LRRK2 in Parkinson's disease – drawing the curtain of penetrance: a commentary

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, and has been regarded as a prototypic non-genetic disorder for a long time. First insight into the genetic contribution to the typical Parkinsonian phenotype came from large families with classical Mendelian inheritance of the disease trait. The discovery of disease-causing mutations in the alpha-synuclein gene for autosomal dominant forms and in the Parkin, PINK1 and DJ-1 gene for autosomal recessive forms of PD allowed a first insight into the molecular mechanisms that mediate neurodegeneration in PD. Heterozygous mutations in the alpha-synuclein gene and homozygous mutations in the Parkin, PINK1 and DJ-1 gene show, in general, full penetrance and account for less than 1% of all PD cases [1]. Thus, due to the small proportion of PD patients that carried mutations in the respective genes, genetic testing and counselling of presymptomatic mutation carriers was not relevant and common recommendations limited molecular testing to symptomatic individuals. These premises have now changed due to the identification and characterization of mutations in the LRRK2 gene in PD [2,3].Mutations in the LRRK2 gene encoding the leucine-rich, repeat kinase 2 protein are the most frequent genetic cause of PD known to date. Clinicogenetic studies clearly showed that the majority of all LRRK2 mutation carriers present with symptoms indistinguishable from idiopathic PD [4-7]. To date, more than 50 variants are known, but only about 10% have been proven pathogenic and account for approximately 2% of sporadic and 10% of familial PD cases [4,8,9]. Initially identified in large families with autosomal dominant inheritance of PD, it turned out that mutations in the LRRK2 gene were also found in patients with the sporadic form of the disease (without positive family history). Among these mutations, a glycine to serine exchange in position 2019 of the peptide sequence (G2019S) is the most frequent worldwide with f