Brain Expression Levels of Commonly Measured Blood Biomarkers of Neurological Damage Differ with Respect to Sex, Race, and Age

Abstract

It is increasingly evident that blood biomarkers have potential to improve the diagnosis and management of both acute and chronic neurological conditions. The most well-studied candidates, and arguably those with the broadest utility, are proteins that are highly enriched in neural tissues and released into circulation upon cellular damage. It is currently unknown how the brain expression levels of these proteins is influenced by demographic factors such as sex, race, and age. Given that source tissue abundance is likely a key determinant of the levels observed in the blood during neurological pathology, understanding such influences is important in terms of identifying potential clinical scenarios that could produce diagnostic bias. In this study, we leveraged existing mRNA sequencing data originating from 2,642 normal brain specimens harvested from 382 human donors to examine potential demographic variability in the expression levels of genes which code for 28 candidate blood biomarkers of neurological damage. Existing mass spectrometry data originating from 26 additional normal brain specimens harvested from 26 separate human donors was subsequently used to tentatively assess whether observed transcriptional variance was likely to produce corresponding variance in terms of protein abundance. Genes associated with several well-studied or emerging candidate biomarkers including neurofilament light chain (NfL), ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCH-L1), neuron-specific enolase (NSE), and synaptosomal-associated protein 25 (SNAP-25) exhibited significant differences in expression with respect to sex, race, and age. In many instances, these differences in brain expression align well with and provide a mechanistic explanation for previously reported differences in blood levels.

Keywords: blood biomarkers; concussion; demographics; neurodegenerative disease; stroke; traumatic brain injury.

Fig. 1.. Comparison of brain mRNA expression between specimens from male and female donors.

Median expression levels of the 28 genes of interest in male (n = 1,914) and female (n = 728) brain specimens, according to brain region. Expression levels are presented as log transformed TPM values unity scaled within each brain region. Overall fold differences in expression levels between male and female specimens are indicated, and were calculated from weighted median TPM values, equally weighted by brain region. 95% confidence intervals and p-values were calculated using the percentile bootstrap method and 1,000 bootstrap samples. P-values were adjusted for multiple comparisons via the Benjamini-Hochberg method. Alternate p-values assessed using Mann-Whitney U test can be found in Supplemental Table 1. *Statistically significant.

Fig. 2.. Comparison of brain mRNA expression between specimens from White and African American donors.

Median expression levels of the 28 genes of interest in White (n = 2,395) and African American (n = 236) brain specimens, according to brain region. Expression levels are presented as log transformed TPM values unity scaled within each brain region. Overall fold differences in expression levels between White and African American specimens are indicated, and were calculated from weighted median TPM values, equally weighted by brain region. 95% confidence intervals and p-values were calculated using the percentile bootstrap method and 1,000 bootstrap samples. P-values were adjusted for multiple comparisons via the Benjamini-Hochberg method. Alternate p-values assessed using Mann-Whitney U test can be found in Supplemental Table 1. *Statistically significant.

Fig. 3.. Associations between donor age and brain mRNA expression.

Relationships between the expression levels of the 28 genes of interest and donor age across the total pool of brain specimens (n = 2,642). The direction and strength of correlations were calculated via stratified Spearman’s rho (r), with stratification by brain region. 95% confidence intervals and p-values were calculated using the percentile bootstrap method and 1,000 bootstrap samples. P-values were adjusted for multiple comparisons via the Benjamini-Hochberg method. *Statistically significant.

Fig. 4.. Comparison of brain mRNA expression between specimens from younger and older donors.

Median expression levels of the 28 genes of interest in younger (n = 699) and older (n = 701) brain specimens, according to brain region. Expression levels are presented as log transformed TPM values unity scaled within each brain region. Overall fold differences in expression levels between younger and older specimens are indicated, and were calculated from weighted median TPM values, equally weighted by brain region. 95% confidence intervals and p-values were calculated using the percentile bootstrap method and 1,000 bootstrap samples. P-values were adjusted for multiple comparisons via the Benjamini-Hochberg method. Alternate p-values assessed using Mann-Whitney U test can be found in Supplemental Table 1. *Statistically significant.

Fig. 5.. Comparisons of brain protein levels across specimens from donors of differing sex and age.

(A) Median abundance of the proteins of interest in male (n = 19) and female (n = 7) cerebral cortex specimens. (B) Median abundance of the proteins of interest in younger (n = 7) and older (n = 7) cerebral cortex specimens. Fold differences are indicated, along with associated 95% confidence intervals, which were calculated using the percentile bootstrap method and 1,000 bootstrap samples. Intergroup statistical comparisons were carried out using Mann-Whitney U test, and p-values were adjusted for multiple comparisons via the Benjamini-Hochberg method. Uncorrected p-values can be found in Supplemental Table 2. *Statistically significant.

Fig. 6.. Agreement between fold differences observed in transcriptomic and proteomic comparisons.

(A) Correlation between the fold differences in mRNA levels and the fold differences in protein levels observed between cerebral cortex specimens from male and female donors in our respective transcriptomic and proteomic analyses. (B) Correlation between the fold differences in mRNA levels and the fold differences in protein levels observed between cerebral cortex specimens from younger and older donors in our respective transcriptomic and proteomic analyses. Notable well-studied biomarkers are indicated. The strength and significance of correlations were evaluated via Spearman’s rho (r). *Statistically significant.