Musladin-Lueke Syndrome (MLS) is an inherited disorder in which dogs have increased amounts of connective tissue in their joints and skin. Clinical signs are usually obvious from birth. Signs include stiff, sometimes contracted joints that cause dogs to walk upright and often on their tip-toes, giving them a “ballerina walk”. Affected dogs are smaller than littermates with creased ears and thick, tight skin. Other features of this disorder include short outer toes, broad flat foreheads with wide-set slanted eyes and high-set ears. Dogs may also exhibit a failure to thrive, have seizures or experience “phantom pains”. The disease progresses for the first year of life, and then clinical signs stabilise. Affected dogs typically have a normal lifespan, but often develop arthritis.
The single nucleotide substitution in the gene called ADAMTSL2 that causes Musladin-Lueke Syndrome (MLS) in many breeds is recessive. This means that dogs that carry two copies of the mutation (homozygotes) will almost certainly develop MLS during their lives. Dogs that carry a single copy of the mutation (also known as carriers or heterozygotes) will not develop MLS as a result of the ADAMTSL2 mutation, but they will pass the mutation onto about half of any offspring they have. Breeding dogs that will not develop MLS should be the breeder’s priority, with a reduction in mutation frequency within the whole breed being the secondary, longer-term target.
Carriers can be bred from safely, provided they are mated to a dog that has also been tested and is clear of the ADAMTSL2 mutation (i.e. carry no copies of the mutation). If a carrier is mated to a clear dog approximately half of the resulting puppies will also be carriers, so should be tested themselves prior to breeding. Breeding carriers to tested, clear dogs is safe, in terms of avoiding dogs affected with MLS, and will help to maintain the genetic diversity of a breed. It is therefore encouraged, particularly in the first few generations following the availability of a new genetic test, so that other desirable characteristics and traits can be preserved before the frequency of the disease mutation within the breed is gradually reduced.