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The Roc domain of the Lrrk2 protein harbors two pathogenic mutations which cause autosomal dominant parkinsonism (R1441C and R1441G). A third putatively pathogenic variant (R1441H) has been identified in four probands of diverse ethnicity with parkinsonism. Herein we show that the R1441H substitutions lie on different haplotypes within our patients, confirming this codon as a mutational hotspot. The absence of this variant in 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.
Leucine-rich repeat kinase 2 (LRRK2) variants cause familial and sporadic parkinsonism. Pathogenic substitutions are distributed across different domains of the Lrrk2 protein; Ras of complex proteins (Roc; R1441G/C), C-terminal of Roc (COR; Y1699C) and mitogen-activated protein kinase kinase kinase (MAPKKK G2019S & I2020T), and recently, substitutions in the COR (R1628P) and WD40 domain (G2385R) have been associated with disease-risk in ethnic Chinese patients [1–3].
The Roc domain contains two confirmed and one putative pathogenic variant at a single codon, arginine (R)1441. The original descriptions of Lrrk2 substitutions in parkinsonism described R1441C (LRRK2 4321C>T) and R1441G (LRRK2 4321C>G), and subsequent studies identified R1441H (LRRK2 4322G>A) [4, 5]. Although, the R1441G substitution appears to be geographically restricted to Northern Spain with evidence of a common founder, R1441C appears to be worldwide and have occurred as at least three independent mutational events [6, 7].
Four families with a history of parkinsonism have been identified to harbor R1441H [4, 5, 8, 9], and given its amino acid position and absence in over 3500 controls screened in studies to date, it is most likely the cause of their disease. However, the families are not large enough to demonstrate definitive co-segregation with disease. The four families are of diverse ethnicity (Portuguese, Greek, US and Taiwanese) and do not share a known ancestral lineage. This study set out to infer haplotypic structure for the chromosomal 12q12 region flanking LRRK2 in R1441H carriers to establish if there is any evidence of a founder-effect.
Parkinsonism was diagnosed by movement disorder neurologists, according to published criteria and family members of the index case were examined if available . The institutional review boards at each institution approved the study and each participant provided signed informed consent. Genomic DNA was extracted from peripheral blood using standard protocols. Direct sequencing of exon 31 was used to verify LRRK2 4322G>A (R1441H) mutation carrier status. For the Taiwanese and US Lrrk2 R1441H carriers all 51 exons of the LRRK2 gene were sequenced with no other putative pathogenic variants observed. Adjacent genetic markers (14 single nucleotide polymorphisms (SNPs) and eight microsatellites) spanning ~6 Mb across the LRRK2 locus were selected to infer haplotype structure in families harboring the Lrrk2 R1441H substitution as previously described . All PCR primers and conditions are available on request.
The inferred haplotype data suggest that the R1441H substitution has arisen on multiple independent occasions (Table). The haplotypes of the R1441H carriers of European descent show diversity which may indicate a number of independent founders (Table). Even though it appears the R1441H carriers do not have a single common founder the clinical presentation of affected carriers appears to be similar to typical Parkinson’s disease with an age at onset range of 32–64 years (median 54.5 years). All initially display levodopa responsive parkinsonism, however disease in one of the siblings from the Greek R1441H family appeared to transition into a progressive supranuclear palsy-like disorder . These observations are reminiscent of one patient from a family with the Lrrk2 R1441C mutation (Family D), who displayed predominant tau rather than a-synuclein pathology, on post-mortem examination . Neuropathologic studies in Lrrk2 R1441H carriers may provide further insight.
The occurrence of Lrrk2 R1441H in four kindreds, the absence in >3500 healthy controls and the pathogenicity of other variants occurring at this amino acid (R1441C and R1441G) collectively support the contention that R1441H is a pathogenic substitution. Longitudinal studies of these families may provide the necessary evidence for R1441H pathogenicity with disease co-segregation, and help elucidate the pathophysiology of Lrrk2 R1441H-associated parkinsonism.
We would like to thank all those who have contributed to our research. This work is supported by a Morris K. Udall Parkinson’s Disease Research Center of Excellence (NINDS P50 #NS40256), the Department of Veterans Affairs (Merit Review Award), the Taiwan National Science Council (96-2628-B-002-103-MY2), by a grant from the “Internationaal Parkinson Fonds” (The Netherlands) to VB, and by a research grant from the “Parkinson Disease Foundation” to CS.
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