Introduction Hypomyelinating leukodystrophies are a group of genetic disorders, characterised by severe permanent myelin deficiency. Their clinical features include developmental delay with or without neuroregression, nystagmus, central hypotonia, progressing to spasticity and ataxia. HSPD1 encodes the HSP60 chaperonin protein, mediating ATP-dependent folding of imported proteins in the mitochondrial matrix. Pathogenic variants in HSPD1 have been related to a number of neurological phenotypes, including the dominantly inherited pure hereditary spastic paraplegia (MIM 605280) and the recessively inherited hypomyelinating leukodystrophy 4 (MIM 612233). Subsequently, an additional phenotype of hypomyelinating leukodystrophy has been reported due to de novo heterozygous HSPD1 variants.

In the current work, we expand the clinical and genetic spectrum of this hypomyelinating disorder by describing a cohort of three patients, being heterozygous for HSPD1 variants involving residue Ala536 of HSP60 (the novel p.Ala536Pro variant and the previously reported p.Ala536Val).

Methods Clinical and radiological evaluation; whole exome sequencing, in vitro reconstitution assay and patient fibroblast cell lysate analysis.

Results Clinical manifestation was of early-onset nystagmus, tremor and hypotonia evolving into spasticity and ataxia and childhood-onset neuroregression in one case. Brain MRI studies revealed diffuse hypomyelination.

The 3D protein structure showed these variants to lie in spatial proximity to the previously reported Leu47Val variant, associated with a similar clinical phenotype. In vitro reconstitution assay and patient fibroblast cell lysate analysis demonstrated that these mutants display aberrant chaperonin protein complex assembly.

Discussion We provide evidence that impaired oligomerisation of the chaperonin complex might underlie this HSPD1-related phenotype, possibly through exerting a dominant negative effect.