Homocarnosinosis: A historical update and findings in the SPG11 gene

OBJECTIVES

A family with homocarnosinosis was reported in the literature in 1976. Three affected siblings had spastic paraplegia, retinitis pigmentosa, mental retardation, and cerebrospinal fluid (CSF) homocarnosine concentrations 20 times higher than in controls. Based on the clinical findings and new genetic techniques, we have been able to establish a precise genetic diagnosis.

METHOD

The medical records were re-evaluated, and genetic analyses were performed post-mortem in this original family. SNP array-based whole genome homozygosity mapping and Sanger sequencing of the SPG11 gene were performed. Seven additional Norwegian SPG11 patients and their disease-causing variants and clinical findings were evaluated. Homocarnosine levels in CSF were measured in four of these seven patients.

RESULTS

A homozygous pathogenic splice-site variant in the SPG11 gene, c.2316 + 1G>A, was found. The clinical findings in the original family correlate with the heterogeneous SPG11 phenotype. The same variant was found in seven other Norwegian SPG11 patients, unrelated to the original family, either as homozygous or compound heterozygous constellation. Normal homocarnosine levels were found in the CSF of all unrelated SPG11 patients.

CONCLUSIONS

A re-evaluation of the clinical symptoms and findings in the original family correlates with the SPG11 phenotype. The increased levels of homocarnosine do not seem to be a biomarker for SPG11 in our patients. Homocarnosinosis is still a biochemical aberration with unknown clinical significance.

A founder mutation p.H701P identified as a major cause of SPG7 in Norway

BACKGROUND AND PURPOSE

SPG7 is one of the most common forms of autosomal recessive hereditary spastic paraplegia. The phenotype has been shown to be heterogeneous, varying from a complex spastic ataxia to pure spastic paraplegia or pure ataxia. The aim of this study was to clinically and genetically characterize patients with SPG7 in Norway.

METHODS

Six Norwegian families with a clinical diagnosis of hereditary spastic paraplegia were diagnosed with SPG7 through Sanger sequencing and whole-exome sequencing. Haplotypes were established to identify a possible founder mutation. All patients were thoroughly examined and the clinical and molecular findings are described.

RESULTS

The core phenotype was spastic paraparesis with ataxia, bladder disturbances and progressive external ophthalmoplegia. The variant p.H701P was identified in homozygous state in one family and in compound heterozygous state in three families. Haplotype analysis of seven surrounding single nucleotide polymorphisms supports that this variant resides on a founder haplotype. Four of the families were compound heterozygous for the previously well-described p.A510V variant.

CONCLUSION

SPG7 is a common subgroup of hereditary spinocerebellar disorders in Norway. The broad phenotype in the Norwegian SPG7 population illustrates the challenges with the traditional dichotomous classification of hereditary spinocerebellar disorders into hereditary spastic paraplegia or hereditary ataxia. A Norwegian founder mutation p.H701P was identified in four out of six families, making it a major cause of SPG7 in Norway.