Clinical features of LRRK2 parkinsonism

Parkinsonism and Related Disorders 15S3 (2009) S205–S208

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Parkinsonism and Related Disorders journal homepage: www.elsevier.com/locate/parkreldis

Clinical features of LRRK2 parkinsonism Kristoffer Haugarvolla , Zbigniew K. Wszolekb, * a Department b Department

of Neurology, Haukeland University Hospital, Bergen, Norway of Neurology, Mayo Clinic Jacksonville, Jacksonville, Florida, USA

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Keywords: Parkinson disease Genetics R1441C R1441G R1441H Y1699C G2019S I2020T R1628P G2385R

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene were initially identified in large families with autosomal dominant Parkinson disease (PD). These mutations (p.R1441C, p.R1441G, p.Y1699C and p.I2020T) revealed that genetic mutations could cause clinically typical, late-onset PD. Subsequently, the p.G2019S mutation was found to be a frequent cause of both autosomal dominant and “sporadic” PD, particularly in populations in North Africa or the Middle East. Two Lrrk2 protein substitutions (p.R1628P and p.G2385R) have since been associated with susceptibility to PD in Asian populations. More than a hundred variants have been identified in the LRRK2 gene, but pathogenicity is most convincing for the p.R1441H substitution. The role in PD remains unknown for other variants because segregation with disease has not been shown. Screening these variants in very large patient-control series may help clarify their role in PD. Lrrk2 is a large, multidomain protein with pathogenic mutations occurring in several functional domains. Cell biological experiments have shown that the p.G2019S mutation increase kinase activity. This is consistent with the observation that homozygous p.G2019S carriers do not have earlier disease onset or more severe disease compared with heterozygous carries. It is now necessary to identify the regulators and substrates of Lrrk2 in order to understand the effect of each LRRK2 mutation. The identification of a large number of presymptomatic LRRK2 mutation carriers provides a unique possibility for future studies on neuroprotection. However, more insight into the basic function of Lrrk2 is needed in order to exploit this potential for translational research. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Parkinson disease (PD) is a common neurodegenerative disease, expected to become significantly more prevalent as a large proportion of the population will reach advanced age [1]. Genetics may help identify the causes of disease, and hence help to unravel the molecular mechanisms underlying disease. This genetic contribution may be crucial in order to provide better treatment for future patients [2]. The PARK8 locus was mapped to chromosome 12q12 in a large Japanese kindred known as the Sagamihara family. The presentation in this family was consistent with autosomal dominant, late-onset Parkinson disease (LOPD) [3]. This lead to the positional cloning of missense mutations in the leucine-rich repeat kinase 2 (LRRK2) gene in several families with clinically typical LOPD [4–6]. LRRK2 is a large gene spanning 144 kb and encoding the Lrrk2 protein containing 2527 amino acids. Numerous missense variants have been identified in the LRRK2 gene over the last years. However, only five mutations lead to protein amino acid substitutions and cosegregate with disease in families (p.R1441C, p.R1441G, p.Y1699C, * Corresponding author. Zbigniew K. Wszolek, MD. Department of Neurology, Mayo Clinic Jacksonville, Cannaday Building 2E, 4500 San Pablo Road, Jacksonville, FL 32224, USA. 1353-8020/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.

p.G2019S and p.I2020T). In addition, the p.R1441H substitution is likely to be pathogenic as it is located adjacently to two pathogenic substitutions in an important functional domain of the Lrrk2 protein. Additionally, genetic association studies have identified two novel LRRK2 variants (p.R1628P and p. G2385R) that are consistently associated with PD in Asia [7–11], see Table 1. The p.G2019S substitution is of special significance as it is frequently identified not only in autosomal dominant, but also “sporadic” PD. Thus, being the most common cause of PD. The mutation is particularly frequent in PD patients residing in, or having genealogical ties to North Africa or the Middle East. This phenomenon can be explained by the fact that most Lrrk2 p.G2019S substitution carriers originate from a common founder [12–14]. Herein, we review the clinical feature of LRRK2 mutations and discuss future challenges. 2. Clinical and pathologic features of LRRK2 parkinsonism 2.1. The LRRK2 c.4321C>T (p.R1441C) mutation The p.R1441C substitution was identified in Family D (Western Nebraska) [5,15]. In an international collaboration we identified 33 affected and 15 unaffected p.R1441C substitution carriers [16]. Mean age of disease onset was 60 years (range, 39–79 years). The

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Table 1 Characteristics of Lrrk2 substitutions associated with parkinsonism Substitution

Protein domain

Age of onset

Main phenotype

Risk ethnicity

p.R1441C

Roc

30–79

PD

Middle European

p.R1441G

Roc

44–>70

PD

Basque

p.R1441H

Roc

32–66

PD

NA

p.Y1699C

COR

35–65

PD

NA

p.G2019S

MAPKKK(kinase)

~25–>90

PD

North Africa, Middle East

p.I2020T

MAPKKK(kinase)

38–74

PD

NA

p.R1628P*

COR

NA

PD

Chinese

p.R2385R*

WD40

NA

PD

East Asian

*Risk-associated variants. Roc: Ras of complex (GTPase). COR: C-terminal of Ras. MAPKKK: mitogen-activated protein kinase kinase kinase.

age-specific cumulative incidence revealed that <20% of carriers had developed PD by age 50 years, while >90% had developed symptoms by age 75 years. The clinical presentation was similar to typical “idiopathic” PD. However, one patient developed features resembling progressive supranuclear gaze palsy (PSP) 5 years after onset of disease, but levodopa response was present throughout the course of disease. There are several founders, but two main haplotypes explain most cases of p.R1441C parkinsonism [16]. Interestingly, all Belgian patients have a common founder and about 10% of familial PD in Belgium can be explained by the p.R1441C substitution [16,17]. Autopsies from four affected p.R1441C carriers from Family D members have been reported [18]. Cell loss and gliosis in the substantia nigra were found in all. Two cases had Lewy body disease (LBD) or diffuse LBD. A third case had no distinctive pathology. The fourth case with clinical PD, and PSP in the course of the disease revealed tau-positive neurofibrillary tangle pathology in the absence of LBD. 2.2. The LRRK2 c.4321C>G (p.R1441G) mutation The p.R1441G substitution was identified in 4 kindreds from the Basque region of Spain and in the British Family PL [6]. Patients have symptoms consistent with PD, including good response to levodopa therapy. This mutation is frequent in the Basque country, where it accounts for about 16% of familial and 4% of sporadic PD cases [19]. It is also prevalent in neighboring regions of northern Spain, however it is very rare elsewhere [20]. This geographical propensity is explained by a common founder of p.R1441G substitution carriers [21,22]. There is one autopsy report on a p.R1441G substitution carrier that revealed isolated neuronal loss in the substantia nigra without LBD pathology [19]. 2.3. The LRRK2 c.4322G>A (p.R1441H) mutation Two pathogenic Lrrk2 protein substitutions (p.R1441C and p.R1441G) are both located in Roc domain of the protein. A third putatively pathogenic variant (p.R1441H) has been identified in five parkinsonism probands of diverse ethnicity [23,24]. The clinical features are consistent with PD, however one Greek p.R1441H carrier developed features consistent with PSP during the course of disease [23]. No pathology reports are available for p.R1441H substitution carriers. The absence of this variant in >3500 control subjects and the presence of two other pathogenic variants at this amino acid position collectively support the contention that R1441H is a pathogenic substitution [23].

2.4. The LRRK2 c.5096A>G (p.Y1699C) mutation Family A (German-Canadian) members with PD typically presented with resting tremor as initial sign, and they all developed other cardinal signs (bradykinesia, rigidity and postural instability). Average age of onset was 53 years (range, 35–65). Those who received treatment responded favorably to levodopa therapy, but later developed motor complications. The clinical range also included pure dementia (two patients) and parkinsonism also with amyotrophy (muscle weakness, atrophy and fasciculations) [25]. The p.Y1699C substitution is also the cause of PD in a large British family known as the Lincolnshire kindred [26]. Pathology in two Family A Y1699C substitution carriers included neuronal loss with depigmentation and gliosis in the substantia nigra and ubiquitin-immunoreactive intranuclear and cytoplasmatic inclusions in one case, respectively. 2.5. The LRRK2 c.6055G>A (p.G2019S) mutation p.G2019S is located in the mitogen-activated protein kinase (MAP) kinase kinase domain of the Lrrk2 protein. The identification of p.G2019S substitutions as the most common cause of both familial and sporadic PD has been a major breakthrough [12–14,27–29]. The frequency of p.G2019S substitutions differ remarkably throughout the world [30]. This is due to a common founder for most p.G2019S carriers, originating from the Middle East or North Africa [14,31]. Two large studies on Lrrk2 p.G2019S parkinsonism conclude that the phenotype of Lrrk2 p.G2019S can not be distinguished from idiopathic PD [28,29]. There are some indications of a more benign course of p.G2019S parkinsonism compared to idiopathic PD with a slower disease progression and less cognitive impairment. However, methodological issues may have contributed to these observations [32]. The penetrance of Lrrk2 p.G2019S has been much debated over the last years. Hulihan et al. investigated sporadic PD in Tunisia and found a lifetime penetrance of 45% (95% CI: 20–100%) for p.G2019S substitution carriers [28,32]. Healy et al. additionally included hereditary patients and estimated that 74% had PD by age 79 years [29]. Interestingly, homozygous p.G2019S carriers do not have more severe disease than heterozygous carriers [33], this lack of gene-dose effect is consistent with the hypothesis that the p.G2019S substitution increases kinase activity. The pathology of p.G2019S parkinsonism is consistent with LBD in most, but not all cases [34–36]. 2.6. p.I2020T The p.I2020T mutation was identified in 2004 [5], and later found to be the cause of disease in the original PARK8 (Sagamihara) kindred [37]. The clinical characteristics of this family are similar to

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sporadic PD. However, gait disturbance is more frequent and tremor is less frequent than in sporadic PD [38]. Affected family members respond well to long-term levodopa treatment. The mean age at onset is 56 years (range, 38–74 years). The neuropathology within the Sagamihara kindred was originally showed to be pure nigral degeneration [37]. However, a later study of this family has identified one patient with LBD and one patient with glial cytoplasmic inclusions (GCIs) consistent with multiple system atrophy pathology [38]. This further highlights the pleomorphic pathology that can be encountered in LRRK2 parkinsonism and indicates common pathways in neurodegeneration.

Due to the frequency of LRRK2 mutations, new families should be whole-gene screened for mutations for research purposes. Clinical and pathological studies of all LRRK2 mutations are important as they may contribute important insights. However, further functional insights are required to better serve the large number of asymptomatic carriers that that are now available, and who may benefit from future neuroprotective treatments.

3. The LRRK2 risk-variants

References

In addition to the pathogenic substitutions mentioned above, the missense variants p.R1628P and p.G2385R are associated with susceptibility to PD in Han Chinese and East-Asians, respectively. The risk-effects of both are modest (odds ratio ~2–3) [7–11,39–41]. However, their identification further highlights the importance of LRRK2 in the etiology of PD and should encourage researchers to embark on collaborative studies to clarify the role of many other LRRK2 variants that have been identified, as their pathogenicity may be resolved by screening them in large patient/control series. 4. Functional insights from the Lrrk2 protein studies Lrrk2 is a member of the ROCO protein family and is characterized by several functional domains, including a GTPase domain and kinase domain. Lrrk2 is widely expressed in the brain, but also in the spleen, the lung and liver [2]. High levels of Lrrk2 are found in the striatum and the hippocampus with relatively low levels in the substantia nigra [42]. The mechanisms whereby missense mutations in the LRRK2 gene lead to neurodegeneration remain unknown. Functional studies indicate that several LRRK2 mutations cause increased Lrrk2 kinase activity and that Lrrk2 kinase activity requires GTPase activity [43]. However, the natural substrate for the Lrrk2 kinase has yet to be identified. Lrrk2-based models of PD will hopefully contribute substantially to understanding the pathophysiology of disease [44]. 5. Conclusions LRRK2 mutations are now established as an important cause of LOPD. However, LRRK2 mutations also cause young-onset PD (YOPD). Genetic testing may be considered in patients with apparent autosomal dominant disease transmission or in populations with high prevalence of LRRK2 mutations (Table 1). Therefore, screening YOPD patients could also be considered. Especially, since it is more cost effective to screen for missense mutations in LRRK2 than whole gene screening in e.g. parkin. Genetic testing may help establishing a diagnosis of PD, particularly in early stages of disease. There are no treatment benefits of genetic testing to date. This may change if future treatments prove most effective in genetically defined subtypes of PD. The genetic testing should be performed by an experienced clinician and if possible in collaboration with clinical geneticist. The involvement of clinical geneticist is crucial for evaluation of asymptomatic genealogically at risk family members (with other family members already tested and with known mutation status) or for the prenatal genetic assessment. The clinical geneticist can provide an extensive information about pattern of inheritance, clinical features and potential benefits and risks of test outcome. The variability in pathology indicates a link between different forms of neurodegeneration. Furthermore, these findings highlight that nigral cell loss and not LBD is the sine qua non in clinical PD.

Conflict of interests No conflicts of interest to declare.

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