Octapeptide repeat alteration mutations of the prion protein gene in clinically diagnosed Alzheimer's disease and frontotemporal dementia
Abstract
Studies focusing on octapeptide repeat alteration mutations in PRNP in Alzheimer's disease (AD) and frontotemporal dementia (FTD) cohorts have been rare. We aim to screen sporadic AD and FTD patients with unknown etiology for the octapeptide repeat insertions and deletions in PRNP. Two hundred and six individuals were screened for alterations to the repeat region in the PRNP gene, including 146 sporadic AD and 60 sporadic FTD patients. Our study showed a 1.5% (3/206) occurrence of the octapeptide repeat alteration mutations in PRNP in a Chinese cohort of sporadic dementia. One late-onset FTD patient and one early-onset AD patient each had a two-octapeptide repeat deletion in PRNP, while one early-onset AD patient had a five-octapeptide repeat insertion mutation. PRNP octapeptide repeat alteration mutations are present in sporadic AD and FTD patients. The genetic investigation for PRNP octapeptide repeat alteration mutations in sporadic dementia patients should be carried out in future clinical studies.
1 INTRODUCTION
The majority of sporadic Alzheimer's disease (AD) and frontotemporal dementia (FTD) cases are unexplained genetically,1, 2 indicating that understudied genetic factors remain to be identified. Genetic prion diseases (gPrDs) are caused by autosomal-dominant mutations in the prion protein gene (PRNP).3, 4 Octapeptide repeat alteration mutations in PRNP are located in a repeating oligopeptide coding sequence, which consists of 27 base pairs (bp) nonapeptide (R1) followed by four 24-bp octapeptide repeats (R2, R2, R3, and R4) with slight variations at the nucleotide level.5 Octapeptide repeat alteration mutations may alter the copper-binding properties of the region and promote the formation of protease-resistant mutant prion protein aggregates.6 Most octapeptide repeat insertion (OPRI) mutations manifest as a genetic Creutzfeldt–Jakob disease (gCJD) or Gerstmann–Straussler–Scheinker (GSS) phenotype.3 In addition, two octapeptide repeat deletions (OPRD) have also been reported as pathogenic variants in patients with gCJD.7-9
To our knowledge, there are no reports focusing on sporadic dementia patients with PRNP OPRI/D mutations. In this study, we aim to screen sporadic AD and FTD patients for OPRI/D mutations in PRNP to elucidate their missing genetics.
2 METHODS
This study included 146 sporadic AD patients and 60 sporadic FTD patients. The demographic features of these cases are shown in Table 1. See also Material and Methods in Data S1.
| Variable | AD | AD | FTD | FTD | ||
|---|---|---|---|---|---|---|
| Early-onset AD | Late-onset AD | Early-onset FTD | Late-onset FTD | |||
| Cases, n | 146 | 134 | 12 | 60 | 47 | 13 |
| Sex (male %) | 59 (40.4%) | 55 (41.0%) | 4 (33%) | 27 (45%) | 21 (44.6%) | 6 (46.1%) |
| Age (years) | 59.9 ± 7.2 | 58.7 ± 6.4 | 71.7 ± 3.1 | 61.3 ± 10.4 | 57.3 ± 9.6 | 71.2 ± 3.0 |
| Age at onset (years) | 56.3 ± 7.4 | 55.0 ± 6.4 | 69.1 ± 2.7 | 58.8 ± 10.1 | 54.9 ± 9.2 | 68.6 ± 3.0 |
| Minimum disease durationa (years) | 3.6 ± 2.9 | 3.7 ± 2.9 | 2.6 ± 1.7 | 2.6 ± 1.8 | 2.5 ± 1.8 | 2.6 ± 1.7 |
| MMSE score | 14.7 ± 7.5 | 14.4 ± 7.4 | 18.2 ± 6.9 | 17.8 ± 6.9 | 18.2 ± 7.1 | 16.8 ± 6.3 |
| MoCA score | 9.7 ± 7.0 | 9.4 ± 7.0 | 12.7 ± 6.1 | 11.5 ± 6.4 | 11.8 ± 6.7 | 10.5 ± 5.7 |
- Abbreviations: AD, Alzheimer's disease; FTD, frontotemporal dementia; MMSE, mini-mental state examination; MoCA, Montreal Cognitive Assessment.
- a Minimum disease duration is the span of time from the onset of the disease to the last follow-up in most cases of our cohort.
3 RESULTS
3.1 Genetic study
We first performed whole exome sequencing (WES) of genomic DNA from 206 dementia patients, but we could not find any mutations in PRNP. The 206 dementia patients were then screened for alterations to the repeat region in the PRNP gene. Size fractionation by agarose electrophoresis of the nested polymerase chain reaction (PCR) products led to the discovery of a patient with a five-octapeptide repeat insertion (5-OPRI) and two patients with two-octapeptide repeat deletions (2-OPRD) from 206 patients (Figure S1). In 206 sporadic dementia patients, we found a 1.5% occurrence of alterations to the repeat region, a substantial difference from the occurrence in the general East Asian population.
Amplification of the octapeptide repeat region of PRNP in Patient 1 by nested PCR revealed the presence of an insertion mutation in one of the PRNP alleles (Figure 1A). The sequence analysis showed a mutated allele carrying a 120-base pair insertion consisting of five octapeptide repeats. The repeats of the mutant allele were arranged as follows: R1-R2-R2-R3g-R3g-R3g-R3g-R2-R3-R4 (Figure 1B). Similarly, a rare heterozygous deletion of two octapeptide repeats was identified in Patient 2 and Patient 3, respectively (Figure 1A). The sequence analysis revealed that the repeat arrangement of the mutated allele was R1-R2-R4c in Patient 2 (Figure 1C), and R1-R2-R4 in Patient 3 (Figure 1D).
We could not find any variations of the causative genes associated with dementia for the three patients on WES study (Data S1: Material and Methods). Finally, the complete PRNP open reading frame was reconfirmed by Sanger sequencing in 206 dementia patients. No single base pair variations or small indels were detected. All three patients with OPRI/D in PRNP were found to be homozygous for methionine at codon 129 [c.385A (p.129M), NM_000311.5].
3.2 Patient 1 (5-OPRI)
The patient, a 55-year-old female, presented with progressive short-term memory loss for 18 months. At the same time, personality changes characterized by dampened emotions, taciturnity, lack of motivation, and poor personal hygiene behavior were observed. Brain MRI revealed atrophy in the bilateral temporal lobes and hippocampi, with no specific abnormalities noted in the diffusion-weighted image (DWI). 18F-FDG-PET indicated a significant hypometabolism in bilateral parietal and temporal lobes (Figure 1E–G). Cerebrospinal fluid (CSF) analyses showed a decreased Aβ-42/Aβ-40 ratio. The patient was diagnosed with probable AD.10 On a repeated electroencephalogram (EEG) recorded at 21 months after onset, triphasic waves against a slowed background rhythm were noted.
3.3 Patient 2 (2-OPRD)
A 74-year-old woman presented with progressive language problems and personality change for 4 years. She began to experience word-finding difficulties, speech reduction, and personality change at the age of 70. The patient became less restrained with behaving obsessively and acquired 7 kg in weight in 3 months due to hyperphagia. Language function assessment at age 71 revealed fluency aphasia. CSF t-tau level was increased. With no specific abnormalities found in DWI, cranial MRI revealed mild atrophy of the right temporal lobe. The 18F-FDG-PET also showed mild hypometabolism in the aforementioned areas (Figure 1H–J). A diagnosis of FTD was made.11 At the age of 72, her language capacity continued to diminish with very limited comprehension. At the age of 73, she was already non-verbal, but could still recognize familiar faces. After our genetic study, the presence of a misfolded pathogenic form of the prion protein (PrPSc) was detected using a real-time quaking-induced conversion (RT-QuIC) assay on the CSF.
3.4 Patient 3 (2-OPRD)
The patient, a 70-year-old woman, first complained of visual disturbances at 59 years old. This disturbance manifested as object agnosia and difficulties telling the time and reading clocks. At the age of 65, she developed memory loss, visuospatial confusion, and a progressive decline in reading and writing. The patient showed impairment in daily tasks, forgot how to operate a phone, became lost while walking in familiar surroundings, and was found with clothes on backward at the age of 68. On examination at the age of 70, severe simultanagnosia and prosopagnosia were revealed. Brain MRI showed bilateral parietal and occipital cortex atrophy with no specific abnormality detected in the DWI (Figure 1K,L). EEG showed generalized slowing of brain activity. The patient was initially identified as having a visual variant of AD.10
The clinical information for the three patients is summarized in Table 2.
| ID | Sex | Family history | Clinical diagnosis | AAO (year) | Disease duration | Symptoms | Neurological examination | MMSE/Moca/CDR | Brain MRI | CSF | Ethnicity | PRNP Mutation |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Patient 1 | Female | No | AD | 54 | 18 months (alive) | Progressive short-term memory loss, personality changes | Cognitive deficits | 9/2/2 | Atrophy in the bilateral temporal lobes and hippocampi | Aβ-42: 704.6 pg/mL Aβ-40: 12244.4 pg/mL t-Tau: 53.2 pg/mL p-Tau: 27.3 pg/mL | Han Chinese | 5-OPRI |
| Patient 2 | Female | No | FTD | 70 | 4 years (alive) | Word-finding difficulties, personality change, hyperphagia | Cognitive deficits, fluency aphasia | 25/20/1 | Mild atrophy of the right temporal lobe | Aβ-42: 1204.8 pg/mL Aβ-40: 11036.3 pg/mL t-Tau: 771.4 pg/mL p-Tau: 65.6 pg/mL | Han Chinese | 2-OPRD |
| Patient 3 | Female | No | AD | 59 | 11 years (alive) | Visual disturbance, memory loss, visuospatial confusion, reading and writing disability | Cognitive deficits, object agnosia, simultanagnosia, prosopagnosia | 5/2/2 | Bilateral parietal and occipital cortex atrophy | Not done | Han Chinese | 2-OPRD |
- Abbreviations: AAO, age at onset; AD, Alzheimer's disease; CDR, global Clinical Dementia Rating; CJD, Creutzfeldt–Jakob disease; CSF, cerebrospinal fluid; EEG, electroencephalogram; FTD, frontotemporal dementia; MMSE, mini-mental state examination; MoCA, Montreal Cognitive Assessment; MRI, magnetic resonance imaging; RT-QuIC, real-time quaking-induced conversion.
4 DISCUSSION
To date, over 40 sporadic or familial cases associated with PRNP mutations all over the world were reported to be initially diagnosed as AD or FTD (Table S1). The mutations found in AD or FTD carriers were also frequently identified in patients with gPrDs.12, 13 The individuals diagnosed with AD or FTD with PRNP mutations are more likely to present auxiliary features of prion disease, including seizures, ataxic gait, myoclonus, and pyramidal/extrapyramidal signs. Frequently, these auxiliary symptoms appear in the middle or late stages of the disease.5, 14 Therefore, it may be difficult to clinically identify AD and FTD patients with PRNP OPRI/D mutations at an early stage.
OPRI and ORPD have highly heterogeneous clinical presentations and prion diseases are well-known clinical imitators of AD and FTD.15 To date, the frequency of PRNP octapeptide repeat alterations in sporadic AD or FTD patients remains poorly understood.13 The three patients identified in our study suggest that they might be more common than previously thought. Nearly 90% of dementia patients in our cohort have an early-onset age, which may partially explain the relatively high occurrence of mutations. However, as the patients with known monogenic causes range only from 5% to 15% in total sporadic AD and FTD patients,1, 2 investigation of the human PRNP octapeptide repeats should be recommended in clinically diagnosed sporadic AD and FTD patients. De novo mutations, incomplete penetrance, and insufficient clinical assessment of parents could be the most common reasons for sporadic patients with PRNP OPRI/D mutations.
Before the genetic study was performed, none of our three patients with PRNP octapeptide repeat alteration mutations fulfilled the diagnostic criteria for gCJD (Table 2).4 None of them presented any symptoms of cerebellar signs, myoclonus, akinetic mutism, or extrapyramidal abnormalities. Brain MRI of all three patients revealed no typical patterns of restricted diffusion on DWI (Figure 1). According to the current diagnostic criteria for familial CJD substantiated by the Centre for Disease Control and Prevention (CPC): “Neuropsychiatric disorder plus disease-specific PrP gene mutation” (2018, https://www.cdc.gov/prions/cjd/diagnostic-criteria.html), the clinical diagnosis of the three cases in our study could be revised to inherited prion diseases after the genetic study. It is worth noting, however, that the clinical phenotype of OPRI mutations does not fit easily into the canonical CJD and GSS syndromes. The phenotype of a larger insertion (5 or more) is typically a slowly progressive dysexecutive syndrome associated with defects of the parietal lobe and particularly dyspraxia.3 Therefore, we suggest the use of the term “gCJD” should be restricted to the presence of a PRNP mutation and a rapidly progressive phenotype as in sporadic CJD, which does not apply to any of our cases.
2-OPRD has been recognized as a pathogenic variant for over 20 years, but very few reports are available.7-9, 16 Three prion disease cases with 2-ORPD were discovered in the Netherlands from 1993 to 2013.16 2-OPRD as the monogenetic cause of gPrDs has been reported in detail in two prior cases with the classic phenotype of gCJD.7, 8 The disease durations of both patients reported previously were less than 2 years (18 and 23 months). Both presented with rapidly progressive dementia with pyramidal/extrapyramidal signs or myoclonus. In contrast, Patient 2 and Patient 3 in our study had long disease durations without any auxiliary features of prion disease (Table 2). It is noteworthy that Patient 2 and Patient 3 are from different regions of China that are very far apart, and their repeat arrangement of the mutant allele differs by one nucleotide (R4 and R4c). There is thus little chance that Patient 2 and 3 have a recent common ancestor.
Technically, WES and ordinary Sanger sequencing could not detect insertions with four or more octapeptide repeats in PRNP.5 The PRNP octapeptide repeat alteration mutations in sporadic dementia patients may be underdiagnosed. A limitation of this study was that we could not perform a brain biopsy on the patient. In addition, further functional studies are needed to determine how these mutations exert their pathogenic effect.
In summary, our study showed a 1.5% occurrence of the octapeptide repeat alteration mutations in PRNP in Chinese sporadic dementia patients. Future clinical studies should investigate sporadic dementia patients for OPRI/D mutations in PRNP.
AUTHOR CONTRIBUTIONS
Haitian Nan and Liyong Wu designed and conceptualized the study. Li Liu, Zhongyun Chen, Min Chu, Yihao Wang, Jieying Li, and Donglai Jing provided the patients of the study. Haitian Nan performed the genetic study. Haitian Nan and Liyong Wu drafted and revised the manuscript. The authors have read and approved the final manuscript.
ACKNOWLEDGMENTS
The authors appreciate all cohort individuals and their families for their participation in this study.
FUNDING INFORMATION
This work was supported by grants from National Natural Science Foundation of China [no. 82201573] and Beijing Postdoctoral Research Foundation [2022-ZZ-015].
CONFLICT OF INTEREST STATEMENT
The authors declare no competing interests.
ETHICS STATEMENT
Written informed consent for publication was obtained from the guardian of each patient.
Open Research
DATA AVAILABILITY STATEMENT
All data relevant to the study are included in the article or uploaded as Supporting Information.
