography has shown significant defects in several patients. Different dystrophin isoforms are expressed in the retina but do not overlap in their distribution, which suggests non-redundant different functional roles.138 The main retinal isoform is the Dp260,34, 138 which localises to the outer plexiform layer together with Dp427.138 In contrast, Dp71 is localised to the inner limiting membrane and to retinal blood vessels. Several

Conclusion

The dystrophin gene is a huge and fascinating gene with a complexity in transcriptional regulation, function, and protein–protein interactions that we are only beginning to fully appreciate. The relation between genotype and phenotype is particularly important not only to diagnostic and counselling viewpoints, but also to the understanding of the pathways and mechanisms that regulate expression. Improvements in knowledge about these features point the way towards a future treatment for this

Search strategy and selection criteria

Data for this review were identified by searches of MEDLINE with the search terms “dystrophin”, “gene”; “genomic organisation”, “mutations”, “gene expression”, “mental retardation”, “brain”, “dilated cardiomyopathy”, and “retina”. This search identified 942 articles. Orginal articles were selected wherever possible; subsequent papers publishede in high-impact-factor journals(eg, Naturegenetics, Human Molecular Genetics, AmericanJournal of Human Genetics) were also included. Important

References (149)

  • S White et al.

    Comprehensive detection of genomic duplications and deletions in the DMD gene, by use of multiplex amplifiable probe hybridization

    Am J Hum Genet

    (2002)
  • C Angelini et al.

    Prognostic factors in mild dystrophinopathies

    J Neural Sci

    (1996)
  • MA Melis et al.

    Elevation of serum creatine kinase as the only manifestation of an intragenic deletion of the dystrophin gene in three unrelated families

    Eur J Paediatr Neural

    (1998)
  • AP Monaco et al.

    An explanation for the phenotypic differences between patients bearing partial deletions of the DMD locus

    Genomics

    (1988)
  • K Arahata et al.

    Preservation of the C-terminus of dystrophin molecule in the skeletal muscle from Becker muscular dystrophy

    J Neural Sci

    (1991)
  • DR Love et al.

    Sequences of junction fragments in the deletion-prone region of the dystrophin gene

    Genomics

    (1991)
  • LA Blonden et al.

    242 breakpoints in the 200–kb deletion-prone P20 region of the DMD gene are widely spread

    Genomics

    (1991)
  • A Pizzuti et al.

    A transposon-like element in the deletion-prone region of the dystrophin gene

    Genomics

    (1992)
  • J Brown et al.

    Analysis of three deletion breakpoints in Xp21.1 and the further localization of RP3

    Genomics

    (1996)
  • L Toffolatti et al.

    Investigating the mechanism of chromosomal deletion: characterization of 39 deletion breakpoints in introns 47 and 48 of the human dystrophin gene

    Genomics

    (2002)
  • F Gualandi et al.

    Genomic definition of a pure intronic dystrophin deletion responsible for an XLDC splicing mutation: in vitro mimicking and antisense modulation of the splicing abnormality

    Gem

    (2003)
  • M Sironi et al.

    Trans-acting factors may cause dystrophin splicing misregulation in BMD skeletal muscles

    FEES Lett

    (2003)
  • SD Wilton et al.

    Specific removal of the nonsense mutation from the mdx dystrophin mRNA using antisense oligonucleotides

    Neuromuscul Disord

    (1999)
  • A Aartsma-Rus et al.

    Targeted exon skipping as a potential gene correction therapy for Duchenne muscular dystrophy

    Neuromuscul Disord

    (2002)
  • PA Roest et al.

    Protein truncation test (PTT) to rapidly screen the DMD gene for translation terminating mutations

    Neuromuscul Disord

    (1993)
  • LM Kunkel et al.

    Molecular genetics of Duchenne and Becker muscular dystrophy: emphasis on improved diagnosis

    Clin Chem

    (1989)
  • JL Mandel

    Dystrophin: the gene and its product

    Nature

    (1989)
  • E Manole

    The dystrophin gene and its product: a view

    Rom J Neural Psychiatry

    (1995)
  • RD Bies et al.

    Expression and localization of dystrophin in human cardiac Purkinje fibers

    Circulation

    (1992)
  • RD Bies et al.

    Human and murine dystrophin mRNA transcripts are differentially expressed during skeletal muscle, heart, and brain development

    Nucleic Acids Res

    (1992)
  • U Nudel et al.

    Duchenne muscular dystrophy gene product is not identical in muscle and brain

    Nature

    (1989)
  • DC Gorecki et al.

    Expression of four alternative dystrophin transcripts in brain regions regulated by different promoters

    Hum Mol Genet

    (1992)
  • F Muntoni et al.

    Transcription of the dystrophin gene in normal tissues and in skeletal muscle of a family with X-linked dilated cardiomyopathy

    Am J Hum Genet

    (1995)
  • D Yaffe et al.

    Multiple products of the Duchenne muscular dystrophy gene

    Symp Soc Exp Biol

    (1992)
  • H Nishio et al.

    Identification of a novel first exon in the human dystrophin gene and of a new promoter located more than 500 kb upstream of the nearest known promoter

    J Clin Invest

    (1994)
  • HM Sadoulet-Puccio et al.

    Dystrophin and its isoforms

    Brain Pathol

    (1996)
  • A Surono et al.

    Circular dystrophin RNAs consisting of exons that were skipped by alternative splicing

    Hum Mol Genet

    (1999)
  • RG Roberts

    Dystrophins and dystrobrevins

    Genome Biol

    (2001)
  • RD Cohn et al.

    Molecular basis of muscular dystrophies

    Muscle Nerve

    (2000)
  • V Straub et al.

    Muscular dystrophies and the dystrophin-glycoprotein complex

    Curr Opin Neural

    (1997)
  • KE Davies et al.

    Molecular analysis of Duchenne muscular dystrophy: past, present, and future

    Ann N Y Acad Sci

    (1995)
  • TA Rando

    The dystrophin-glycoprotein complex, cellular signaling, and the regulation of cell survival in the muscular dystrophies

    Muscle Nerve

    (2001)
  • KP Campbell et al.

    Association of dystrophin and an integral membrane glycoprotein

    Nature

    (1989)
  • KG Culligan et al.

    Role of dystrophin isoforms and associated proteins in muscular dystrophy (review)

    Int J Mol Med

    (1998)
  • M Michalak et al.

    Functions of dystrophin and dystrophin associated proteins

    Curr Opin Neural

    (1997)
  • KM Bushby

    The limb-girdle muscular dystrophies-multiple genes, multiple mechanisms

    Hum Mol Genet

    (1999)
  • BJ Petrof

    Molecular pathophysiology of myofiber injury in deficiencies of the dystrophin-glycoprotein complex

    Am J Phys Med Rehabil

    (2002)
  • R Madhavan et al.

    Calmodulin-activated phosphorylation of dystrophin

    Biochemistry

    (1994)
  • DM Fillers et al.

    Dystrophin expression in the human retina is required for normal function as defined by electroretinography

    Nat Genet

    (1993)
  • VN D'Souza et al.

    A novel dystrophin isoform is required for normal retinal electrophysiology

    Hum Mol Genet

    (1995)
  • Cited by (836)

    View all citing articles on Scopus
    View full text