The COL6A3 gene, encodes a component of type VI collagen, which is a flexible protein in the extracellular space [25]. Previous studies have shown that mutation in COL6A genes (COL6A 1–3) is associated with Bethlem myopathy and Ullrich congenital muscular dystrophy. Now it is believed that these two diseases may be at one end of a phenotypic spectrum, mainly manifesting as muscle weakness and joint contractures [26].
Using whole-exome sequencing, Zech et al. recently reported recessive mutations in the COL6A3 gene in association with early-onset isolated dystonia. Clinical symptoms varied from focal dystonia, mild cervical dystonia, mild segmental dystonia and mild generalized dystonia to severe generalized dystonia [6, 27]. A previous study by Zech et al. detected five variants in five early-onset isolated dystonia patients, and found that at least one of the homozygous variants is located in exon 41 or 42. Furthermore, experiments in Zebrafish embryos and the mouse brain showed the exon skipping mutation in exon 41 in resulted in the development of segmental dystonia, without any muscular disease. Thus, they concluded that variants in COL6A3 may cause dystonia by affecting the extracellular matrix in the central nervous system, and that exons 41 and 42 are hotspots for mutation [6]. However, the role of COL6A3 mutation in isolated dystonia had been challenged by Lohmann et al .[27], who screened 955 patients with combined or isolated dystonia. They only identified one biallelic mutation in a patient with Parkinsonism and dystonia. This patient also carried homozygous mutations in the PINK1 gene, which are considered to be associated with early-onset Parkinsonism. Panda et al. also reported an early-onset isolated dystonia case with two pathogenic compound heterozygous loss-of-function mutations in exons 10 and 12 [28].
Dystonia can be seen in both early- and late-onset PD patients, and can occur precede or after parkinsonism[29]. Two of the seven patients we identified carrying these possible deleterious variants presented with dystonia feature. We assume it may be due to the relatively short disease duration (mean 3.7 years).
In the present study, we reported two recessive mutations (p.A769T and p.D1674N) in the COL6A3 gene in our index PD patient. The variants are located in exons 6 and 8, in contrast to the previous study. However, since the patient currently shows no symptoms of dystonia, we assume variants in some region of COL6A3 may have an association with PD. As a result, we identified seven likely pathogenic variants in COL6A3, but none of them were located in exon 41 or 42 as previously reported. In addition, two patients who carried the variants p.A1031T and p.R1656Q presented with dystonia. SKAT-O analysis showed significant aggregate burden between patients and controls, indicating that variants in the COL6A3 gene may increase the genetic burden in PD.
Conventional single-variant test may not be appropriate in our cohort because of small sample size and variants’ low frequencies. Thus we adapted the SKAT-O to investigate the associations of variants and phenotypes. SKAT-O combined the burden test and SKAT to maximize the power[30]. Besides, by way of small-sample adjustment method of SKAT-O, we can properly control the type I error due to small sample size. However, SKAT-O also has its limitation: in the scenario that there are more neutral variants than actual deleterious variants, it may be slightly less powerful[31].
Our study suggests that variants in the COL6A3 gene may increase susceptibility to PD. Further studies on the function and mechanism of COL6A3 and other dystonia-related genes are needed to unravel the complexity of the association between PD and dystonia.
The limitations of our study are the small sample sizes and lack of function research. Currently, there are few studies of the relationship between COL6A3 and PD. Further clinical studies with a larger sample size more and diverse demographic background are warranted, along with basic studies exploring the biological function of COL6A3 and its associated pathways.