Mucopolysaccharidosis type I is a rare inherited condition that progressively affects many parts of the body due to the buildup of complex sugar molecules inside cells, causing damage that worsens over time.

Understanding the Numbers: Who Gets Mucopolysaccharidosis Type I

Mucopolysaccharidosis type I, commonly known as MPS I, is an uncommon genetic disorder that affects approximately 1 in every 100,000 newborns when looking at the severe form of the condition. This means that in a population of one million people, roughly ten individuals might be born with severe MPS I. The milder, or attenuated form—which means a less severe version—is even rarer, occurring in about 1 in 500,000 newborns.^[1]

This condition does not favor one gender over another, affecting males and females equally. It also does not seem to be more common in any particular ethnic or geographic group, though the condition can occur in any family regardless of background. The rarity of MPS I means that many families have never heard of it until a diagnosis affects their own child.^[3]

Because MPS I is so rare, many healthcare providers may not encounter a case throughout their entire careers. This rarity can sometimes lead to delays in diagnosis, as the early symptoms might be mistaken for more common childhood conditions. The progression and severity can vary widely among affected individuals, making each case unique in its presentation and course.

What Causes Mucopolysaccharidosis Type I

Mucopolysaccharidosis type I happens because of changes—called mutations—in a specific gene known as the IDUA gene. This gene contains the instructions that tell the body how to make an enzyme called alpha-L-iduronidase, or IDUA enzyme for short. Enzymes are special proteins that help break down various substances in the body, acting like molecular scissors that cut larger molecules into smaller, manageable pieces.^[1]

The IDUA enzyme’s job is to break down large sugar molecules called glycosaminoglycans (GAGs), which were once called mucopolysaccharides—the origin of the condition’s name. These GAGs, specifically dermatan sulfate and heparan sulfate, are found throughout the body in connective tissue, which is the material that supports and holds together other body parts and organs.^[6]

When the IDUA gene has mutations, the body cannot produce enough working IDUA enzyme, or sometimes cannot produce any at all. Without this enzyme functioning properly, GAGs cannot be broken down as they should be. Instead, they accumulate inside compartments within cells called lysosomes. Lysosomes normally act as the cell’s recycling and waste management centers, digesting and breaking down molecules that are no longer needed. When GAGs pile up in these lysosomes, the cells become enlarged and cannot function properly, leading to the various symptoms seen in MPS I.^[2]

MPS I is inherited in what doctors call an autosomal recessive manner. This means that for a child to have the condition, they must inherit two copies of the changed IDUA gene—one from their mother and one from their father. Parents who each carry one changed copy of the gene typically do not show symptoms themselves but have a one-in-four chance with each pregnancy of having a child with MPS I.^[11]

Risk Factors for Developing the Condition

The primary risk factor for mucopolysaccharidosis type I is having parents who are both carriers of a mutation in the IDUA gene. Carriers have one normal copy and one changed copy of the gene, which means they produce enough enzyme to function normally themselves but can pass the changed gene to their children. When both parents are carriers, there is a 25 percent chance with each pregnancy that their child will inherit both changed copies and develop MPS I.^[2]

A family history of MPS I significantly increases risk. If a couple has already had one child with the condition, their risk of having another affected child remains at 25 percent with each subsequent pregnancy. Similarly, if either parent has siblings or other close relatives with MPS I, the likelihood that they are carriers increases.

Unlike many health conditions, lifestyle factors, environmental exposures, or behaviors during pregnancy do not cause or increase the risk of MPS I. The condition is purely genetic, determined at the moment of conception when the child inherits their genetic material from both parents. Nothing a parent does or does not do during pregnancy can cause or prevent the genetic changes that lead to MPS I.

Certain populations with higher rates of marriages between relatives may see slightly higher rates of autosomal recessive conditions like MPS I, simply because relatives are more likely to carry the same genetic changes. However, the condition remains rare even in these populations.^[11]

Signs and Symptoms of Mucopolysaccharidosis Type I

Children with mucopolysaccharidosis type I often appear completely normal at birth. However, some babies may show soft bulges around the belly button, called umbilical hernias, or in the lower abdomen, known as inguinal hernias. These hernias occur when tissue pushes through weak spots in the abdominal wall.^[1]

The severe form of MPS I typically reveals itself within the first year of life. Parents might notice that their baby has frequent upper respiratory infections, a persistently runny nose, or noisy breathing. The child’s facial features may begin to look different from other children, described by doctors as “coarse.” This includes a larger head (called macrocephaly), widely spaced eyes, a flattened nose bridge, thick lips, and a large tongue that may protrude from the mouth.^[3]

As these children grow, many develop problems with their skeleton and joints. A curved spine, particularly a hump in the lower back called a gibbus deformity, often appears within the first year. The joints throughout the body become stiff and difficult to move, a condition called contractures. By age three, children with severe MPS I typically stop growing at a normal rate, resulting in short stature.^[4]

The eyes are commonly affected in MPS I. Clouding of the clear front surface of the eye, called the cornea, develops in many individuals and can significantly reduce vision. Hearing loss is also frequent, often due to recurrent ear infections that damage the structures of the ear over time.^[1]

Internal organs become enlarged as GAGs accumulate within their cells. The liver and spleen grow larger than normal, a condition called hepatosplenomegaly, which can cause the abdomen to appear swollen. The heart is also affected, with problems in the heart valves that control blood flow. These valve abnormalities can lead to heart failure if left untreated.^[3]

Breathing problems are common in MPS I. The airway can become narrow due to several factors: enlarged tongue, thickened tissues in the throat, and accumulation of GAGs in the respiratory tract. This narrowing leads to frequent respiratory infections and can cause sleep apnea, where breathing repeatedly stops and starts during sleep.^[1]

In children with severe MPS I, intellectual development is profoundly affected. Developmental delays usually become apparent by age one, and children eventually lose skills they had previously gained, a process called developmental regression. They may lose the ability to walk, talk, or care for themselves. This progressive loss of mental function is one of the defining features that distinguishes severe MPS I from the milder forms.^[1]

Individuals with attenuated MPS I experience many of the same physical symptoms but usually not until later in childhood, typically between ages three and ten years. Their symptoms progress more slowly. Some may have learning disabilities, but many have normal intelligence and do not experience the profound intellectual decline seen in severe cases. These individuals can often live into adulthood, though they face significant challenges from progressive joint disease and heart problems.^[4]

Preventing Mucopolysaccharidosis Type I

Because mucopolysaccharidosis type I is caused by genetic mutations present from conception, there is no way to prevent the condition from developing once a child inherits two changed copies of the IDUA gene. However, families can take steps to understand their risk and make informed decisions about future pregnancies.

Carrier testing is available for individuals who have a family history of MPS I or who have had one affected child. This testing looks for changes in the IDUA gene and can tell prospective parents whether they carry a mutation. When both partners learn they are carriers before or during pregnancy, they can better prepare for the possibility of having an affected child.^[2]

Prenatal testing options exist for pregnant women whose partners are both known carriers or who have previously had a child with MPS I. Tests such as amniocentesis or chorionic villus sampling can determine whether the developing baby has inherited two changed copies of the gene. These procedures involve taking samples of fluid around the baby or tissue from the placenta, which can then be tested for the genetic mutations.^[4]

Newborn screening programs have been implemented in some regions to identify MPS I shortly after birth, before symptoms appear. These programs test a few drops of blood from the baby’s heel for low levels of the IDUA enzyme. Early detection through newborn screening allows treatment to begin immediately, which can prevent or reduce some of the severe complications of the disease.^[5]

Genetic counseling provides valuable support for families considering their reproductive options. Counselors can explain the inheritance pattern, discuss the likelihood of having affected children, and present available testing options. They also provide emotional support and help families understand the implications of test results. For couples at high risk who wish to have biological children, options such as preimplantation genetic diagnosis may be discussed, though this involves complex assisted reproductive procedures.

How Mucopolysaccharidosis Type I Changes Normal Body Function

Understanding what happens inside the body with mucopolysaccharidosis type I requires looking at how cells normally manage their waste. Every cell in the body contains lysosomes, which are like tiny compartments that serve as recycling centers. These lysosomes contain enzymes that break down various molecules the cell no longer needs, converting large molecules into smaller pieces that can be reused or safely eliminated.^[1]

In a healthy person, the IDUA enzyme inside lysosomes breaks down two specific types of glycosaminoglycans: dermatan sulfate and heparan sulfate. These GAGs are long chains of sugar molecules that play important roles in connective tissue throughout the body. Connective tissue provides structure and support, cushions organs, and helps cells communicate with each other. As cells naturally turn over and replace old GAG molecules with new ones, the old GAGs must be broken down and removed.^[6]

When someone has MPS I, their cells lack sufficient IDUA enzyme activity. Without this enzyme, dermatan sulfate and heparan sulfate cannot be properly broken down. Instead of being recycled or eliminated, these GAGs accumulate inside the lysosomes. As more and more GAG molecules pile up, the lysosomes swell and grow larger. This enlargement of lysosomes affects the entire cell’s ability to function normally.^[7]

The accumulation of GAGs causes cells throughout the body to malfunction or die. In bones and joints, this buildup interferes with normal growth and development, leading to the skeletal abnormalities seen in MPS I. The characteristic “dysostosis multiplex” visible on X-rays shows multiple skeletal problems including thickened bones, abnormally shaped vertebrae, and poorly formed joints. These changes explain why children with MPS I have short stature, stiff joints, and spinal deformities.^[4]

In the heart, GAG accumulation in valve tissue makes the valves thick and stiff. Heart valves normally open and close smoothly to control blood flow through the heart chambers. When GAGs build up in valve tissue, the valves cannot open fully or close completely, forcing the heart to work harder to pump blood. Over time, this extra work can lead to heart failure, one of the leading causes of death in people with MPS I.^[9]

The airways and lungs are similarly affected. GAG deposits in the tissues of the throat, windpipe, and airways cause these passages to narrow. The vocal cords become thickened, resulting in the characteristic deep, hoarse voice. Narrowed airways make breathing difficult, especially during sleep, and increase susceptibility to respiratory infections. The lungs themselves may become stiff as GAGs accumulate in lung tissue, making it harder to breathe deeply.^[1]

In the brain and nervous system, GAG accumulation has particularly devastating effects in severe MPS I. While the exact mechanisms are not completely understood, researchers believe that GAGs interfere with normal brain cell function and development. They may disrupt communication between brain cells, damage the protective coating around nerve fibers, or trigger inflammatory responses that harm brain tissue. This explains the progressive intellectual decline and loss of developmental milestones seen in children with severe MPS I.^[1]

The cornea of the eye becomes cloudy as GAGs accumulate in its normally transparent tissue. The cornea must be clear to allow light to pass through to the back of the eye. When GAG deposits make it cloudy, vision becomes increasingly impaired. Similarly, GAG buildup in the structures of the ear contributes to hearing loss, both by causing recurrent infections and by directly damaging the delicate bones and tissues involved in hearing.^[1]

In the spinal canal, GAG accumulation can cause the canal to narrow, a condition called spinal stenosis. The spinal cord runs through this canal, and when the space becomes too narrow, the cord can be compressed and damaged. This compression causes numbness, weakness, or pain and can affect the person’s ability to walk or control bladder and bowel function.^[1]

The progressive nature of MPS I reflects the fact that GAGs continue to accumulate throughout life. Early in infancy, before much accumulation has occurred, children may appear normal. As GAG deposits increase with age, more and more cells throughout the body become dysfunctional, and symptoms worsen. The rate of accumulation varies among individuals, which explains why some people have rapidly progressive severe disease while others have slowly progressive attenuated forms.^[18]