What is so special about DVL2?
The screw tail gene (aka DVL2) is the gene responsible for the cork screw tail in French and British Bulldogs, and Boston Terriers. It has been proven that this gene is the link to the presence of hemivertebrae in these breeds. The gene causes the tail vertebrae to be deformed (called hemivertebrae), which allows the tail to curl around on itself. Unfortunately, the deformed vertebrae are not limited to the tail, and can also occur anywhere along the entire spine, causing twists (kyphosis) and instability.
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It is widely accepted that vertebral abnormalities have an extremely high degree of heritability meaning that if the parents are affected, the offspring are very likely to be affected. In fact, up to 93.5% of French Bulldogs have been found to have vertebral abnormalities along the entire length of their spine. It is important to note that these vertebral abnormalities can vary in number and location, and there is no way to predict the location or how pups will be affected. Some of these abnormalities in the spine will have little consequence to the health of the dog whilst others will have a severe impact resulting in pain, paralysis and even death.
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Until recently, researchers believed that all Frenchies have 2 copies (called homozygous) of the DVL2 gene. However, through genetic screening, a handful of dogs have been identified as having only a single copy (called heterozygous or carriers). This means that it is now possible to breed this gene out of the population without having to outcross to a different breed. From recent research studies, there is a very high likelihood that the DVL2 gene also causes heritable heart defects and increases the degree of Brachycephalic Obstructive Airway Syndrome (BOAS) through the morphological changes the gene causes on the skull.
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So what does this all mean? It means that Frenchies with one or no copies of the DVL2 gene should have no hemivertebrae in their spine, although some will still have kinks in their tails. YES! They will have a fully functional tail! A normal spine also means a longer spine, so their body will be longer and should not have a roached appearance.
Right lateral x-ray of Vader showing the absence of hemivertebrae along the cervical and thoracic spine.
A congenital vertebral malformation at the seventh thoracic vertebra (T7) causing kyphosis of the spine.
Health Priority Vader - heterozygous DVL2
Daylorana Melrose Miley - heterozygous DVL2
A schematic line drawing showing the radiographic appearance of the vertebral malformation classification system according to Guitierrez et al. (2014).
Another piece of the spinal puzzle - CDDY
The short leg phenotype seen in French Bulldogs (and other breeds like Dachshunds, Corgis, Shih Tzu and Pekingese) is called chondrodystrophy and is genetic. Now bare with me as I explain the genetics of this. There are two known gene mutations that can cause short legs in dogs, these are CDDY and CDPA. In CDDY, there is a genetic mutation in the FGF4 gene (Functional Fibroblast Growth Factor 4 - responsible for bone development) on chromosome 12 which is called the CFA12 FGF4 retrogene. In CDPA the FGF4 mutation is seen on chromosome 18 and is called the CFA18 FGF4 retrogene. CDDY is classified as a risk factor for developing a disease called IVDD Type 1 (Intervertebral Disc Disease Type 1), where as CPDA just causes short legs. Frenchies typically have CFA12, where as Dachshunds have both.
How does CDDY cause IVDD? CFA12 FGF4 causes the premature calcification of the nucleus pulposus. The nucleus pulposus is the soft gel-like centre of the intervertebral disc in-between vertebral segments (See anatomy picture below). With normal aging, the nucleus is gradually replaced by fibrocartilage, but in Frenchies (and other breeds with CDDY), this commences at a young age and is accelerated. When the nucleus pulposus becomes mineralised, it loses its ability to absorb concussive forces between 2 vertebrae and then extrudes into the spinal canal that courses above it. The extruded disc material then causes bruising and or compression of the spinal cord. There is further compression due to haemorrhage in the canal from the extruded disc material rupturing through the vertebral venous plexus. Symptoms correspond with the location and compression of the spinal cord, including pain, ataxia or paralysis of limbs. Surgery to attempt to reduce the compression is called a hemilaminectomy and can cost approximately $10,000, with no guarantee. A study by Batcher et.al. (2019) identified the average incidence of IVDD surgery in Frenchies to be 4.1 years. Other studies confirm this very young age of onset, and hypothesise that the DVL2 gene may be a contributing factor. There is also strong evidence that a dog with IVDD will likely have another episode in a different location within the following 12 months. Furthermore, the gene works in an autosomal dominant fashion which means both a double copy (homozygous) and a single copy (heterozygous) is enough to cause an increased risk to developing IVDD Type 1. Currently it is thought that the only known way to remove this risk is to be clear of the CDDY gene (have no copies).
The above images show a normal spinal segment and intervetebral disc (A.) and how an extruded disc causes compression on the spinal cord above it (B.). Taken from Fossum's Small Animal Surgery textbook.
The above image shows a French Bulldog in France that lives in a wheel chair due to several disc herniations that left her permanently paralysed.