Autosomal dominant spastic paraplegia linked to chromosome 2p: clinical and genetic studies of a large Japanese pedigree

Clinical and Genetic Spectrum in a Large Cohort of Hereditary Spastic Paraplegia

BACKGROUND

Next-generation sequencing-based molecular assessment has benefited the diagnosis of hereditary spastic paraplegia (HSP) subtypes. However, the clinical and genetic spectrum of HSP due to large fragment deletions/duplications has yet to be fully defined.

OBJECTIVE

We aim to better characterize the clinical phenotypes and genetic features of HSP and to provide new thoughts on diagnosis.

METHODS

Whole-exome sequencing (WES) was performed in patients with clinically suspected HSP, followed by multiple ligation-dependent probe amplification (MLPA) sequentially carried out for those with negative findings in known causative genes. Genotype-phenotype correlation analyses were conducted under specific genotypes.

RESULTS

We made a genetic diagnosis in 60% (162/270) of patients, of whom 48.9% (132/270) had 24 various subtypes due to point mutations (SPG4/SPG11/SPG35/SPG7/SPG10/SPG5/SPG3A/SPG2/SPG76/SPG30/SPG6/SPG9A/SPG12/SPG15/SPG17/SPG18/SPG26/SPG49/SPG55/SPG56/SPG57/SPG62/SPG78/SPG80). Thirty patients were found to have causative rearrangements by MLPA (11.1%), among which SPG4 was the most prevalent (73.3%), followed by SPG3A (16.7%), SPG6 (3.3%), SPG7 (3.3%), and SPG11 (3.3%). Clinical analysis showed that some symptoms were often related to specific subtypes, and rearrangement-related SPG3A patients seemingly had later onset. We observed a presumptive anticipation among SPG4 and SPG3A families due to rearrangement.

CONCLUSIONS

Based on the largest known Asian HSP cohort, including the largest subgroup of rearrangement-related pedigrees, we gain a comprehensive understanding of the clinical and genetic spectrum of HSP. We propose a diagnostic flowchart to sequentially detect the causative genes in practice. Large fragment mutations account for a considerable proportion of HSP, and thus, MLPA screening acts as a beneficial supplement to routine WES. © 2024 International Parkinson and Movement Disorder Society.

Anticipation Can Be More Common in Hereditary Spastic Paraplegia with SPAST Mutations Than It Appears

BACKGROUND AND OBJECTIVE

Hereditary spastic paraplegia (HSP) is a heterogeneous neurodegenerative disorder with lower-limb spasticity and weakness. Different patterns of inheritance have been identified in HSP. Most autosomal-dominant HSPs (AD-HSPs) are associated with mutations of the SPAST gene (SPG4), leading to a pure form of HSP with variable age-at-onset (AAO). Anticipation, an earlier onset of disease, as well as aggravation of symptoms in successive generations, may be correlated to SPG4. Herein, we suggested that anticipation might be a relatively common finding in SPG4 families.

METHODS

Whole-exome sequencing was done on DNA of 14 unrelated Iranian AD-HSP probands. Data were analyzed, and candidate variants were PCR-amplified and sequenced by the Sanger method, subsequently checked in family members to co-segregation analysis. Multiplex ligation-dependent probe amplification (MLPA) was done for seven probands. Clinical features of the probands were recorded, and the probable anticipation was checked in these families. Other previous reported SPG4 families were investigated to anticipation.

RESULTS

Our findings showed that SPG4 was the common subtype of HSP; three families carried variants in the KIF5A, ATL1, and MFN2 genes, while five families harbored mutations in the SPAST gene. Clinical features of only SPG4 families indicated decreasing AAO in affected individuals of the successive generations, and this difference was significant (p-value <0.05).

CONCLUSION

It seems SPAST will be the first candidate gene in families that manifests a pure form of AD-HSP and anticipation. Therefore, it may be a powerful situation of genotype-phenotype correlation. However, the underlying mechanism of anticipation in these families is not clear yet.

[Molecular and genetic analysis of spinocerebellar ataxia type 10 (SCA10)]

Spinocerebellar ataxia type 10 (SCA10) is a dominantly inherited neurodegenerative disease caused by expansion of the ATTCT pentanucleotide repeat in intron 9 of a novel gene, ATXN10, on chromosome 22q13.3. It is clinically characterized by progressive ataxia, seizures, and anticipation, which can vary within and between families. The length of the expanded ATTCT repeats is highly unstable on paternal transmission and shows a variable degree of somatic and germline instabilty, revealing complex SCA10 genetic mechanisms. Moreover, the purity of the expanded repeat element may be a disease modifier. ATTCT repeats have been recently shown to form unpaired DNA structure and may serve as an aberrant DNA replication origin, potentially contributing to repeat instability and cell death. How this untranslated ATTCT expansion leads to neurodegeneration has been still controversial. We discuss several possible pathogenic mechanisms for SCA10, and growing number of evidence indicates a gain-of-function RNA mechanism, similar to the myotonic dystrophies caused by non-coding CTG or CCTG repeat expansions.

Possible anticipation in hereditary spastic paraplegia type 4 (SPG4)

OBJECTIVE

We report a multigenerational family with uncomplicated hereditary spastic paraplegia type 4 and apparent anticipation. Genetic analysis of the proband revealed a frame shift mutation (5 base pair deletion) in exon 9 of the SPG4 gene encoding the spastin protein. We hypothesized that this deletion mutation may be dynamic and variability in the size of the deletion could account for the anticipation.

METHODS

Clinical and genetic analysis of this family and the deletion mutation.

RESULTS

In this family, the age of onset, which ranges from 3 to 50 years shows an average decrease in the age of onset of 21.8 years per transmission over three generations. Genetic analysis of multiple family members indicates that all affected members carry the same c.1340_1344delTATAA mutation and that it is not dynamic.

CONCLUSION

In this family, other molecular mechanisms may contribute to development of anticipation.

A novel mutation in the spastin gene in a family with spastic paraplegia

Hereditary spastic paraplegia (HSP) is a degenerative neuromuscular disease characterized by progressive lower extremity weakness, spasticity and hyperreflexia. Inheritance of HSP is commonly autosomal dominant, spastin was identified as the defective gene in chromosome 2p-linked autosomal dominant hereditary spastic paraplegia (AD-HSP). In a large American family with AD-HSP, we have identified a novel spastin mutation at a splice-acceptor site in intron 6 (1130-1 g--> a) and detected a corresponding aberrant transcript generated from a cryptic splice site. This is predicted to cause a frameshift and premature truncation of the abnormal spastin protein. Our data are the first to confirm that a mutation in an acceptor site in the spastin gene results in activation of a cryptic acceptor site and a translational frameshift. The clinical phenotype of this pedigree is also discussed.