Recessive mutations in DDHD2 cause SPG54, a complex hereditary spastic paraplegia (HSP) with less than forty patients reported worldwide. In this retrospective, multicenter study we describe eight additional SPG54 cases harboring homozygous or compound heterozygous DDHD2 variants. Finally, we reviewed literature data on SPG54, with the aim to better define the phenotype and the brain magnetic resonance imaging (MRI) pattern as well as genotype-phenotype correlations. SPG54 is typically characterized by early-onset (i.e., congenital or, more frequently, infantile) delay in motor and cognitive milestones, coupled or followed by appearance of spasticity. Cognitive impairment is absent in adult-onset cases. Spasticity progresses over time. Abnormal eye movement, found in about 50% of cases, is the feature most frequently associated with spasticity and developmental delay. Cerebellar ataxia is a prominent sign in several patients, including one adult of this study, suggesting to include SPG54 in the differential diagnosis of spastic-ataxia syndromes. Brain MRI shows thin corpus callosum and non-specific periventricular white matter lesions in about 90% and 70% of cases, respectively. Brain MR spectroscopy reveals abnormal lipid peak in 90% of investigated patients. Twenty-one pathogenic changes have been reported so far, many of which are nonsense or small deletion/duplication. Most mutations appear to be private, with only two mutations recurring in three (i.e., R287*) or more families (i.e., D660H). The identification of nine novel variants expands the molecular spectrum of DDHD2-related HSP and corroborates the notion of a quite homogeneous clinical and neuroradiological phenotype in spite of different genotypes.
Keywords: HSP; Hereditary spastic paraparesis; Leukodystrophy; SPG54; Thin corpus callosum.