Duchenne muscular dystrophy (DMD) is a genetic disorder that causes progressive muscle weakness as individual muscle cells die. The cause of DMD's progressive muscle wasting can be traced back to a small mutation in one gene on the X chromosome. Inside this gene, many separate chunks, called exons, contain instructions for producing an important muscle protein called dystrophin.  Each exon must be read in order, from 'start' to 'end,' to properly construct the protein. Most boys with DMD (about 75%) are missing one of these exons. Not only does this remove some of the instructions for making the muscle protein, it also garbles the code inside exons further downstream.   A small bit of the garbled code stops further reading of the gene. A closer look at this process shows that the stop signal arises because the exon's removal shifts the remaining genetic code out of its usual alignment, as represented by the gray boxes. After the code shifts, different letters appear in the boxes.  In DMD, one of these boxes contains the code letters T-A-G; this code tells dystrophin production to stop. As dystrophin's production machinery tries to read the gene’s instructions — starting at exon 1 and proceeding to exon 75 — the new stop signal halts the machine in mid-gene.  Only a shortened version of dystrophin is formed. Since the rest of the exons are ignored, the dystrophin is much shorter than normal.  

The cell recognizes the protein isn’t right and immediately degrades it. In the other cases of Duchenne, all exons are present, but smaller changes in the gene — like a change in one letter of code — critically alter the shape of the dystrophin protein, or generate a stop signal in the middle of the gene.  (In the latter case, gentamycin may help by letting the dystrophin production machinery skip over the premature stop signal.) Without dystrophin, vital connections between proteins in the cell membrane and other proteins inside the cell are lost.  No one knows exactly how, but these changes weaken the membrane of the muscle cell and make it more prone to rupture. When the membrane ruptures, molecules that normally stay inside the cell flow out, and molecules that normally stay outside the cell flow in.  One of the molecules that flows in (calcium) causes the muscle cell to contract in the area near the damage. The localized contraction (unlike a contraction along the entire length of the muscle) breaks the fibers inside the cell.  When damage to the cell is severe, the cell dies.  Macrophages and other cells arrive to clean up the remnants. After the clean-up, many "satellite" cells appear and group together to build a new cell. Though dead cells are replaced by new ones when a boy is very young, as he gets older the number of dying cells overwhelms the repair capacity of the satellite cells.  The cells simply stop creating new muscle.  Instead, fat and connective tissue fill in the spaces left by the dead muscle cells. Becker muscular dystrophy (BMD) is a similar disorder that is also caused by a mutation — frequently an exon deletion — in the dystrophin gene.  The progression of the disorder is much the same except it proceeds more slowly.  The critical difference is that the exon deletion in BMD does not produce a stop signal inside the gene. A closer look at the exon loss in Becker shows that the stop signal fails to appear, because the genetic code stays in alignment after the deletion. When the protein production machinery reads the gene, all remaining exons are read to form the protein.  The dystrophin produced from this gene is slightly shorter than normal and does not function as well.  But because it manages to perform some of dystrophin's duties, muscle is lost more slowly than in Duchenne. If a physician suspects Duchenne or Becker muscular dystrophy after examining the boy, she will use a CPK test to determine if the muscles are damaged.  This test measures the amount of the CPK enzyme (creatine phosphokinase) in the blood.  In a boy with Duchenne or Becker, CPK leaks out of the muscle cells into the bloodstream, so a high level confirms that muscle damage is present. The level of CPK in the blood can be measured in several different ways.  All of them show that a boy with DMD or BMD will have 50 to 100 times more CPK in his bloodstream than a boy without DMD.  Because most boys with DMD or BMD have lost an exon from their dystrophin gene, a rapid DNA test can quickly identify the missing exon and confirm a diagnosis. The test produces a vertical "line-up" of the exons within the boy's gene.  When an exon is present, it appears as a dark rectangular-shaped stain at a particular height in the picture.  When an exon is absent, the stain is missing.  The test below shows a boy who's missing exon 50. This picture is produced after DNA is isolated from a small blood sample taken from the boy.  The test is based on just a small portion of the gene around an exon.  A geneticist can detect the presence of this particular exon by using a chemical reaction called PCR.  In PCR, two short "primers" are added to the boy's DNA sample. When the exon is present, the two primers attach to the DNA.  The red primer attaches to the right side of the exon, while the blue primer attaches to the left side. When the exon is absent, the primers fail to attach. In essence, after the primers attach, the DNA between the two primers is copied millions of times.  Millions of DNA pieces accumulate when the exon is present, but nothing happens when the exon is absent. 

The geneticist will see the difference between these two tubes when he concentrates the DNA pieces in one spot in a slab of gel. In practice, many different pairs of primers — each pair specific for a separate exon — are thrown into the same reaction mixture with the boy's DNA.  These primers produce bands on the gel in different locations. The full series of bands shown on the left will appear if the boy does not have Duchenne, or has a different type of mutation.  If the boy is missing an exon, a blue spot will be missing from his series. Once the mutation in the boy is identified, the same process can be used to determine if his female relatives carry the mutation. MALE WITH DMD FEMALE CARRIER MALE WITHOUT DMD FEMALE Both carriers and non-carriers of an exon-deficient gene will produce a spot where the boy has none.  A carrier's spot will be half as dark as a non-carrier's. This happens because a carrier has a second, complete dystrophin gene, while a non-carrier has two complete dystrophin genes.  The PCR reaction amplifies the DNA from the complete genes, so PCR generates twice as much DNA from a non-carrier than a carrier.  If DNA testing fails to find the mutation causing the boy's disorder, a physician may confirm the diagnosis with a muscle biopsy.  To remove a muscle sample, a needle is inserted into a muscle while the boy is under local anesthetic. Several features of DMD/BMD muscle will be markedly different from unaffected muscle.  These include:  a wider range of cell sizes, fibers that stain darkly due to excess calcium ions, fat and connective tissue between the fibers, and possibly, macrophages engulfing dying muscle cells. (PHOTOS COURTESY OF ALAN PESTRONK, M.D., WASHINGTON UNIV.) The most striking difference is seen when a fluorescent antibody is added to the muscle sample.  The antibody binds only to the dystrophin protein and lights up the edges of unaffected muscle cells. (PHOTO COURTESY OF SIMON WATKINS, Ph.D., UNIV. of PITTSBURGH) There is no dystrophin and no fluorescence in muscle from a boy with DMD.  On occasion, one or two cells (called revertants) have dystrophin and glow.  Researchers believe that a second mutation in the dystrophin gene in these cells has reversed the effect of the first mutation. In boys with BMD, most cells light up — indicating the presence of dystrophin — but they glow at a lower intensity. When BMD is suspected, the geneticist may also want to measure the size of the dystrophin protein from the muscle biopsy, because BMD dystrophin is usually smaller than normal dystrophin.  Normal-sized dystrophin is shown below on the left.  Its vertical position indicates it is 479 kd (a kilodalton is a unit of molecular size). The second column shows the dystrophin from another boy, but the protein's lower position indicates it is smaller than normal dystrophin. This boy has Becker muscular dystrophy. Duchenne muscular dystrophy, or the milder form Becker muscular dystrophy, is like any other sex-linked disorder.  The dystrophin gene that causes the disorder is on the X chromosome, one of two types — X and Y — that determine sex.  Girls have two X chromosomes (making a girl a girl), and boys have one X and one Y (making a boy a boy). This "mismatch" in the sex chromosomes of boys makes them more susceptible to disorders caused by genes on the X.  

A girl has two Xs, and therefore, two dystrophin genes.  If one is mutated, she can fall back on the other gene.  A boy has only one X and only one dystrophin gene.  If he has a mutated dystrophin gene, he has no other copy to fall back on. A boy gets muscular dystrophy when he inherits an X chromosome with a mutated dystrophin gene (XD) from his mother. He also inherits a Y chromosome from his father, but the Y does not contain the dystrophin gene. Therefore, he can only make dystrophin from the mutated gene he got from his mother. A boy can also get Duchenne or Becker even if his mother does not carry a mutated dystrophin gene. This happens if the gene mutates in the mother's eggs, or if the gene mutates early in the boy's embryonic development.  (About one third of boys get Duchenne this way.) A girl can also inherit a mutated dystrophin gene from her mother, but because she gets a normal gene from her father, she does not develop the disorder.  She is, however, a carrier of the disorder and can pass it to her future sons. Carriers of Becker muscular dystrophy can be produced when a girl inherits a mutated gene from her mothor OR when a girl inherits a mutated dystrophin gene from a father with Becker.  (Unlike males with Duchenne, men with Becker can father children). If a couple carries a mutated dystrophin gene, their chance of producing a child with muscular dystrophy, or a child who is a carrier, can be calculated with a Punnett square.  Let's start with a common situation:  a female carrier and her unaffected male partner. To use the square, we first move the parents' chromosomes to the outer edges of the box. Each parent donates only one of their two sex chromosomes to the child, so we place one from the father and one from the mother into each box.  Each completed box shows a potential combination (a.k.a. genotype) in the child, and the entire square contains all possible combinations. Next, we count the boxes that contain the Duchenne-causing genotype (XDY), and divide this number by the total number of boxes.  There is only one Duchenne genotype, so every child of this couple has a 1-in-4 (25%) chance of being a boy with Duchenne.  Each child also has a 25% chance of being a girl who carries the mutated gene (XXD). The most important thing to remember about these odds is that they apply to every child this couple has.  

It may be useful to think of the Punnett square as a roulette wheel.  Each child is a separate "spin of the wheel," so each child has a 25% chance of being a boy with muscular dystrophy (or an unaffected boy, or an unaffected girl, or a carrier girl). In this family, two out of four children have muscular dystrophy.  Other couples with the mutation may have one, three, four, or even no children with the disorder. A Punnett Square also shows us the children a man with Becker muscular dystrophy can have.  Usually, his partner will be a woman who does not carry a dystrophin mutation. As before, we move the parents' chromosomes to the outer edges of the square, and then copy and paste them into the inner boxes. Two out of four boxes are unaffected boys, so there is a 50% chance this couple will have an unaffected boy (rollover the XY boxes).  The other two boxes contain a carrier (XDX), so there is a 50% chance of having a girl who carries the mutation for Becker muscular dystrophy. Duchenne muscular dystrophy (DMD) is a typical sex-linked disorder affecting only boys.  Becker muscular dystrophy (BMD) is a milder form of the same disorder.  A boy inherits DMD or BMD when he receives an X chromosome with a mutated dystrophin gene from his mother. Boys with Duchenne lose muscle throughout their lives, but this is usually not detected until a parent notices unusual walking or running and/or a difficulty talking around the age of 3.  The calves may also be unusually large and firm.  The muscle weakness in Becker is usually noticed later in childhood. Duchenne muscular dystrophy is the most common of the more than 20 different muscular dystrophies.  About 1 in every 3,500 boys is born with Duchenne, and about 1 in every 18,000 boys is born with Becker – a milder form of the disorder.  All ethnic groups are equally affected by both disorders. Three tests are commonly used to diagnose Duchenne and Becker.  A CPK (or CK) assay will detect muscle damage, but not its source. A DNA test or muscle biopsy can point directly to Duchenne or Becker.  CPK and DNA tests can also detect many carriers. Duchenne and Becker muscular dystrophies are caused by a mutation in a gene that produces an important muscle protein called dystrophin. 

 In Duchenne, no dystrophin is produced, while in Becker, a distorted version is produced.  Without fully functional dystrophin, the muscle cells gradually die. There are no cures for Duchenne or Becker muscular dystrophies.  Treatment consists of physical therapy to avoid tightening of the muscles, braces and wheelchairs to keep the boy mobile, spinal surgery for scoliosis, and breathing aids.  Steroids may be used to slow the progression of the disorder. Clinical Features: Dr. Alfred Spiro discusses the first physical signs of DMD, how a doctor tests muscle strength during an office visit and differences between Duchenne and Becker. CPK Assay (Flash Animation) This blood test determines if muscles are being damaged by measuring the amount of muscle enzyme (CPK) spilled into the bloodstream. DNA Testing: A DNA test used after a CPK assay frequently confirms a diagnosis of DMD in boys who are missing an exon. Muscle Biopsy ( Flash Animation) If the DNA test fails to detect a missing exon, DMD or BMD and be confirmed with a sample of muscle. Prenatal Testing: Testing prior birth via amniocentesis is not a routine test except in women with a family history of muscular dystrophy. Initial Reactions: Suzanne Burger and Margaret Cohan discuss their first reactions after getting the diagnosis and how they dealt with it. Grief and Sadness: Parents of boys with Duchenne discuss cycles of grief and how they cope with the diagnosis. Support Groups: Support groups can help parents deal with anger, school issues, and personal relationships. Your friends: Suzanne Burger comments on changing friendships and how to help if a friend has a son with muscular dystrophy. Your Child’s Questions: Margaret Cohan and Suzanne Burger discuss how they answer their sons’ question and when they give them information. His Friends: Margaret Cohan discusses her son’s friends and what she does to help stimulate new friendships. Protecting Your Child: Margaret Cohan warms against over-protecting your son and discusses the importance of Joe’s activities to her family. School Advocacy: Getting your son’s school to respond to his needs may be hard, but there are groups and people that can help. Special School Events: Margaret Cohan relates her son’s experience on a team building field trip and how the teachers responded. Wheelchairs: Suzanne Burger and Margaret Cohan discuss planning for the wheelchair and how to make the transition. Getting Away: Suzanne Burger discusses the importance of getting away with short vacations or through your own activities. Progression of DMD: Though muscles of the boy are affected by DMD at birth, overt signs are not obvious until the boy is between 4 and 7. Dr. Spiro discusses the progression of the disorder after age 7. Contractures: Dr. Spiro discusses contractures, the tightening of the boy’s joints, and what can be done about them. Dardiac and Respiratory Muscles: In later stages, heart and breathing-related muscles weaken. Dr. Spiro discusses the first signs of cardiac and respiratory involvement and way to address them. Slowing Progression: Dr. Spiro discusses the use of prednisone to slow progression of muscles weakness and Suzanne Burger relates her son’s experience with the drug. 

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