Guidelines for Evaluating the Comparability of Down-Sampled GWAS Summary Statistics

Abstract

Proprietary genetic datasets are valuable for boosting the statistical power of genome-wide association studies (GWASs), but their use can restrict investigators from publicly sharing the resulting summary statistics. Although researchers can resort to sharing down-sampled versions that exclude restricted data, down-sampling reduces power and might change the genetic etiology of the phenotype being studied. These problems are further complicated when using multivariate GWAS methods, such as genomic structural equation modeling (Genomic SEM), that model genetic correlations across multiple traits. Here, we propose a systematic approach to assess the comparability of GWAS summary statistics that include versus exclude restricted data. Illustrating this approach with a multivariate GWAS of an externalizing factor, we assessed the impact of down-sampling on (1) the strength of the genetic signal in univariate GWASs, (2) the factor loadings and model fit in multivariate Genomic SEM, (3) the strength of the genetic signal at the factor level, (4) insights from gene-property analyses, (5) the pattern of genetic correlations with other traits, and (6) polygenic score analyses in independent samples. For the externalizing GWAS, down-sampling resulted in a loss of genetic signal and fewer genome-wide significant loci, while the factor loadings and model fit, gene-property analyses, genetic correlations, and polygenic score analyses are robust. Given the importance of data sharing for the advancement of open science, we recommend that investigators who share down-sampled summary statistics report these analyses as accompanying documentation to support other researchers' use of the summary statistics.

Fig 1.

LD Score genetic correlations and heritability estimates for the seven indicator phenotypes of the single-factor models of EXT and EXT-minus-23andMe (see Step 1). The left panel displays the analysis of the original study with 23andMe data, the middle panel displays the down-sampled analysis excluding 23andMe data, and the right panel displays the difference in estimates computed by subtracting the values in the middle panel from those in the left panel. The lower and upper triangles display pairwise genetic correlation (r_g) estimates and standard errors, respectively. The diagonals display the observed-scale heritability (h²; see Table 1 for standard errors). These results are also reported in Table S1. Abbreviations: ADHD = attention-deficit/hyperactivity disorder; ALCP = problematic alcohol use; CANN = lifetime cannabis use; FSEX = age at first sexual intercourse (reverse coded); NSEX = number of sexual partners; RISK = risk tolerance; SMOK = lifetime tobacco initiation.

Fig 2.

Path diagram of a single-factor model with seven indicator phenotypes, of which SMOK and CANN are down-sampled, as estimated with Genomic SEM. These results are also reported in Table S2. The same figure displaying the results of the original study is available here: https://www.nature.com/articles/s41593-021-00908-3/figures/1 Abbreviations: EXT g = genetic externalizing factor; ADHD = attention-deficit/hyperactivity disorder; ALCP = problematic alcohol use; CANN = lifetime cannabis use; FSEX = age at first sexual intercourse (reverse coded); NSEX = number of sexual partners; RISK = risk tolerance; SMOK = lifetime tobacco initiation; AIC = Akaike Information Criterion; CFI = comparative fit index; SRMR = standardized root mean square residual.

Fig 3.

Scatterplot of genetic correlations (r_g) and marginal density plots between EXT (y-axis) or EXT-minus-23andMe (x-axis) with 77 other phenotypes. Each point corresponds to the genetic correlation coefficient with its 95% confidence intervals (r_g ± 1.96 × SE) estimated with bivariate LD Score regression. Table S5 reports the estimates, their standard errors, and confidence intervals. The Spearman rank correlation reported in the figure is rounded from r = 0.9995. No particular shape, such as a normal distribution, is expected for the marginal density because the figure displays an arbitrary selection of traits.

Fig 4.

Comparison of the down-sampled polygenic score (PGS) analyses in Add Health (29 phenotypes) and the Collaborative Study on the Genetics of Alcoholism (COGA; 26 phenotypes). Panel A displays the standardized difference between the coefficient estimates (i.e., a Z-statistic) of the down-sampled PGS for EXT-minus-23andMe versus the PGS for EXT from the original study. Absolute values were evaluated so that a negative standardized difference refers to an attenuation towards zero in the down-sampled analysis. Panel B displays the same measure but as a histogram. Four coefficient estimates were significantly (at the 5% level) attenuated in the down-sampled analysis: lifetime smoking initiation (Add Health and COGA; P = 3.18×10^–5 and 4.17×10^–5, respectively), the phenotypic externalizing factor (Add Health; P = 0.046), and lifetime cannabis use (Add Health, P = 0.03). None of the coefficients were significantly larger in the down-sampled analysis. Panel C displays a scatter plot of the absolute value of the coefficient estimates divided by their respective standard errors (i.e., a Z-statistic). These results are also reported in Table S6.