FGF14 Repeat Expansions Identified in Patients with Parkinson's Disease

Expanding the Genetic Spectrum of Parkinsonian Syndromes

The genetic architecture of Parkinson's disease remains partially unexplained, as many patients lack identifiable mutations despite a suggestive family history. Clinicians are increasingly looking toward short tandem repeat expansions (repetitive DNA sequences that can expand to pathogenic lengths) as potential drivers of this missing heritability [1, 2]. These genetic variants often exhibit pleiotropy, the ability of a single genetic variant to cause multiple different clinical phenotypes, leading to significant overlap between ataxia and parkinsonism [3, 4]. Such complexity frequently results in diagnostic challenges, as patients may present with features that mimic multiple neurodegenerative syndromes [5, 6]. Identifying these hidden genetic contributors is essential for improving diagnostic accuracy and tailoring patient management. A recent study investigates whether a repeat expansion previously associated with late-onset ataxia might also contribute to the development of Parkinson's disease.

Long-Read Sequencing of Diverse Patient Cohorts

To identify these complex genetic variants, the researchers employed long-read whole-genome sequencing, a technology that reads extended segments of DNA to detect complex structural variants often missed by standard sequencing methods. This approach allowed for the precise characterization of the GAA repeat expansion in the FGF14 gene, a structural anomaly that is notoriously difficult to resolve using traditional short-read techniques. The study analyzed a primary cohort of 411 individuals with Parkinson's disease and 197 neurologically healthy controls sourced from the Parkinson's Progression Markers Initiative (PPMI). By utilizing this high-resolution sequencing technique on a well-characterized clinical cohort, the authors sought to identify pathogenic variations that might explain the underlying etiology in patients who meet standard diagnostic criteria. To ensure the robustness of their findings and account for genetic background variations, the researchers expanded their analysis to include 1429 additional controls. These subjects were drawn from several large-scale genomic databases, including the National Institutes of Health (NIH) Center for Alzheimer's Disease and Related Dementias (CARD) initiative, the 1000 Genomes Project, and the All of Us program. These control groups represented globally diverse populations, providing a broad genetic baseline to compare against the Parkinson's disease cases. This extensive control dataset was critical for determining the prevalence of FGF14 expansions in the general population and confirming whether the identified variants were specifically associated with the parkinsonian phenotype.

Clinical and Pathological Characteristics of FGF14 Carriers

The researchers identified pathogenic FGF14 GAA repeat expansions in five individuals with Parkinson's disease, while only one control subject across the combined cohorts carried the variant. For the practicing clinician, the most notable aspect of these cases is their presentation. All five individuals with the expansion met the clinical criteria for Parkinson's disease, demonstrating that this genetic variant does not exclusively result in the cerebellar ataxia typically associated with FGF14 mutations. Instead, these patients presented with the classic motor symptoms and progression expected in a standard clinical setting, suggesting that FGF14 expansions may be a rare but unrecognized cause of the typical parkinsonian phenotype. Objective diagnostic markers further confirmed that these patients followed a standard pathological course. All five individuals showed typical patterns of neurodegeneration on DaTSCAN imaging, a specialized scan used to visualize dopamine transporters in the brain, which revealed the characteristic loss of dopaminergic neurons. To investigate the underlying molecular pathology, the authors utilized a seeding assay, a highly sensitive laboratory test that detects the misfolding of specific proteins. Alpha-synuclein aggregation was confirmed by a positive seeding assay in four individuals with available data, providing definitive evidence of the protein misfolding that characterizes Parkinson's disease. These findings indicate that FGF14 expansions are associated with the hallmark alpha-synuclein pathology and dopaminergic deficit seen in idiopathic cases, rather than representing a distinct or atypical neurological syndrome.

Implications for Genetic Testing and Diagnosis

The identification of these variants suggests that the genetic architecture of parkinsonism is more complex than previously understood. While pathogenic GAA repeat expansions in FGF14 are an established cause of late-onset cerebellar ataxia, these genetic markers have not previously been linked to Parkinson's disease. This discovery aligns with emerging evidence that repeat expansions in other ataxia-associated genes, such as RFC1 (replication factor C subunit 1), can contribute to atypical or familial forms of Parkinson's disease. By demonstrating that FGF14 expansions can manifest as a classic parkinsonian phenotype rather than cerebellar ataxia, this study provides the first report implicating FGF14 in Parkinson's disease and establishes it as a rare, previously unrecognized genetic contributor to the condition. These findings significantly broaden the phenotypic spectrum of FGF14 repeat-associated disease, indicating that clinicians might eventually need to consider this genetic factor even in the absence of cerebellar signs. For the practicing neurologist, the study underscores the utility of long-read sequencing for detecting hidden forms of pathogenic variation in unresolved cases. As these advanced diagnostic tools become more accessible in clinical settings, they may reveal that a subset of patients currently diagnosed with idiopathic Parkinson's disease actually carry rare repeat expansions that were previously undetectable by standard genetic screening methods, potentially opening the door to more precise genetic counseling and targeted management.

References

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