Genetic basis of Degenerative Myelopathy in Bernese Mountain Dogs - is caused by mutations in the SOD1 gene. Two distinct mutations are identified in Bernese Mountain Dogs: SOD1-A mutation (c.118G>A in exon 2) and SOD1-B mutation (c.52A>T in exon 1). Inheritance is autosomal recessive with incomplete penetrance—dogs with two copies of the mutated gene (homozygous) have increased risk but may not always develop the disease. Heterozygous dogs (carriers) usually do not show clinical signs but can pass the mutation to offspring.
Pathophysiology - The mutation affects the superoxide dismutase 1 enzyme that protects cells from oxidative damage. Deficiency leads to progressive degeneration of the spinal cord white matter, particularly affecting nerve cells controlling hind limb movement. Results in muscle atrophy, weakness, and loss of coordination starting in the hind limbs.
Clinical Presentation - Symptoms usually appear between 6 and 14 years of age, although earlier onset is possible. Early signs include hind limb weakness, dragging paws, stumbling, and loss of coordination. Progression leads to paralysis of the hind limbs, involvement of the front limbs, respiratory difficulties, and ultimately euthanasia due to reduced quality of life.
Diagnosis and Testing - Genetic testing for both SOD1-A and SOD1-B mutations is recommended for Bernese Mountain Dogs to determine carrier or affected status. Diagnosis is supported by clinical signs and ruling out other causes of neurological impairment.
Management - There is no cure; treatment is supportive and includes physical therapy and assistive devices to maintain mobility. Disease progression can vary; some dogs maintain mobility longer with intensive management.