Duchenne Muscular Dystrophy Mutation Types

Duchenne Muscular Dystrophy (DMD) is caused by mutations, changes in the DNA, within the dystrophin gene. DNA is a chain made up of four types of bases joined together, A (adenosine), C (cytosine), G (guanine) and T (thymine). The order of the DNA bases, or DNA sequence, specifies how the protein is made. Genes serve as instructions for making proteins in the body. DMD mutations are like mistakes in the instructions for making dystrophin protein, which is why the bodies of people with DMD cannot make any fully functional dystrophin protein. Genetic testing is an important part of DMD diagnosis and for developing the best treatment plan for a child with DMD.

In muscle, dystrophin protein performs the critical job of anchoring part of the muscle contraction machinery to the membrane that surrounds the muscle fiber. Like the springs around the edge of a trampoline, dystrophin absorbs shock from muscle contractions. Without dystrophin, muscles become damaged and weakened because they cannot withstand the force of contractions that occur with muscle use.

Individuals with DMD generally do not have any functional dystrophin. The severity of disease symptoms depends partially on the type of dystrophin mutation present. Becker muscular dystrophy (BMD) is a less severe form of muscular dystrophy that occurs when partially functional dystrophin is present. Knowing the type of mutation will also determine whether certain mutation-specific therapies are suitable treatment options.

Duchenne Muscular Dystrophy Mutation Types

The dystrophin gene is one of the largest genes in the body and different types of mutations that cause DMD are found at various locations within the gene. Areas of the dystrophin gene is more likely to have mutations are called “hot spots”. Large gaps in the DNA sequence called large deletions, are found in 60-70% of DMD cases. Duplications are parts of the DNA sequence that are repeated like if you copied part of a sentence and pasted it next to where it was copied. Large duplications are found in 10% of DMD cases.

15-30% of DMD cases are caused by smaller changes in the DNA sequence. Duchenne muscular dystrophy point mutations are one of these smaller types of mutations. A point mutation is a substitution, insertion, or deletion of a single DNA base. Nonsense mutation is one of the most common types of DMD point mutations. Nonsense mutations cause the protein-building instructions to be read “stop” when there should still be more protein-building blocks to add on. DMD nonsense mutations result in little or no dystrophin protein.

A missense mutation is a point mutation that substitutes one letter of the genetic code for another. This causes the substitution of a different protein building block at that position in the protein. Missense mutations usually result in a partially functional form of dystrophin and the milder disease, BMD. In rare cases, the more severe disease, DMD, can be due to a missense mutation. This occurs when the missense mutation in an essential part of the dystrophin protein, such as where it connects to the muscle contraction machinery.

In-Frame Versus Out-of-Frame DMD Mutations

One of the most important determinants of how a dystrophin gene mutation affects dystrophin protein is whether the mutation is in-frame or out-of-frame. In the genetic code, each set of three DNA bases is like a word. Each word codes for one protein building block. In-frame mutations have an addition or deletion of a multiple of three letters. While this will add an extra protein building block or take one away, the rest of the genetic code is not changed. In-frame mutations in dystrophin are typical in individuals with BMD who produce an altered form of dystrophin that has partial functionality.

Out-of-frame mutations have an addition or deletion of letters not in a multiple of three. Unlike the words on this page, the three-letter words in the genetic code have no spaces in between. Out-of-frame mutations cause the protein-building machinery to read a completely different sequence of words. For example, satpendendot reads “sat pen den dot”; but with p deleted the reading frame shifts to “sat end end ot”. Often out-of-frame mutations will result in multiple stop signals, producing truncated dystrophin, missing a critical part that connects to the membrane surrounding the muscle fiber. Generally, this type of abnormal, incomplete dystrophin protein is disposed of by protein complexes inside the cell that act like a clean-up crew. Out-of-frame mutations typically result in little or no functional dystrophin and cause the more severe disease, DMD.

Splicing Mutations

Splicing is an intermediate step in protein production where a copy of the gene instructions, called the RNA transcript, is processed. Genes are comprised of segments called exons that code for protein building blocks with intervening regions that are removed. Splice sites designate the joining sites between exons. When point mutations, small deletions and small insertions disrupt a splice site, a single exon may be excluded. This results in the joining of two exons that are not normally next to each other, and the connection can be in-frame or out-of-frame. In-frame splice site mutations can produce partially functional dystrophin protein that is missing the protein section corresponding to the exon that was left out. Non-functional dystrophin is produced when the joining of two exons causes the genetic code to be out-of-frame.

Predicting DMD Disease Severity

Genetic testing can determine if a child has an in-frame or out-of-frame mutation. Following the reading frame rule, milder BMD tends to occur with in-frame mutations, and more severe DMD is more likely with out-of-frame mutations. The out-of-frame status of dystrophin gene mutations is a strong predictor for DMD no matter the location or size of deletions or duplications that cause out-of-frame shift. However, there are exceptions where an out-of-frame mutation causes BMD and an in-frame mutation causes DMD. Approximately 10% of mutations do not follow the reading frame rule.

For in-frame mutations, the location and size of mutations have more influence on disease severity. In this case, the level of function of the resulting dystrophin protein depends on which part of the protein is disrupted by the mutation. The two ends of the dystrophin protein are usually more critical to function than the middle section.

Knowing which mutation is present in your child can provide guidance on what to expect but it is important to remember that not everyone with the same mutation will experience the same timing and severity of disease symptoms. A Duchenne genetic mutation is part of a person’s genetic makeup, called a genotype. The phenotype is the effect of the Duchenne mutation on the body, which depends on other parts of the individual’s genetic makeup as well as environmental factors.

Genetic testing is used to diagnose Duchenne and Becker muscular dystrophy and predict the phenotype—the severity and progression of symptoms. The type of mutation and the details of location within the gene are detected by genetic testing. To make a diagnosis, doctors consider information from genetic testing along with muscular dystrophy symptoms and progression experienced by the person. Disease severity cannot always be predicted based on genetic testing alone and some individuals may show disease severity that is in between the typical phenotypes for DMD and BMD.

Duchenne Muscular Dystrophy Genetics and Carrier Testing

Most boys with DMD inherit the mutation from their mother. Confirming whether the mother of a child with DMD is a carrier of the mutation is important for family planning because she could pass it on to subsequent children. Female relatives of the mother may also be at risk of being a carriers of DMD. The dystrophin gene is located on the X chromosome and boys only have one X chromosome. Females, having two X chromosomes, can carry a DMD mutation and still have one normal copy of dystrophin. Most female carriers do not show symptoms of muscular dystrophy.

Mutation-Specific Therapies

Individuals with nonsense mutations, causing a premature stop signal, may be candidates for nonsense mutation read-through therapies. These therapies are molecules that cause the protein-production machinery to read through and continue building dystrophin protein even when they encounter a codon that signals stop.

Exon skipping therapies modify the splicing of exons together, allowing mutated exons to be skipped. There are many different exon skipping therapies that target specific mutations. People with certain out-of-frame deletions are candidates for exon-skipping therapies. Exon-skipping treatment allows a person who previously produced no functional dystrophin to produce a partially functional dystrophin protein that is missing a section corresponding to the exon skipped. Having genetic testing can determine if a person with DMD is suitable for an exon-skipping therapy that is approved or in clinical trials.

Genetic testing is used for the diagnosis of Duchenne muscular dystrophy, predicting disease severity and progression, as well as for treatment planning. Knowing the exact mutation can help match the person with DMD to therapies that may be most beneficial to them. Individuals with DMD are likely to have inherited their dystrophin mutation from their mother, which makes genetic testing for DMD important for family planning.

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