Clinical-Genetic Associations in the Prospective Huntington at Risk Observational Study (PHAROS): Implications for Clinical Trials.

IMPORTANCE: Identifying measures that are associated with the cytosine-adenine-guanine (CAG) expansion in individuals before diagnosis of Huntington disease (HD) has implications for designing clinical trials. OBJECTIVE: To identify the earliest features associated with the motor diagnosis of HD in the Prospective Huntington at Risk Observational Study (PHAROS). DESIGN, SETTING, AND PARTICIPANTS: A prospective, multicenter, longitudinal cohort study was conducted at 43 US and Canadian Huntington Study Group research sites from July 9, 1999, through December 17, 2009. Participants included 983 unaffected adults at risk for HD who had chosen to remain unaware of their mutation status. Baseline comparability between CAG expansion (≥37 repeats) and nonexpansion (<37 repeats) groups was assessed. All participants and investigators were blinded to individual CAG analysis. A repeated-measures analysis adjusting for age and sex was used to assess the divergence of the linear trend between the expanded and nonexpanded groups. Data were analyzed from April 27, 2010, to September 3, 2013. EXPOSURE: Huntington disease mutation status in individuals with CAG expansion vs without CAG expansion. MAIN OUTCOMES AND MEASURES: Unified Huntington's Disease Rating Scale motor (score range, 0-124; higher scores indicate greater impairment), cognitive (symbol digits modality is the total number of correct responses in 90 seconds; lower scores indicate greater impairment), behavioral (score range, 0-176; higher scores indicate greater behavioral symptoms), and functional (Total Functional Capacity score range, 0-13; lower scores indicate reduced functional ability) domains were assessed at baseline and every 9 months up to a maximum of 10 years. RESULTS: Among the 983 research participants at risk for HD in the longitudinal cohort, 345 (35.1%) carried the CAG expansion and 638 (64.9%) did not. The mean (SD) duration of follow-up was 5.8 (3.0) years. At baseline, participants with expansions had more impaired motor (3.0 [4.2] vs 1.9 [2.8]; P < .001), cognitive (P < .05 for all measures except Verbal Fluency, P = .52), and behavioral domain scores (9.4 [11.4] vs 6.5 [8.5]; P < .001) but not significantly different measures of functional capacity (12.9 [0.3] vs 13.0 [0.2]; P = .23). With findings reported as mean slope (95% CI), in the longitudinal analyses, participants with CAG expansions showed significant worsening in motor (0.84 [0.73 to 0.95] vs 0.03 [-0.05 to 0.11]), cognitive (-0.54 [-0.67 to -0.40] vs 0.22 [0.12 to 0.32]), and functional (-0.08 [-0.09 to -0.06] vs -0.01 [-0.02 to 0]) measures compared with those without expansion (P < .001 for all); behavioral domain scores did not diverge significantly between groups. CONCLUSIONS AND RELEVANCE: Using these prospectively accrued clinical data, relatively large treatment effects would be required to mount a randomized, placebo-controlled clinical trial involving premanifest HD individuals who carry the CAG expansion.

X-linked cone-rod dystrophy (locus COD1): identification of mutations in RPGR exon ORF15.

X-linked cone-rod dystrophy (COD1) is a retinal disease that primarily affects the cone photoreceptors; the disease was originally mapped to a limited region of Xp11.4. We evaluated the three families from our original study with new markers and clinically reassessed all key recombinants; we determined that the critical intervals in families 2 and 3 overlapped the RP3 locus and that a status change (from affected to probably unaffected) of a key recombinant individual in family 1 also reassigned the disease locus to include RP3 as well. Mutation analysis of the entire RPGR coding region identified two different 2-nucleotide (nt) deletions in ORF15, in family 2 (delAG) and in families 1 and 3 (delGG), both of which result in a frameshift leading to altered amino acid structure and early termination. In addition, an independent individual with X-linked cone-rod dystrophy demonstrated a 1-nt insertion (insA) in ORF15. The presence of three distinct mutations associated with the same disease phenotype provides strong evidence that mutations in RPGR exon ORF15 are responsible for COD1. Genetic heterogeneity was observed in three other families, including the identification of an in-frame 12-nt deletion polymorphism in ORF15 that did not segregate with the disease in one of these families.