When a horse is tested for PSSM2 they can come back heterozygous (N/K1) or homozygous (K1/K1) but what exactly is K1?

What Is K1?

K1 is muscle myopathy that affects the integrity of muscle structure

What Is A Myopathy

A myopathy is a muscle disease where the muscle fibres do not function normally. Myopathies affect skeletal and cardiac muscle, and can cause motor delay, muscle weakness, muscle disfunction and respiratory issues.

What Does The K1 variant Affect?

The K1 variant is a missense allele of COL6A3 that affects something called the Collagen alpha-3(VI) chain

What Is The Collagen Alpha-3(VI) Chain?

The Collagen alpha-3 (VI) chain is one of the three alpha chains of type VI beaded filament collagen that’s encoded as the COL6A3 gene. Collagens are the most abundant protein in mammals and are the main structural components of connective tissue. The Collagen alpha-3 (VI) chain aids in Myofibril formation- Myofibrils are rod like units within the muscle cell.

The K1 Variant Of PSSM2 Explained
“An image showing Myofibrils in the muscle cell- image By BruceBlaus.
Image from Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”

What Is The Function Of The Collagen Alpha-3(VI) Chain?

The Collagen alpha-3(VI) chain provides instructions for making the flexible protein type VI collagen. Type VI microfibrillar (microfibril) collagen is found in many extracellular matrices including those of muscle, skin, tendon, and vessels.

What Happens When There Is A Gene Mutation In The Collagen Alpha-3(VI) Chain?

When a genetic mutation occurs in the Collagen alpha-3(VI) chain the degenerative nature of the myopathy will weaken muscles over time.

How Does Having A Mutation In The Collagen Alpha-3(VI) Chain Cause A Horse To Be Symptomatic?

Lets look at this in an easy way- having mutations in the Collagen alpha-3(VI) chain is like building a house with no cement; you can still build it but at some point in time the house will crumple and fall to the ground. This is what happens in PSSM2 horses, the mutation in that one gene affects the rest of the muscle structure, it might work for a little while but the connective tissue that wraps the muscle fibre is weakened, and as PSSM2 is degenerative as time goes on that connective tissue will become weaker, and weaker, leading a horse to eventually become symptomatic.

Why Are Horses Affected By K1 At Different Ages?

Because every horses body is different. Some horses bodies are stronger, some horses bodies are weaker, some horses are immature, while others are more mature, some horses are able to withstand pain and even grow accustomed to it, and some horses simply cant take the slightest ounce of pain and will let you know that something is wrong- Every single horse is an individual and different just like us.

For more information on the K1 variant of PSSM2 check out our YouTube video by clicking this link