Muscular dystrophy is caused by mutations in genes responsible for building and maintaining muscle fibers. These mutations prevent the body from producing key structural proteins that muscles need to function, leading to progressive weakness and degeneration. There are more than 30 types of muscular dystrophy, each tied to a different gene, but they all share a common thread: the genetic instructions for keeping muscle cells intact are broken or incomplete.
How Healthy Muscles Stay Intact
To understand what goes wrong in muscular dystrophy, it helps to know what muscles need to survive the constant stress of contraction. Every time a muscle fiber contracts, it generates enormous mechanical force. A protein called dystrophin acts as a molecular spring, connecting the internal skeleton of the muscle cell to the protective outer membrane and the surrounding tissue. This connection absorbs and distributes the force of each contraction so the cell membrane doesn’t tear.
Dystrophin doesn’t work alone. It’s part of a larger group of proteins called the dystrophin-glycoprotein complex, which forms a bridge between the inside and outside of the muscle cell. When any protein in this bridge is missing or defective, the membrane becomes fragile and prone to damage during normal movement.
The Gene Behind Duchenne and Becker MD
The most well-known forms of muscular dystrophy, Duchenne and Becker, both stem from mutations in the same gene: the DMD gene, which provides the blueprint for dystrophin. What separates the two is how severely the mutation disrupts that blueprint.
In Duchenne muscular dystrophy, the mutation throws off the entire reading sequence of the gene (called a frameshift mutation), so the body produces essentially no functional dystrophin, typically less than 3% of normal levels. Without this critical shock absorber, muscle fibers are extremely vulnerable and begin breaking down in early childhood.
In Becker muscular dystrophy, the mutation preserves the gene’s reading sequence, so the body still produces a shortened or reduced version of dystrophin. This partially functional protein offers some protection, which is why Becker MD tends to appear later in life and progress more slowly. About 10% of patients don’t follow this pattern neatly, with some Duchenne-type mutations producing milder disease and vice versa.
Roughly one-third of Duchenne cases arise from spontaneous (de novo) mutations, meaning the child’s mother did not carry the mutation. Another 19% of sporadic cases result from germline mosaicism, where the mutation exists in some of the mother’s egg cells but not in her blood or other tissues, making it invisible on standard genetic testing. The remaining cases are inherited directly from a carrier mother.
How These Mutations Are Inherited
Muscular dystrophy follows three main inheritance patterns, depending on the type.
- X-linked recessive: The DMD gene sits on the X chromosome. Because boys have only one X chromosome, a single defective copy is enough to cause disease. Girls have two X chromosomes, so a working copy on the second X usually compensates, though some female carriers experience mild symptoms. This pattern applies to Duchenne and Becker MD.
- Autosomal dominant: Only one copy of the mutated gene (from either parent) is needed to cause disease. Myotonic dystrophy follows this pattern and affects males and females equally.
- Autosomal recessive: Both parents must carry and pass on a faulty copy for their child to be affected. Many forms of limb-girdle muscular dystrophy follow this pattern. At least 17 autosomal recessive subtypes of limb-girdle MD have been identified.
What Causes Myotonic Dystrophy
Myotonic dystrophy, the most common form of adult-onset muscular dystrophy, has a very different genetic mechanism. Instead of a missing protein, the problem is a stutter in the genetic code. A short DNA sequence (three letters: CTG) that normally repeats a handful of times in the DMPK gene expands dramatically, ranging from 50 to several thousand repeats. The longer the expansion, the more severe the disease tends to be and the earlier symptoms appear.
This expansion doesn’t destroy a structural protein the way Duchenne mutations do. Instead, the expanded repeats produce abnormal RNA molecules that get stuck inside the cell’s nucleus and interfere with the processing of many other genes. The result is a multisystem disease that affects not just muscles but also the heart, eyes, and endocrine system. The repeat length also tends to grow with each generation, which is why children of affected parents often develop more severe symptoms than their parents did.
What Causes Facioscapulohumeral MD
Facioscapulohumeral muscular dystrophy (FSHD) has one of the more unusual genetic mechanisms. On chromosome 4, there’s a region of repeating DNA segments called D4Z4. In healthy individuals, these repeats are long enough to keep a gene called DUX4 tightly locked down and silent. DUX4 is normally active only in very early embryonic development and is supposed to be permanently switched off in mature muscle.
In FSHD, the D4Z4 repeat region contracts to an abnormally short length, loosening the molecular packaging that keeps DUX4 silenced. The gene reactivates in scattered muscle cells, producing a protein that is toxic to mature muscle fibers. Additional genetic modifiers can worsen or stabilize this process. Mutations in a gene called SMCHD1, for example, further disrupt the silencing mechanism and can push borderline cases into active disease.
Limb-Girdle MD and Sarcoglycan Deficiencies
Limb-girdle muscular dystrophy is not a single disease but a family of at least 22 distinct genetic conditions that primarily weaken the muscles around the hips and shoulders. A major subgroup involves mutations in the genes for sarcoglycan proteins, which are part of the same dystrophin-glycoprotein complex that anchors muscle fibers. Four sarcoglycan genes (SGCA, SGCB, SGCG, and SGCD) each correspond to a different autosomal recessive subtype of limb-girdle MD.
When any one of these sarcoglycan proteins is missing, the entire complex destabilizes. The result is similar to what happens in Duchenne MD: the muscle cell membrane loses its structural support and becomes vulnerable to contraction-related damage. Other limb-girdle subtypes involve completely different proteins, including enzymes involved in sugar modification of cell-surface molecules and proteins that help repair torn membranes.
How Damaged Genes Lead to Muscle Loss
Regardless of which gene is mutated, the path from genetic defect to muscle wasting follows a broadly similar sequence. When structural proteins are missing or defective, the muscle cell membrane tears during normal contractions. These micro-tears allow calcium to flood into the cell from the surrounding tissue.
Calcium is essential for muscle contraction in controlled amounts, but unregulated calcium influx is destructive. Excess calcium activates enzymes called calpains that break down the cell’s internal structures. It also causes mitochondria, the cell’s energy producers, to swell and rupture. The combined effect kills the muscle fiber.
Healthy muscle has a remarkable ability to regenerate. Stem cells called satellite cells can divide and fuse to repair damaged fibers. In muscular dystrophy, the relentless cycle of damage and repair eventually exhausts this regenerative capacity. Over time, dead muscle fibers are replaced by fat and scar tissue, which is why affected muscles may initially look enlarged (from fat infiltration) before visibly wasting away.
Early Signs of Muscle Breakdown
One of the earliest measurable signs of muscular dystrophy is a dramatic rise in creatine kinase (CK), an enzyme normally contained inside muscle cells. When muscle fibers break down, CK leaks into the bloodstream. In Duchenne MD, blood levels of CK are 10 to 100 times higher than normal from birth, peaking between ages 2 and 5. Some patients show levels nearly 47 times the upper limit of normal. This spike often appears before a child shows any obvious weakness, which is why an unexpectedly high CK level on a routine blood test sometimes triggers the first referral for genetic testing.
As the disease progresses and more muscle is replaced by non-contractile tissue, CK levels actually decline because there are fewer muscle cells left to release the enzyme. This counterintuitive drop does not mean improvement; it reflects advancing loss of functional muscle.