MC1R variants and faster motor decline in Parkinson disease

Design

Longitudinal cohort (PPMI) with replication in participants from three randomised trials; up to 12 years of follow-up

Population

809 PPMI participants with Parkinson disease (383 sporadic); 587 in replication; 53 prodromal

Primary outcome

Rate of motor decline (MDS-UPDRS Part III); phenoconversion risk

Effect

Sporadic PD: +0.57 points/year (95% CI 0.16 to 0.98) in carriers; replication +1.37 (95% CI 0.28 to 2.46); prodromal SHR 4.75 (95% CI 1.48 to 15.27)

MC1R is involved in pigmentation and oxidative stress, and loss-of-function variants are carried by more than 60% of people of European descent. This longitudinal cohort used the Parkinson Progression Markers Initiative (PPMI) and a replication cohort drawn from three US trials, with follow-up of up to 12 years.

In the main analysis of 383 people with sporadic Parkinson disease, carriers had about 30% faster motor decline on the MDS-UPDRS Part III than non-carriers (0.57 points per year, 95% CI 0.16 to 0.98). In the replication cohort of 587 people, the figure was about 50% (1.37 points per year, 95% CI 0.28 to 2.46). In a very small prodromal group of 53, carriers had a subdistribution hazard ratio of 4.75 (95% CI 1.48 to 15.27) for converting to Parkinson disease.

The populations were of European descent, and the authors suggest the variants could help with prognosis and trial enrichment. A variant carried by most people is not a practical clinical test, and the prodromal result rests on 34 carriers and 19 non-carriers. There is no treatment implication.

• Do not order MC1R testing for patients; there is no validated clinical use or treatment consequence.
• Tell patients that progression varies widely and that genetic markers do not yet guide management.
• Continue to monitor motor progression with a standardised scale at each visit.
• Consider this research-stage; it may matter more for the design of trials than for the clinic.

Why it matters

It suggests a common genetic variant may help explain why some people progress faster, which matters for trial design.

Don't overread it

The cohorts were of European descent and observational; an association with progression does not establish a cause or a treatment target.

The statistics, in plain English

A 0.57-point-per-year difference is small against a scale with a 132-point range, and the confidence interval (0.16 to 0.98) is wide. The prodromal hazard ratio of 4.75 has an interval from 1.48 to 15.27 because there were only 53 participants, so its size is very uncertain.

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