The present study investigated the prevalence of SCA10, 12, 31, and 36 using comprehensive methodologies, including RP-PCR and LRS based on the repeat ranges of full penetrance alleles as well as repeat structures. As a result, expanded alleles of SCA36 were identified in 11.9% of undiagnosed cerebellar ataxia families (cohort 1) and 1.0% of unselected ataxia patients (cohort 2). These findings underscore the potential underdiagnosis of SCA36, especially in Korean populations, due to the lack of a convenient diagnostic method. In addition, our study unveiled distinct clinical features in the Korean SCA36 patients, distinguishing them from their global counterparts. Notably, Korean patients exhibited a heightened frequency of hyperreflexia.
SCA36 has been discovered across diverse populations, with its prevalence in genetic ataxia cohorts ranging from 0.3% to 6.3% [16, 17, 20-23]. Interestingly, some studies have failed to detect SCA36 in certain European countries, including Germany, the United Kingdom, Greece, and Portugal [24]. However, the prevalence of SCA36 has been found to be particularly high in the Galicia region of Spain (6.3%) [23], and also high in the Japan (1.5% of all Japanese SCAs) [16] and China (1.6% of autosomal dominant SCAs) [22]. Our current investigation revealed that the presence of SCA36 in 11.9% of cohort 1 and 1.0% of cohort 2. The frequency observed in cohort 1 significantly surpasses that reported in other populations. This higher frequency may be attributable to the exclusion of other common SCAs, and some of the patients were ruled out for other genetic conditions through exome sequencing. Moreover, the inclusion of patients with a family history of ataxia may have contributed to the observed prevalence. Although not all patients' family histories were available, at least all patients diagnosed had a family history. The prevalence observed in cohort 2, which includes unselected ataxia patients, may align more closely with findings from previous studies in Japan and China. However, further studies utilizing a larger number of patients would be necessary, as cohort 2 may not sufficiently represent the Korean ataxia patients, especially considering that SCA3, one of the most prevalent subtypes, was not observed in cohort 2. Furthermore, given the reported founder effect in specific regions such as the Asidan area of Japan and the Galicia region of Spain for SCA36 [16, 17, 23], further research is warranted to explore the genetic relatedness of SCA36 between Japan and South Korea. In terms of clinical features, Korean patients with SCA 36 demonstrated a higher prevalence of hyperreflexia, and SCA 36 patients in China and Western Japan also reported a higher prevalence of hyperreflexia from 79 to 100%, which supports the forementioned genetic relatedness.
This study could not identify any patients with SCA10, 12, or 31, consistent with the previous literature findings. SCA10 is recognized as the most common SCA in Peru, the second most common subtype in Mexico, and prevalent in certain regions of Southern Brazil [25]. However, its occurrence in other regions is known to be rare, although isolated case reports have been documented in Japan and China [26-28]. Similarly, SCA12 is predominantly found in a single ethnic group from northern India [29]. while a very limited number of cases have been reported in China. [30] While SCA31 is known as a prevalent ataxia in Japan [14, 31, 32]. it has been shown rare incidences in surrounding regions of Asia such as Korea, Taiwan, and China [33-35].
Recently, the usefulness of LRS in the diagnosis of repeat expansion disease has been gaining considerable interest in medical research [10]. Indeed, Wang et al. diagnosed patients with SCA36 using LRS [19], and it also be used for Cerebellar Ataxia, Neuropathy, and Vestibular Areflexia Syndrome (CANVAS), another repeat expansion disease [36]. It can effectively address the limitation of existing diagnostic methods such as Southern blot and NGS, which fall short in detecting large repeat expansions that cause certain SCAs. However, the challenges of complex data interpretation, coverage depth, cost, and accessibility underscore the need for further research to optimize this technology for routine clinical applications [37]. Notably, the repeat numbers of one of the expanded alleles in this study were lower than 650 repeats, the typically recognized pathogenic threshold by Southern blot [17, 38]. Despite the lower repeat number, those patients presented with characteristic clinical symptoms of SCA36 such as cerebellar ataxia, tongue fasciculation, muscle fasciculation, hearing impairment and hyperreflexia. It is uncertain whether symptoms can appear even with repeat numbers below 650, or whether the low count was due to technical differences between the Southern blot and LRS, which may have differing resolutions. Additionally, assigning repeat numbers to alleles in LRS may not be intuitive and could vary depending on algorithms, as the repeat numbers in each sequence read are distributed broadly as shown in Figure 2, resembling the smearing pattern in Southern blot due to instability of the expansions [17, 24]. Future research utilizing LRS may help refine our understanding of the thresholds for repeat expansions that could trigger diseases such as SCA36. Moreover, standardization of repeat measurement will be necessary for the use of LRS as a diagnostic tool.
This study does have certain limitations. Some participants, due to the severity of their disability, could not undergo a comprehensive examination. Furthermore, we couldn't apply LRS to four patients suspected of having SCA36 based on RP-PCR results due to limited amounts or poor quality of DNA. Their diagnoses, however, remain strongly supported by their clinical symptoms and RP-PCR findings. Another limitation is the potential for sampling bias in cohort 1. The study cohort was derived from two hospitals in South Korea, which may restrict the generalizability of our findings to broader populations. potentially limiting the generalizability of our findings to wider populations. Furthermore, cohort 2 lacks thorough clinical characterization. Subsequent studies may consider incorporating a more diverse population sample to validate these findings.
The implementation of LRS with the assistance of ONT and Cas9 target enrichment emerges as a viable alternative to the traditional Southern blot method for diagnosing SCA36. This innovative approach not only demonstrates effective diagnostic capability but also circumvents some of the limitations inherent in conventional techniques. Given the considerable prevalence of SCA36 revealed in this study, incorporating this test into the routine diagnostic panel for SCAs could significantly improve the diagnostic yield, particularly in the Korean patient population. Furthermore, these findings emphasize the necessity for increased awareness and recognition of SCA36 among healthcare professionals, which could facilitate earlier diagnosis and improve patient management.