Expansion of clinico-genetic spectrum of PRDX3 disease: a literature review with two additional cases

Graphical Abstract Graphical Abstract


Graphical Abstract Introduction
Peroxiredoxin III (PRDX3) is a crucial antioxidant protein that plays a vital role in the mitochondria by detoxifying reactive oxygen species.Recently, a study shed light on the bi-allelic variations found in the PRDX3 gene that causes spinocerebellar ataxia, autosomal recessive type 32 [SCAR32; Online Mendelian Inheritance in Man (OMIM) #619862], often accompanied by characteristic MRI signal changes in the infratentorial region. 1,23][4] In this report, we aim to further expand the genomic and clinical spectrum of PRDX3 disease by presenting two unrelated Korean patients with cerebellar ataxia.These patients exhibited both novel compound heterozygous variants and distinct infratentorial MRI signal changes, further contributing to the growing body of knowledge on this rare disease.

Materials and methods
Whole-exome sequencing (WES) was performed for 2 unrelated probands including 1 trio and 1 proband-only case from 154 undiagnosed ataxia patient cohorts.The detailed method for data generation and processing was described elsewhere. 5Briefly, generated reads were aligned to the human reference genome hg38 and processed according to the best practice of Genome Analysis Toolkit. 6The ANNOVAR program was used to annotate genomic information 7 ; population allele frequencies were obtained from the Genome Aggregation Database (gnomAD) 8 and Korean Variant Archive 2 (KOVA2). 9Variants were interpreted based on the American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) criteria.Informed consent was obtained from the participants according to the Declaration of Helsinki, and the study was approved by the Institutional Review Board of Seoul National University Hospital (IRB No.1406-081-588).

Clinical presentations and MRI findings
Patient 1 is a 46-year-old Korean male patient.His older sister had no neurological symptoms and reported no other familial history (Fig. 1A).He did not exhibit any signs of motor or cognitive developmental delay during his childhood.At the age of 25, he began to experience both hand tremors and gait ataxia, which later progressed to dysarthria at the age of 35.At age 45, the patient visited our clinic due to a worsening of hand tremors, gait ataxia and dysarthria.During the neurological examination, Babinski's sign was present but deep tendon reflexes were within the normal range.Routine cerebellar function tests showed normal results, but bilateral swaying gait was observed.No abnormal laboratory findings were observed, including in the cerebrospinal fluid (CSF) exam.The patient did not exhibit hypokinetic/hyperkinetic movements or any sensory/cognitive/ hearing impairments during the last follow-up at the age of 46.The Scale for the Assessment and Rating of Ataxia (SARA) score was 6 at age 46, as shown in Table 1.In the MRI taken at age 45, T2 hyperintensity in reticular formation and dentate nucleus as well as transverse stripes of the pons were observed (Fig. 1B).
Patient 2 is a 24-year-old Korean female patient.Her younger sister had no neurological symptoms and reported no other family history (Fig. 1A).Ataxic gait was prominently observed from the age of 11 years, followed by the onset of both hands tremors at age 13 and dysarthria at age 15.She exhibited mild cognitive impairment at the age of 13, as indicated by a fullscale intelligence quotient (FSIQ) score of <75.No definite motor delays were noted during her early development.The patient's neurological symptoms stopped progressing at age 19 and remained stationary thereafter.At age 24, the patient had a SARA score of 8 (Table 1).The patient did not exhibit hypokinetic/hyperkinetic movements or any sensory/cognitive/hearing impairment during the last follow-up at the age of 24.Imaging studies revealed a T2 hyperintensity in the reticular formation, and dentate nucleus, as well as transverse stripes of the pons, was observed (Fig. 1B).

Genetic findings
Targeted sequencing for known 41 ataxia genes and evaluation for spinocerebellar ataxia (SCA) short-tandem repeats including SCA1, 2, 3, 6, 7, 8 and 17, and dentatorubropallidolusian atrophy (DRPLA) provided negative results for both patients.To further investigate the genetic basis of their condition, we performed WES and analysed ∼7,000 Mendelian disorders reported in the OMIM database.
For Patient 1, our analysis revealed two compound heterozygous variants in the PRDX3 gene located in trans conformation: c.508C > T, p.Arg170Ter, a known disease-causing variant found in previous studies in homozygous alleles, 1 and a novel variant c.512A > G, p.Asp171Gly, which was located 4 bp away from the reported nonsense variant.The configuration of these two alleles could be identified without family studies, as they were phased in different reads (see Supplementary Fig. 1).Initially, the novel variant (p.Asp171Gly) was classified as a variant of uncertain significance (VUS).However, subsequent computational predictions strongly suggested a highly deleterious effect for this variant, as indicated by a Combined Annotation Dependent Depletion (CADD) score of 27.6.Furthermore, analysis of evolutionary conservation revealed that the amino acid residue at position 171 is highly conserved among various species, as denoted by a GERP++ score of 5.19.Considering these in silico predictions, coupled with the presence of phenotypic similarities, distinctive MRI findings and segregation analysis, we reclassified this variant as a likely pathogenic variant.For Patient 2, we conducted a trio WES, and we identified two nonsense variants in the PRDX3 genes.Our analysis confirmed that the previously reported p.Arg170Ter variant was inherited from her father, whereas the novel p.Arg207Ter variant was inherited from her mother.Her asymptomatic younger sister was not evaluated for genetic testing.Population frequencies of all reported pathogenic variants to date based on gnomAD, KOVA2 and our in-house database of 4,880 Koreans are in Supplementary Table 1.

Discussion
Since the initial description of PRDX3 disease in 2021, nine patients with varying onset ages ranging from infantile onset to 55 years have been reported.All of these patients presented with gait ataxia, and most eventually developed upper limb ataxia, dysarthria and/or dysphagia.The disease appeared to progress in a caudal-to-rostral spatial pattern, as suggested by Rebelo et al. 1 as well as the patients in this study.Most patients displayed oculomotor signs, with a few also exhibiting hypokinetic features, hyperkinetic features and muscle weakness.Sensory, cognitive and hearing impairments as well as pyramidal signs during neurological examination were rarely reported (Table 1).Our study reports two additional patients who displayed caudal-to-rostral spatial disease spread, with one showing mild cognitive impairment and the other exhibiting pyramidal signs.Although most patients displayed a 'neurodegenerative' phenotype, a single case with a 'neurodevelopmental' component was reported, wherein the overall disease severity trajectory improved slowly over time. 1 Patient 2 also displayed a caudal-to-rostral spatial spread of disease; however, the overall disease severity trajectory showed improvement as no symptoms worsened after the age of 19.The inclusion of our patient with an improving phenotype provides strong evidence for expanding the phenotypic spectrum of PRDX3 disease.Only a few reports have described the MRI features of PRDX3 disease, including T2 signal changes in the reticular formation, dentate nucleus and transverse stripes of the pons, which are not typically seen in other hereditary degenerative ataxia.Our patients also exhibited distinctive T2 signal changes which therefore strengthen the MRI spectrum of PRDX3 disease.
The genetic findings identified in our study indicated that the p.Arg170Ter variant is the most frequent pathogenic hit (three different cases have been identified including our two cases) in the PRDX3 disease (Supplementary Table 1).Additionally, our study has revealed two novel variants, p.Asp171Gly and p.Arg207Ter, in the Korean population.Knockdown of PRDX3 in a cellular model results in reduced levels of glutathione peroxidase activity-elucidating its key role in detoxifying pathways of H 2 O 2 .The importance of reactive oxygen species in the pathomechanism of many neurodegenerative disorders suggests the importance of its pathway.A single case of clinical improvement after treatment with idebenone in a patient with homozygous TXN2 variants, a gene closely related to the efficient cycling of PRDX3, suggests a possible therapeutic option may exist in PRDX3 disease patients and calls for increased awareness of the disease. 10n this study, we aimed to expand the understanding of PRDX3 disease by investigating its phenotypic and imaging spectrum.Our findings not only contribute to a better characterization of the disease but also highlight the need for increased awareness of PRDX3 disease in undiagnosed cerebellar ataxia patients.While the majority of cerebellar ataxia cases follows an autosomal dominant inheritance pattern, patients diagnosed with recessive PRDX3 disease may feel relieved that the disease is not likely to be passed on to their offspring.This underscores the importance of raising awareness about the disease, as early diagnosis and treatment can potentially help affected individuals both in genetic counselling and exploring possible therapeutic options.To our best knowledge, this is the first report of PRDX3 disease in the East Asian population.PRDX3 disease should be strongly suspected in cerebellar ataxia patients with caudal-to-rostral spatial spread pattern and/or distinctive infratentorial MRI findings regardless of one's ethnical background.

Figure 1
Figure 1 Radio-genomic findings of ataxia patients with PRDX3 disease identified in this study.(A) Pedigree and variant information of our patients.(B) Brain MRI findings of Patients 1 and 2. Transverse stripes of the pons are marked with arrows in both patients (upper panel).T 2 hyperintensity of the reticular formation and dentate nucleus are marked with arrows in both patients (lower panel).