Juvenile neurodegenerative encephalopathy of the Bullmastiff caused by a mutation in the MFF gene, which encodes the mitochondrial fission factor. Loss of function leads to hyperfused mitochondria, with energy dysfunction and symmetric bilateral lesions in cerebellar and basal nuclei. It manifests around 6 months of age as progressive ataxia and behavioural alterations, with a fatal course.

Affected breed: Bullmastiff. In the cohort of unrelated controls the carrier frequency was 18 % (12/65 dogs), indicating that the allele is relatively widespread in the breed and justifying systematic screening of breeding animals.

- Progressive ataxia with onset around 6 months
- Behavioural alterations and multifocal neurological deficits
- Affected vision and vestibulocerebellar signs
- Progressive course towards death or euthanasia
- Symmetric bilateral lesions in cerebellar and basal nuclei on MRI

A familial cerebellar ataxia with hydrocephalus was described in the Bullmastiff almost 40 years ago as an autosomal recessive trait. In 2022, Christen and colleagues revived the study with two young cases, sequenced the genome of an affected animal against 782 controls and found a homozygous frameshift variant in MFF, XM_038574000.1:c.471_475delinsCGCTCT (p.Glu158Alafs*14), which truncates 55 % of the protein. The variant co-segregated perfectly with the phenotype in four cases and 70 controls, with a carrier frequency of 18 %. By analogy with the human phenotype, they proposed renaming the disease mitochondrial fission encephalopathy (MFE).

- Test breeding animals with the MFF test before mating
- Do not cross two carriers: 25 % risk of affected homozygotes
- A carrier can be crossed with a clear individual; the offspring intended for breeding must be tested
- Exclude affected animals and their known carrier parents from breeding
- Given the carrier frequency, prioritise progressive replacement with clear offspring without narrowing the gene pool

Differential diagnosis with other juvenile mitochondrial and neurodegenerative encephalopathies of the dog (AHE due to SLC19A3, Yorkshire SNE, Lagotto LSD). MRI shows symmetric bilateral lesions in cerebellar nuclei and, in recent cases, in olivary nuclei. Histology with elongated mitochondria points to a defect in mitochondrial dynamics. By analogy with humans, it is classified as encephalopathy due to a defect in mitochondrial and peroxisomal fission 2.

1. Christen M et al. Mitochondrial fission factor (MFF) frameshift variant in Bullmastiffs with mitochondrial fission encephalopathy. Anim Genet 53(6):814-820, 2022. PMID: 36085405
2. OMIA:002551-9615. Mitochondrial fission encephalopathy in Canis lupus familiaris. https://omia.org/OMIA002551/9615/