A pigmentation gene variant most Europeans carry marks faster Parkinson's decline

Design

Longitudinal cohort study with replication in three randomised trial cohorts

Population

383 participants with sporadic Parkinson disease in PPMI, 587 in the replication cohort, 53 with prodromal disease; up to 12 years of follow-up

Primary outcome

Rate of motor decline on MDS-UPDRS Part III; phenoconversion risk in the prodromal group

Effect

30% faster decline in carriers (β 0.57 points/year, 95% CI 0.16–0.98); replication 50% faster (β 1.37, 0.28–2.46); prodromal phenoconversion sHR 4.75 (1.48–15.27)

MC1R regulates pigmentation and oxidative stress, and loss-of-function variants are carried by more than 60% of people of European descent. This cohort study asked whether carriers progress faster. It used the Parkinson Progression Markers Initiative with up to 12 years of follow-up, then sought replication in three US randomised trial cohorts.

Among 383 participants with sporadic Parkinson disease, carriers declined 30% faster on the MDS-UPDRS Part III motor score (β 0.57 points per year, 95% CI 0.16–0.98, P = .006) after adjustment for age at onset, sex, race, baseline score and levodopa equivalent dose. In the 587-participant replication cohort, the gap was 50% (β 1.37, 95% CI 0.28–2.46, P = .01). In a prodromal subgroup of 53, carriers had more than four times the risk of phenoconversion (subdistribution hazard ratio 4.75, 95% CI 1.48–15.27, P = .009).

The immediate use is in research rather than clinic: a variant this common, this easy to genotype and this clearly associated with progression is a ready-made enrichment strategy for trials that need to detect slowing of decline. The prodromal finding rests on 53 people and should be treated as preliminary. And the whole analysis is in people of European descent, which limits what it says about Indian patients.

• Do not order MC1R genotyping in clinic — there is no intervention that follows from the result.
• Note the finding if you refer patients into disease-modification trials; enrichment by MC1R status is likely to appear in protocols.
• The association does not change management of an individual's motor symptoms today.
• Ask about melanoma history and sun exposure in patients with Parkinson disease for the reason that already exists — melanoma risk is raised in this population — not because of this study.
• Generalisability to Indian patients is unknown; MC1R variant frequencies differ substantially by ancestry.

Why it matters

Parkinson's trials fail partly because progression rates vary so much between participants, and a common genotype that predicts them is worth having.

Don't overread it

This is an observational association in people of European descent; it does not show that MC1R dysfunction causes faster decline.

The statistics, in plain English

A difference of 0.57 MDS-UPDRS points per year is small in one year and substantial over a decade, which is why the follow-up length matters. The replication cohort's estimate (1.37 points per year) is much larger with a wide interval (0.28 to 2.46), so the two cohorts agree on direction rather than on size. The prodromal hazard ratio of 4.75 has an interval from 1.48 to 15.27 — derived from 53 people, it establishes little more than that the effect is probably not zero.

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