Which are the causes of Larsen syndrome?

TL;DR: Larsen syndrome is a rare genetic disorder primarily caused by mutations in the FLNB gene, which provides instructions for making the protein filamin B. These mutations disrupt skeletal development and joint stability, leading to the characteristic features of the condition, such as multiple joint dislocations and distinct facial characteristics.

What are the primary causes of Larsen syndrome?

The primary cause of Larsen syndrome is a genetic mutation affecting the FLNB gene located on chromosome 3. The FLNB gene is responsible for producing the protein filamin B. Think of filamin B as a "molecular scaffold" or a structural glue that helps cells maintain their shape and assists in the proper development of the skeleton. When this gene is mutated, the protein does not function correctly, which interferes with the formation of bones and cartilage throughout the body. In our DiseaseMaps community, where 58 people with Larsen syndrome have shared their experiences, we see firsthand how these structural differences manifest in unique ways across different families.

Is Larsen syndrome hereditary?

Yes, Larsen syndrome is an inherited condition. It typically follows an autosomal dominant pattern of inheritance. This means that a person only needs one copy of the mutated FLNB gene—either from their mother or father—to develop the disorder. However, it is also possible for Larsen syndrome to occur as a "de novo" or spontaneous mutation. In these instances, the genetic change occurs for the first time in the affected individual, even if neither parent carries the mutation. Because it is a genetic condition, there are no known environmental, lifestyle, or dietary triggers that cause Larsen syndrome; it is present from the moment of conception.

How do genetic mutations lead to the symptoms of Larsen syndrome?

The clinical presentation of Larsen syndrome is a direct result of how the defective filamin B protein affects connective tissue and bone growth. Because filamin B is essential for cartilage-to-bone transformation (ossification), the skeletal system becomes more flexible and prone to instability than in individuals without the condition. The following factors highlight the specific mechanisms involved:

• Joint Instability: Mutations lead to lax ligaments and abnormal bone shapes, resulting in frequent dislocations of the hips, knees, and elbows.


• Skeletal Malformation: Abnormalities in the growth plates can cause short stature and scoliosis (curvature of the spine).


• Craniofacial Differences: The protein deficiency affects the development of the midface, often resulting in a flattened appearance and a prominent forehead.


• Foot Structure: Many individuals with Larsen syndrome are born with clubfoot (equinovarus), which is a common developmental consequence of the underlying connective tissue weakness.

Are there other causes or risk factors involved?

It is important to distinguish between the cause of Larsen syndrome and external risk factors. The cause is strictly genetic—the FLNB mutation. There are no known autoimmune, infectious, or metabolic mechanisms that trigger the condition. While researchers continue to study the etiology of Larsen syndrome, there is currently no evidence that external environmental factors (such as maternal health during pregnancy) contribute to the development of this specific genetic disorder. Research is currently focused on understanding why the severity of the symptoms varies so greatly, even among family members who share the exact same genetic mutation.

Next steps

• Consult with a clinical geneticist to confirm the diagnosis through molecular genetic testing of the FLNB gene.


• Schedule an evaluation with an orthopedic specialist who has experience with rare skeletal dysplasias to manage joint stability.


• Connect with the 58 members of the DiseaseMaps community to share experiences and learn about ongoing research initiatives.


• Request a referral to a physical therapist specializing in connective tissue disorders to improve mobility and muscle strength.

Medical disclaimer: This information is for educational purposes only and does not constitute medical advice, diagnosis, or treatment; always seek the advice of your physician or other qualified health provider with any questions regarding a medical condition.

References

• NIH Genetic and Rare Diseases Information Center (GARD): Larsen syndrome overview.


• Orphanet: Rare disease database entry for Larsen syndrome (ORPHA:487).


• OMIM (Online Mendelian Inheritance in Man): #150250 - Larsen syndrome; LRSN.


• PubMed: Clinical studies on FLNB gene mutations and skeletal dysplasia.