以嗅覺測驗、黑質超音波影像和多巴胺系統核子影像[18F]-dopa PET作為巴金森氏病運動前期診斷工具來檢測LRRK2 G2385R變異帶因者之病例對照研究

Abstract

巴金森氏病(簡稱巴病)是僅次於阿茲海默症最常見的神經退化性疾病。其致病機轉是由於位在中腦黑質的突觸前多巴胺神經元的退化，造成投射至紋狀體的多巴胺減少而導致運動障礙。座落在染色體12q12上，由51個exons組成的leucine-rich repeat kinase 2 (簡稱LRRK2)基因，是顯性遺傳巴病最常見的致病基因，也是目前和傳統老年型巴病病患最相關的一個基因。在漢人的基因相關性研究(genetic association study)中發現，LRRK2 G2385R變異是漢人中獨特的巴病基因危險因子。在臨床表現上，帶有LRRK2 G2385R變異的巴病患者和典型偶發性巴病患者幾乎無法區分。在體外的細胞實驗中發現，在有外來毒物的環境下G2385R變異會加速細胞凋亡。病理的研究顯示運動障礙症狀出現時，約有60%的多巴胺神經元已凋亡，由此可知運動前期的診斷(premotor diagnosis)對於巴病的預防以及神經保護藥物的發展甚為重要。病理研究亦顯示巴病神經元的退化起始於延腦與顱底的嗅覺構造，而在臨床上嗅覺測驗工具UPSIT可檢測出早期的嗅覺功能異常。再者，運用多巴胺系統核子影像工具如18F-dopa PET，可發現多巴胺系統在影像上所呈現的變化要比運動障礙症狀的出現提早4-6年。此外，更有學者指出縱使臨床尚未出現運動障礙的症狀，穿顱超音波影像上出現的黑質hyperechogenicity可作為多巴胺系統的易感標記。基於以上的備景說明，我們進行此ㄧ病例對照研究(case-control study)。研究之目在於，以帶有漢人族群特有巴病的基因危險因子LRRK2 G2385R變異的無症狀帶因者為對象，評估嗅覺測驗、多巴胺系統核子影像18F-dopa PET和黑質之穿顱超音波影像在巴金森氏病運動前期診斷的價值。 本研究中共收錄了526健康受試者進行基因篩檢，其中有24位是G2385R變異的帶因者 (男性6位，女性18位)，占所有健康志願者的4.6%。在帶因者中符合納入條檢者有15位，並同時收錄15位年齡與性別相符之非帶因者作為對照組。本研究發現華人特有巴病危險因子LRRK2 G2385R變異在巴病的運動前期可能導致多巴系統的退化。同時，也發現多巴胺核子影像工具18F-dopa PET的被殼指標不論在巴病的診斷或是運動前期的運用上都是較為敏感的指標。和對照組相較，在G2385R變異帶因者的運動前期，並未發現明顯的嗅覺異常或代表鐵質代謝異常的黑質hyperechogenicity的情形，暗示前述兩者運動前期的臨床表現可能與LRRK2 G2385R變異無關，亦獨立於多巴胺系統之外。此外，在G2385R帶因者中，某些在運動前期已出現多巴胺系統退化現象、嗅覺異常或是帶有黑質hyperechogenicity的受試者，則需進ㄧ步追蹤日後是否出現巴金森氏病的症狀，以期能早期治療。

Parkinson’s disease(PD) is a relentlessly progressive neurodegenerative disorder caused by loss of dopaminergic neurons in the substantia nigra(SN). It is characterized by motor symptoms of bradykinesia, rest tremor and rigidity. Mutations in the gene leucine-rich repeat kinase 2 (LRRK2), located on chromosome 12p11.2-q13, are the most frequent causes of familial autosomal dominant PD. Recently genetic association studies identified LRRK2 G2385R variant is a common risk polymorphism for sporadic PD in the Chinese population. In vitro studies of transfected cells have proved that G2385R variant iss more vulnerable to oxidative stress and associated with a higher rate of apoptosis. Pathological studies have shown that at the time when PD motor symptoms appear, a significant amount with approximate 60% of dompaminergic neurons in the SN has already occurred. In the clinical aspects, hyposmia is considered as premotor sign of PD. Degeneration in the olfactory system has been also proved pathologically to be early in the stage of this degenerative disease. Moreover, based on in vivo PET and SPECT imaging of the dopaminergic system, the onset of dopaminergic neuronal loss antedates the motor symptoms of PD by approximately 4 to 6 years. Recently increased echogenicity of SN detected by transcranial sonography has been related to a functional impairment of the nigrostriatal system. It has also been proved to be a susceptibility marker for the development nigral injury that can be detected early in life prior to the onset of motor symptoms. These facts signify the importance of early diagnosis of PD in the premotor phase, in which neuroprotective measures can possibly intervene. With this background, we conducted this case-control cohort study in a hospital based cohort. We intended to evaluate various tools of premotor diagnosis with olfactory test, 18F-dopa PET and transcranial sonography of SN in the healthy subjects carrying LRRK2 G2385R variant, which has been proved to be specific for ethnic Han Chinese population. 526 healthy subjects were recruited from the PD center of National Taiwan University hospital for genetic screening of LRRK2 G2385R polymophism. Twenty four of them are G2385R variants. The prevalence rate of G2385R polymophism in our study group is 4.6%, which is similar to previous studies. Fifteen G2385R variants fulfilling the criteria of our study were then included for premotor diagnosis study. Meanwhile, fifteen aged and sex matched non-variants were included as control group. In the study of 18F-dopa PET, G2385R variants manifested a non-significant tendency to have decreased 18F-dopa uptake in the striatum, particularly in bilateral putamens. Our study has also proved that putaminal indicator of 18F-dopa PET imaging is a more sensitive indictor than the caudate indicator in terms of both clinical diagnosis and premotor diagnosis of PD. Furthermore, the G2385R variants did not show hyposmia or SN hyperechogenicity in comparison to the control group. It suggested that these two aforementioned premotor signs of PD seemed not related to LRRK G2385R polymophism. We should further follow particular subjects in this study, who presented with hyposmia, dopaminergic dysfunction shown by 18F-dopa PET, or SN hyperechoenicity to see if motor symptoms merge in the future and take in-time therapeutic intervention if PD is once diagnosed.