Serum 6-Bromotryptophan Levels Identified as a Risk Factor for CKD Progression.

Metabolite levels reflect physiologic homeostasis and may serve as biomarkers of disease progression. Identifying metabolites associated with risk alleles-genetic variants associated with CKD risk commonly present in persons of African descent-may reveal novel markers of CKD progression relevant to other populations. We evaluated associations between the number of risk alleles and 760 serum metabolites identified untargeted profiling in participants of the African American Study of Kidney Disease and Hypertension (AASK) (=588; Bonferroni significance threshold <6.5×10) and replicated findings in 678 black participants with CKD in Bio, an electronic medical record-linked biobank. We tested the metabolite association with CKD progression in AASK, Bio, and the Modification of Diet in Renal Disease (MDRD) Study. One metabolite, 6-bromotryptophan, was significant in AASK (=4.7×10) and replicated in Bio (=5.7×10) participants, with lower levels associated with more risk alleles. Lower levels of 6-bromotryptophan were associated with CKD progression in AASK and Bio participants and in white participants in the MDRD Study, independent of demographics and clinical characteristics, including baseline GFR (adjusted hazard ratio per two-fold higher 6-bromotryptophan level, AASK, 0.76; 95% confidence interval [95% CI], 0.64 to 0.91; Bio, 0.61; 95% CI, 0.43 to 0.85; MDRD, 0.52; 95% CI, 0.34 to 0.79). The interaction between the risk alleles and 6-bromotryptophan was not significant. The identity of 6-bromotryptophan was confirmed in experiments comparing its molecular signature with that of authentic standards of other bromotryptophan isomers. Serum 6-bromotryptophan is a consistent and novel risk factor for CKD progression.