Genome-Wide Analysis of the Parkinsonism-Dementia Complex
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ORIGINAL CONTRIBUTION Genome-Wide Analysis of the Parkinsonism- Dementia Complex of Guam Huw R. Morris, MB, PhD; John C. Steele, MD; Richard Crook, BSc; Fabienne Wavrant-De Vrièze, PhD; Luisa Onstead-Cardinale, BS; Katrina Gwinn-Hardy, MD; Nick W. Wood, MD, PhD; Matthew Farrer, PhD; Andrew J. Lees, MD; P. L. McGeer, MD, PhD; Teepu Siddique, MD, PhD; John Hardy, PhD; Jordi Perez-Tur, PhD Background: Parkinsonism-dementia complex (PDC) went conventional linkage analysis in 5 families with PDC. is a neurofibrillary tangle degeneration involving the depo- One marker, D20S103, generated a logarithm of odds score sition of Alzheimer-type tau, predominantly in the me- of greater than 1.5. Multipoint association analysis also sial temporal cortex, brainstem, and basal ganglia. It oc- highlighted 2 other areas on chromosome 14q (adjacent curs in focal geographic isolates, including Guam and the to D14S592, 59.2 megabases [M]) and chromosome 20 Kii peninsula of Japan. The familial clustering of the dis- (adjacent to D20S470, 17.4 M) with multipoint associa- ease has suggested that a genetic factor could be impor- tion logarithm of the odds scores of greater than 2. The tant in its etiology. areas around D20S103, D14S592, and D20S470 were fur- ther analyzed by association using additional microsat- Objective: To determine whether a genetic locus could ellite markers and by conventional linkage analysis. This be identified, linked, or associated with PDC. did not provide further evidence for the role of these areas in PDC. Design and Patients: We performed a genome-wide association study of 22 Guamanian PDC and 19 control Conclusions: This study has not identified a single subjects using 834 microsatellite markers with an ap- gene locus for PDC, confirming the impression of a geo- proximate genome-wide marker density of 4.4 centi- graphic disease isolate with a complex genetic, a genetic/ morgans. environmental etiology, or a purely environmental etiology. Results: Two-point association analysis identified 17 markers (PϽ.015). Each of these markers then under- Arch Neurol. 2004;61:1889-1897 FTER WORLD WAR II, US clarified by the identification of genetic military physicians as- mutations in affected autosomal domi- signed to Guam and Na- nant families.10 These genetic advances and tional Institutes of Health the continuing familial occurrence of PDC (NIH) scientists recog- on Guam led to the consideration of a ge- Anized that familial amyotrophic lateral scle- netic approach to this disease.11 A num- rosis (ALS) and atypical parkinsonism with ber of observations suggest that PDC may dementia (parkinsonism-dementia com- have a genetic etiology. The disease clus- plex [PDC]) were common among Cha- ters in some Guamanian Chamorro fami- morro living on Guam.1-4 These diseases lies but spares others.12-14 The disease has had been present on Guam for more than different prevalences in different vil- a century and were known as lytico, for pa- lages.15,16 The adjacent villages of Umatac ralysis, and bodig, for parkinsonism. It was and Merizo in southern Guam have simi- hoped that finding the cause of these dis- lar environments and their residents have eases would help to explain the causes of identical lifestyles, yet from 1956 to 1965, neurodegenerative diseases in the rest of the prevalence of PDC in Umatac was 274 the world. During the past 50 years, cy- per 100000 population and only 42 per cad neurotoxicity, infectious agents, and 100000 population in Merizo.15,17,18 Tra- calcium deficiency have been investi- ditionally, the populations of these vil- gated as possible causes for PDC but not lages did not intermarry. The disease may been proved to be relevant.5-9 occur in Chamorro migrants many years In recent years, the etiology of familial after they have left Guam.19-21 The dis- Author Affiliations are listed at Alzheimer disease, Parkinson disease, and ease often occurs in successive genera- the end of this article. frontotemporal dementia (FTD) has been tions and affects first-degree relatives. One (REPRINTED) ARCH NEUROL / VOL 61, DEC 2004 WWW.ARCHNEUROL.COM 1889 ©2004 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/29/2021 Chamorro family in Umatac, southern Guam, is re- an immigrant population that now numbers 80000.30 Con- corded to have experienced the disease in every genera- temporary Chamorros are descended from a genetic tion since the late 1700s.2 Furthermore, a recent 40- “bottleneck” occurring 300 years (approximately 13 gen- year follow-up of a National Institutes of Health case- erations) ago, although a substantial part of the subse- control cohort shows that having an affected first- quent population growth relates to immigration. Ge- degree relative remains a risk factor for development of netic analysis of this disease assumes that a common PDC.22 Parkinsonism-dementia complex occurs in Gua- founder introduced a pathogenic mutation at an ances- manian Chamorros, but it does not seem to have af- trally distant point. The founder’s affected descendants fected the other ethnic groups who have immigrated to would share the pathogenic mutation and an adjacent Guam since World War II.23 Thus, among the many hun- chromosomal region. Genotyping of cases and control dreds of pathologically verified cases of this disease in subjects to determine shared alleles among cases should Chamorros during the past 50 years, there is only 1 clini- identify the area in which the pathogenic mutation lies. copathologic report of possible PDC in a Filipino mi- The strength of the association depends on the back- grant.24 In recent decades, the prevalence of PDC has ground frequency of the disease-associated allele in the steadily declined and the age at onset has increased.25-27 control population and decays with increasing distance This may support an exposure to a causal environmen- from the disease mutation, owing to meiotic recombi- tal factor with a cohort effect. Alternatively, this may re- nation. The background allele frequencies are un- flect increased social mobility and marrying out, thereby known a priori and may depend on whether the muta- allowing a decline in the co-occurrence of recessive ge- tion has arisen on an indigenous chromosome or has been netic factors, and/or a decrease in concurrence between introduced by an immigrant to the population. a genetic predisposition and environmental cofactors. A The evidence suggesting a genetic cause for PDC and previous report from our group has excluded the most the population structure of the affected Guamanian Cham- likely candidate gene for PDC, tau, by means of direct orro population led us to further investigate a genetic cause sequencing and microsatellite marker association.28 In the for PDC using an association analysis method. The over- present study, we have used linkage disequilibrium to lap between PDC and ALS is debated, but ALS has de- search for a PDC locus. clined in prevalence on Guam much more rapidly than Population-based disequilibrium studies involve map- PDC, such that there is only a handful of cases on the ping a disease locus by association at adjacent polymor- island at present, and ALS does not have a significantly phic genetic markers. They depend on a single gene mu- greater prevalence of disease on Guam than elsewhere. tation arising on a founder chromosome, accounting for Because of this, we decided to study PDC in isolation. a given disease in the population under study. This ap- We identified 22 apparently unrelated PDC cases and 19 proach has been used successfully in genetically iso- unrelated, neurologically asymptomatic controls for the lated populations, because they are more likely to have case-control study and selected 5 families for confirma- a single disease/single gene and mutation relationship (ge- tory linkage studies. Cases, controls, and family mem- netic and allelic homogeneity).29 They are also more likely bers underwent evaluation and selection by clinical neu- to have a sufficiently large shared chromosomal seg- rologists (J.C.S., H.R.M., K.G-H., and A.J.L.), and autopsy ment around any given marker and around a single dis- confirmation of PDC was subsequently obtained in 3 cases. ease gene mutation, depending on the age of the popu- lation and time of introduction of the mutation. An METHODS isolated population is defined as a population that is de- rived from a limited number of founder members and has expanded through growth rather than immigration. The CASE ASCERTAINMENT Chamorro population of Guam originated from a small number of immigrants from the Indo-Malay peninsula. All 22 cases were Chamorros with neurodegenerative symp- By the time of Magellan’s arrival in Guam in 1521, the toms beginning at ages that ranged from 54 to 74 years (de- population was 100000. However, with Spanish coloni- tailed demographic and clinical material on the 22 cases is avail- zation in the late 17th century, the native population was able from the authors on request). Eleven presented with dementia, and of these parkinsonism subsequently developed severely reduced to just 1608 by the early 1700s. The sur- in 10. Ten presented with parkinsonism, and of these demen- vivors, mostly women and children, were relocated to tia subsequently developed in 6. In the remaining case, demen- Catholic parishes on Guam, the largest and southern- tia (with paranoia) and parkinsonism occurred simulta- most island in the Mariana archipelago. Subsequently, neously. The parkinsonism usually had atypical features for the remaining Chamorro population mixed with Fili- Parkinson disease (ie, unresponsive to levodopa and involv- pino, Mexican, and Spanish immigrants who acted as ad- ing rigidity without rest tremor). In 10 of the 22 cases, fea- ministrators for the government of Guam and European tures of supranuclear gaze palsy, pseudobulbar palsy, and promi- sailors who visited the island during Pacific explora- nent axial rigidity were also seen.