Greater than the sum of its parts: A novel psychiatric multimorbidity metric applied to rare disease

A quantitative multimorbidity score (MMS) for 3q29 deletion and 3q29 duplication syndromes shows markedly lower mean MMS than controls (−8.22±4.67 and −6.47±4.66 vs 1.36±2.35).

• Why it matters: Existing multimorbidity approaches sum diagnoses and miss subthreshold neurodevelopmental and psychiatric impairment, limiting measurement in rare genetic syndromes.
• What they did: The team built an MMS across five functioning domains as the sum of z-scores from six standardized assessments, using cross-sectional remote (2023–2026) and in-person (2017–2020) phenotyping.
• The result: In 3q29 deletion, higher MMS severity is linked to worse Vineland-3 adaptive behavior (R2=0.589, p<2E-16), MMS correlates more strongly than diagnosis-based metrics (R2=0.589 vs 0.447), and is stable longitudinally (R2=0.858, p=8.34E-8).

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