Super Muscular Puppies

Myostatin is a protein that acts as a negative regulator of muscle cell (myocyte) production and growth. However in rare cases, inactivating mutations to the myostatin  gene (MSTN/GDF8) can increase in the number (hyperplasy) and size (hypertrophy) of myocytes, resulting in a 200-300% increase in muscle mass relative to controls. This has been demonstrated experimentally by knocking out MSTN in "mighty mice".

Similar mutations to MSTN have also been demonstrated to be the mechanism underlying massive muscle increase in domesticated animals selectively bred to be "double-muscled", such as Belgian Blue cattle and whippets dogs.

Recently, a group of researchers were able to create transgenic beagles using CRISPR to selectively mutate the MSTN gene. This resulted in two puppies with different sets of MSTN mutations. Puppy #11 (dubbed Tiangou, after the Chinese mythical heaven dog) demonstrated similar muscle mass to littermate controls, due to being chimeric (i.e. some cells carried the mutation while others did not). However puppy #5 (dubbed Hercules after...Hercules) was shown to have biallelic MSTN mutations in all tissues resulting in the double-muscled phenotype observed in the aforementioned domesticated breeds.

Authors of this recent publication note that this could serve as an alternative model in the development of therapies for muscle wasting disorders, but also mention the potential to create "new strains of dogs with favorable traits for other purposes." The implication of designer pets brings to mind ethical concerns. Though essentially, all pure-bred dogs are freakish inbred wolves, mutated through the generations of artificial selection instead of direct genetic manipulation