LRRK2 G2019S and I2020T mutations are not common in Alzheimer's disease and vascular dementia

Dear Sir, Parkinson’s disease (PD) is a progressive neurodegenerative disease characterized by resting tremor, bradykinesia, and rigidity. PD is the second most common neurodegenerative disease after Alzheimer’s disease (AD) [Zimprich et al., 2004]. A number of genes and genetic loci have been linked with PD [Tan et al., 2000]. Recently mutations in the leucine-rich repeat kinase 2 (LRRK2) gene has been reported to be a cause of autosomal dominant PD (PARK8) [Zimprich et al., 2004]. Interestingly, a common 6055G>A transition (G2019S) mutation accounts for about 3–7% of familial PD and 1–1.6% of sporadic PD in a number of European populations (1.3–5). The I2020T mutation was found as the causative mutation in the original PARK8 family in Japan. While the prevalence of G2019S mutation may vary amongst different ethnic races [Di Fonzo et al., 2005; Gilks et al., 2005; Lu et al., 2005; Skipper et al., 2005; Tan et al., 2005], exon 41 has been suggested to be the LRRK2 mutational hotspot. Pleomorphic pathology and unusual clinical presentations that lack distinctive histopathology have been described in PD patients possessing LRRK2 mutations [Zimprich et al., 2004]. Varying pathologies suggestive of dementia with Lewy Body, tauopathy, supranuclear palsy, and AD have been associated with LRRK2 mutations [Zimprich et al., 2004]. The location of the LRRK2 gene on chromosome 12 is close to a linkage peak for late onset AD, and this led to the suggestion that LRRK2 may be a candidate gene for dementias such as AD. Prompted by these observations, we conducted sequence analysis of LRRK2 exon 41 in AD and vascular dementia patients of Asian ethnicity. Two hundred subjects consisting of 125 AD, 50 vascular dementia, and 25 mixed AD and vascular dementia patients who satisfied established clinical criteria were included. The dementia syndrome was diagnosed using the Diagnostic and Statistical Manual, 4th edition (DSM-IV) criteria and the etiological diagnoses followed the NINCDS-ADRDA criteria for AD and the NINDS-AIREN criteria for vascular dementia. There were 170 (85%) Chinese, 19 (9.5%) Indians, 6 (3%) Malays, and 5 (2.5%) of mixed ethnicity, with about equal gender distribution. The mean age of patients was 76.3 (SD 8.78) and mean age at onset was 73.4 (SD 8.78) years. None of the patients gave a positive family history of autosomal dominant dementia. Informed consent was taken from each subject and the institutional ethics committee approved the work. We performed sequence analysis of exon 41 to determine the presence of LRRK2 G2019S and I2020T mutations within the MAPKKK domain, using the primers (foward: 50TTTTGATGCTTGACATAGTGGAC-30; reverse:50-CACATCTGAGGTCAGTGGTTATC-30). 2.5 ml 10 PCR buffer, 2.0 ml of 2.5 mmol/L each dNTP(Promega),1.5 ml of 25 mmol/L MgCl2, 1 ml of 10 mol/L of each primer (Research Biolabs) and 1 unit AmpliTaq Gold in a total volume of 25ml. Cycle conditions were: 948C for 11 min; 30 cycles of 948C for 30 sec, 608C for 45 sec, 728C for 1 min, and final extension step for 728C for 7 min. Sequencing was carried out using BigDye Terminator Cycle Sequencing Kit according to manufacture’s instructions (Applied Biosystems). We recognize that since the frequency of G2019S is very low in Asians with PD, the prior probability of detecting G2019S in Asians with other neurodegenerative diseases (e.g., AD) will probably be low. In our study, genetic analysis revealed none of the 200 study subjects carried the G2019S and I2020T mutations. While a much larger sample size may be required to detect mutations with very low prevalence, our findings suggest that G2019S and I2020T mutations are rare (at least <0.05%) in Asian AD and vascular dementia patients, supporting findings from recent studies in European AD patients [Hernandez et al., 2005; Toft et al., 2005; Zabetian et al., 2006]. Hence routine genetic testing for these two mutations in sporadic AD patients may not be cost effective. However, these results do not exclude a possible role of other genetic variants within the LRRK2 gene in AD or other dementias.