Exome sequencing reveals a novel homozygous mutation in ACP33 gene in the first Italian family with SPG21

Mast Syndrome Outside the Amish Community: SPG21 in Europe

Background:

Mast syndrome is a rare disorder belonging to the group of hereditary spastic paraplegias (HSPs). It is caused by bi-allelic mutations in the ACP33 gene, and is originally described in Old Order Amish. Outside this population, only one Japanese and one Italian family have been reported. Herein, we describe five subjects from the first three SPG21 families of German and Austrian descent.

Methods:

Five subjects with complicated HSP were referred to our centers. The workup consisted of neurological examination, neurophysiological and neuropsychological assessments, MRI, and genetic testing.

Results:

Onset varied from child- to adulthood. All patients exhibited predominant spastic para- or tetraparesis with positive pyramidal signs, pronounced cognitive impairment, ataxia, and extrapyramidal signs. Neurophysiological workup showed abnormal motor and sensory evoked potentials in all the patients. Sensorimotor axonal neuropathy was present in one patient. Imaging exhibited thin corpus callosum and global brain atrophy. Genetic testing revealed one heterozygous compound and two homozygous mutations in the ACP33 gene.

Conclusion:

Herein, we report the first three Austrian and two German patients with SPG21, presenting a detailed description of their clinical phenotype and disease course. Our report adds to the knowledge of this extremely rare disorder, and highlights that SPG21 must also be considered in the differential diagnosis of complicated HSP outside the Amish community.

A homozygous ABHD16A variant causes a complex hereditary spastic paraplegia with developmental delay, absent speech, and characteristic face

Hereditary spastic paraplegia (HSP) is a genetically and clinically heterogeneous genetic disease characterized by progressive weakness and spasticity predominantly affecting the lower limbs. Complex HSP is a subset of HSP presenting with additional neuronal and/or non-neuronal phenotypes. Here, we identify a homozygous ABHD16A nonsense variant in two affected children in a Chilean family. Very recently, two groups reported patients with biallelic ABHD16A whose clinical presentation was similar to that of our patients. By reviewing the clinical features of these reports and our patients, ABHD16A-related HSP can be characterized by early childhood onset, developmental delay, intellectual disability, speech disturbance, extrapyramidal signs, psychiatric features, no sphincter control, skeletal involvement, thin corpus callosum, and high-intensity signals in white matter on T2-weighted brain MRI. In addition, our affected siblings showed a characteristic face, sleep disturbance, and nodular and hyperpigmented skin lesions, which have not previously been reported in this condition.

circSPG21 protects against intervertebral disc disease by targeting miR-1197/ATP1B3

The abnormal expression of circular RNAs (circRNAs) is associated with numerous human diseases. This study investigated the mechanism by which circRNA acts as competitive endogenous RNA in the regulation of degenerative intervertebral disc disease (IVDD). Decreased expression of circSPG21 was detected in degenerated nucleus pulposus cells (NPCs), the function of circSPG21 in NPCs was explored and verified, and the downstream target of circSPG21 was investigated. The interaction between circSPG21 and miR-1197 and its target gene (ATP1B3) was studied by online database prediction and molecular biological verification. Finally, the circSPG21/miR-1197/ATP1B3 axis was verified in the mouse tail-looping model. The expression of circSPG21 in the nucleus pulposus in IVDD was directly related to an imbalance of anabolic and catabolic factors, which affected cell senescence. circSPG21 was found to play a role in human NPCs by acting as a sponge of miR-1197 and thereby affecting ATP1B3. The regulation of circSPG21 provides a potentially effective therapeutic strategy for IVDD.