MRI-phenotyping of Patients With Pathogenic Anoctamin 5 Variants

Background:

The anoctamin 5 gene (ANO5) encodes the anoctamine 5 protein that act as a calcium-sensitive chloride channel. The protein is preferentially expressed in skeletal and cardiac muscle and bone and likely acts in the repair of the cell membrane. Pathogenic ANO5 variants inherited in a autosomal recessive trait give rise to three main phenotypes: Limb-girdle muscular dystrophy type R12 (LGMDR12, formerly classified as LGMD2L), Miyoshi distal muscular dystrophy type 3 (MMD3), and asymptomatic hyperCKemia). As the name implies, patients with LGMDR12 are affected more proximally and patients with MMD3 more distally, but the definition and distinction between the two entities is unclear. Men with anoctaminopathy are more severely affected than women. Cardiac disease such as arrhythmias and cardiomyopathy as well as bulbar symptoms or respiratory failure are very rare in anoctaminopathies. Onset is in adulthood and disease progression is slow, generally with a later onset and disease progression than seen in other LGMDs. Ambulation is preserved until late in the disease course.

However, only few studies based on small case series have investigated the phenotype of patients with ANO5 mutations using MRI. There is therefore a need to investigate a larger international group of patients using MRI to properly describe which muscles are affected in men and women with anoctaminopathy.

The spectrum of phenotypes in anoctaminopathies resembles that seen in dysferlinopathies, and in the latter group, it has been shown that the former division into LGMDR2 (formally LGMD2B) and Miyoshi distal muscular dystrophy type 2 (MMD2) is rather arbitrary. Our hypothesis is that this may very well also be the case for LGMDR12 and MMD3. A large MRI study would be able to shed light on this question. Muscle involvement in patients with ANO5 mutations is said to be asymmetric based on clinical assessments (7,8,10). The proposed study will also elucidate this by studying symmetry of muscle affection. Finally, the diseases severity is said to be marked between the two sexes, but this has not been quantified in any detail before. The proposed study will also be able to shed light on this.

Aim: