What is collagen?

Collagen is the most abundant protein in the body and adds structure, strength, and stability to connective tissue, such as skin, tendons, ligaments, and cartilage.

Connective tissue, as its name suggests, is tissue that connects and binds other tissues together. Two important types of connective tissue that are made from collagen include:

• Ligaments - fibrous bands that connect bone to bone (e.g. the anterior cruciate ligament in the knee (see image below), which connects the femur (thighbone) to the tibia (shin bone)). They act to stabilise joints.
  

• Tendons - tough cord-like structures that attach muscle to bone (e.g. the Achilles tendon in the calf, which connects the soleus and gastrocnemius muscles of the calf to the calcaneus bone of the heel.) They allow muscles to move bones and limbs.

Collagen is an ideal material for tendons and ligaments, as it gives them tensile strength and stiffness, allowing them to generate and withstand strong forces when we move. The molecular structure of collagen gives it these useful properties.

Structure of collagen

A collagen molecule is typically formed of three strings of amino acids (known as alpha chains) which are wound around one another to form a triple helix structure (or tropocollagen).

These tropocollagens then aggregate together to form long, thread-like structures known as fibrils. Individual fibrils are then tightly packed together to form larger collagen fibres, which are, in turn, densely packed to make up tendons, ligaments and other fibrous connective tissue.

As we’ll discuss in later sections, the composition of collagen fibrils can affect the strength and durability of collagen fibres and, more broadly, the susceptibility of tendons and ligaments to injury and degeneration.

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What are the different types of collagen?

There are thought to be at least 28 different types of collagen, which have different structural properties. Of these, type I, II, III, V, and XI collagen are known as fibril-forming collagens, as they assemble into the thread-like structures called fibrils (discussed earlier).

Across the human body, the five main types of collagen are:

• Type I - which makes up 90% of collagen in the body. This is the predominant type of collagen found in tendons and ligaments.
  

• Type II - found mainly in cartilage.
  

• Type III - found in the reticular fibres that make up connective tissue that supports various organs.
  

• Type IV - found in the basement membrane layer of skin.
  

• Type V - found in the cornea, hair and nails.

Collagen fibrils are often made out of a mixture of different types of collagen. For example, collagen fibrils that make up cartilage have a mixture of Type II and III collagen.

Source: Musiime, M., Chang, J., Hansen, U., Kadler, K. E., Zeltz, C., & Gullberg, D. (2021). Collagen assembly at the cell surface: dogmas revisited. Cells, 10(3), 662.

Fibrils making up tendons and ligaments tend to be made predominantly of Type I collagen, with a small proportion (less than 5%) of Type V collagen.

The role of Type V collagen in these fibrils is not well understood, although it is thought to play a role in the assembly of fibrils and in inhibiting their lateral growth. On this note, studies suggest that a higher ratio of Type V: Type I collagen in fibrils is associated with a narrower fibril diameter.

As we’ll discuss in later sections, genetic factors that alter the amount of Type V collagen in fibrils can affect the mechanical durability of tendons and ligaments. This, in turn, can affect susceptibility to musculoskeletal soft tissue injuries such as tendinopathy.

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What are musculoskeletal soft tissue injuries?

Musculoskeletal soft tissue injuries include damage to soft tissues such as muscles, tendons, and ligaments. It includes conditions such as:

• muscle strains
• tendinopathies
• tendon rupture
• ligament strains
• ligament tears

Soft tissue injuries do not include injuries to bone, such as fractures.

Estimates suggest that over 100 million musculoskeletal soft tissue injuries are sustained each year across the globe. Many of these injuries are sustained during sport and exercise, with elite athletes having a greater injury risk. Let’s take a brief look at the various types of these injuries.

- Tendinopathy

Tendinopathy is a broad term that describes the degeneration of a tendon, involving pain, swelling, and decreased performance. It is thought to result from a failed healing response to injury and is associated with repetitive mechanical stress to the tissue (also known as overuse).

The most common muscle tendons to suffer tendinopathy include: the rotator cuff (supraspinatus tendon), wrist extensors (lateral epicondyle) and pronators (medial epicondyle), patellar and quadriceps tendons, and the Achilles' tendon.

The diagram below shows some of the factors that are linked to tendinopathy.

Source: Corinne N. Riggin, Tyler R. Morris, Louis J. Soslowsky, Chapter 5 - Tendinopathy II: Etiology, Pathology, and Healing of Tendon Injury and Disease, Editor(s): Manuela E. Gomes, Rui L. Reis, Márcia T. Rodrigues, Tendon Regeneration, Academic Press, 2015, Pages 149-183, ISBN 9780128015902, https://doi.org/10.1016/B978-0-12-801590-2.00005-3

The term tendinopathy may also cover tendonitis - which involves degeneration and inflammation of tendons, and tendinosis - which involves degeneration, and a failed healing response, without inflammation.

Achilles’ tendinopathy

One of the most common tendinopathies is Achilles tendinopathy, which affects the Achilles’ tendon that connects the gastrocnemius and soleus muscles in the calf, to the calcaneus (or heel bone).

Degeneration of the Achilles’ tendon is strongly related to overuse, with people who are more physically active having a higher incidence of Achilles tendinopathy. For example, about 10% of runners develop the condition, and it is thought that elite athletes have a 5 times higher incidence of Achilles’ tendinopathy compared to non-athletes.

Patients with Achilles’ tendinopathy often complain of pain and tenderness in their heel, which often accompanies a sudden increase in training load.

Tennis elbow (lateral epicondylitis)

Tennis elbow (lateral epicondylitis) refers to tendinopathy of the tendons of the wrist extensor muscles in the forearm (most commonly extensor carpi radialis brevis) that attach to the outside aspect of the humerus.

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It is an overuse injury caused by repetitive mechanical stress to the wrist extensor muscle tendons, such as that experienced by tennis players.  About 1-3% of people experience tennis elbow each year.

Tendon rupture

Tendon fibres can also tear, either partially or completely (full-thickness tear). This is known as tendon rupture and may be caused by an acute high-load injury, or, more, gradually, due to chronic weakening of the tendon from tendinopathy.

- Ligament injuries

Ligaments attach from bone to bone and help to stabilise joints. When subject to high forces, ligaments may stretch excessively (known as a ligament strain) or tear.

A ligament that is commonly damaged in elite athletes is the anterior cruciate ligament (ACL) of the knee. This ligament attaches from the femur to the tibia and is one of several ligaments that stabilise the knee joint.

The ACL can be stretched or torn during sudden movements such as:

• pivoting with the foot firmly planted
• sudden deceleration and changing direction (known as cutting).
• landing improperly from a jump

Direct injury to ligaments e.g. from a collision or blunt trauma can also cause ligament damage.

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What is the COL5A1 gene?

The COL5A1 gene encodes the alpha-1 chain of Type V collagen.

As mentioned earlier, Type V collagen is found in small amounts in the collagen fibrils that make up tendons and ligaments, where it is involved in the formation of fibrils and regulation of fibril diameter. The COL5A1 gene therefore plays an important role in the mechanical durability of tendons and ligaments.

- COL5A1 gene variants

A SNP (single nucleotide polymorphism), designated rs12722, causes a C → T change in the DNA code of the COL5A1 gene. This gives rise to two different COL5A1 gene variants or alleles:

• the ‘C’ variant
• the ‘T’ variant

It is thought that these gene variants may affect how the DNA instructions of the COL5A1 gene are read and made into the alpha-1 chain of Type V collagen. This, in turn, may affect the ratio of Type V to Type I collagen in tendons and ligaments, thereby influencing their susceptibility to injury.

As we’ll elaborate on in the next section, the ‘T’ variant has been linked to a greater risk of musculoskeletal soft tissue injury.

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How do COL5A1 gene variants affect risk of musculoskeletal soft tissue injury?

Studies have linked the ‘T’ variant (rs12722) of the COL5A1 gene to an increased risk of musculoskeletal soft tissue injury.

A 2022 meta-analysis, which pooled together the findings of 18 studies, compared the COL5A1 genotypes of 2164 cases with ligament, tendon, and muscle injuries to those of 5079 healthy uninjured controls.

As illustrated in the Forest plot below, the ‘T’ variant was associated with a 1.14 [1.03 - 1.28] times higher odds of musculoskeletal injury compared to the ‘C’ variant.

Using a dominant genetic model, the authors calculated that carrying the T variant (i.e. having the TT or CT genotype) was linked to a 1.28 [1.08 - 1.52] times greater odds of having a musculoskeletal injury compared to having the CC genotype.

Source: Guo, R., Ji, Z., Gao, S., Aizezi, A., Fan, Y., Wang, Z., & Ning, K. (2022). Association of COL5A1 gene polymorphisms and musculoskeletal soft tissue injuries: a meta-analysis based on 21 observational studies. Journal of Orthopaedic Surgery and Research, 17(1), 1-13.

When the above analysis stratified the results by injury type, however, there was an association between the ‘T’ variant and ligament injury only, with no significant association found for tendon and muscle injuries.

Individuals with the TT and CT genotypes had a 1.52 and 1.30 times higher odds, respectively, of ligament injury compared to those with the CC genotype.

Source: Lv, Z. T., Gao, S. T., Cheng, P., Liang, S., Yu, S. Y., Yang, Q., & Chen, A. M. (2018). Association between polymorphism rs12722 in COL5A1 and musculoskeletal soft tissue injuries: a systematic review and meta-analysis. Oncotarget, 9(20), 15365.

An earlier 2018 meta-analysis, which pooled the results from 9 studies encompassing 1140 cases with musculoskeletal soft tissue injury and 1410 healthy controls, also found that the T variant was associated with a greater risk of tennis elbow (lateral epicondylitis), ACL (anterior cruciate ligament injury), and Achilles’ tendinopathy. This is illustrated in the Forest plot above, which shows the odds ratios for those with the TT genotype vs CT and CC genotypes.

Why do COL5A1 gene variants affect susceptibility to injury?

The underlying mechanism for why the ‘T’ variant (rs12722) of the COL5A1 gene is linked to an increased risk of musculoskeletal soft tissue injury is not fully understood.

Some theories suggest that the ‘T’ variant causes changes in COL5A1 gene expression that lead to a greater amount of Type V collagen in fibrils. This results in a greater ratio of Type V to Type I collagen in fibrils that make up tendons and ligaments, which is thought to lower their tensile strength and make them more susceptible to stretching, tearing, and other damage.

Source: Ribbans, W. J., & Collins, M. (2013). Pathology of the tendo Achillis: do our genes contribute?. The Bone & Joint Journal, 95(3), 305-313.

On a related note, recall that Type V collagen acts to limit the diameter of fibrils in tendons and ligaments. It is possible that increased Type V collagen content caused by the ‘T’ variant results in smaller diameter fibrils that are more densely packed. Tendons and ligaments made from these smaller diameter fibrils are then more susceptible to injury.

Supporting this hypothesis, biopsies taken from people with tendon degeneration show that their tendons have a higher content of Type V collagen.

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