In this first GWAS on melatonin secretion, we identified six suggestive loci that were associated with variation in log UMCR. rs142037747 is located near GALNT15 (polypeptide N-acetylgalactosaminyltransferase 15). GALNT15 catalyzes initiation of mucin-type O-linked glycosylation by adding N-acetylgalactosamine to serine or threonine residues of the polypepide chain [36]. Glycosylation is associated with cell adhesion, signal transduction, molecular trafficking and differentiation in central nervous system development [37]. It remains to be determined whether and how GALNT15 significantly affects melatonin level.
rs17681554 is located within ZFHX3 (Zinc Finger Homeobox 3). ZFHX3 is a transcriptional regulator which contains four homeodomains and seventeen zinc fingers [38]. During neuronal differentiation, there is certain preferential expression pattern of ZFHX3 isoforms [39]. In addition, circadian behavior alteration is shown in inducible conditional Zfhx3 knockout adult mice [40]. Further studies are needed to elucidate if there is a direct linkage between ZFHX3 and melatonin.
rs74760291 locates in the intronic region of NKAIN2 (sodium/potassium- transporting ATPase subunit beta-1-interacting protein). NKAIN2 is previously named T-cell lymphoma breakpoint-associated target protein 1 due to its association with lymphoma [41]. Genome-wide analyses demonstrated NKAIN2 gene in association with sleep quality and insomnia [42, 43]. Since darkness is the most important regulator of melatonin secretion and light drastically reduced melatonin across the day, melatonin also reciprocally affected sleep quality and insomnia in large-scale meta-analysis [44]. From our study, it is highly likely that genetic variation of NKAIN2 affects sleep via affecting melatonin secretion. However, further study is warranted as we cannot exclude the reverse causation that NKAIN2 variation-associated sleep order results in disturbed melatonin secretion.
rs7433686 is located near MME (membrane metalloendopeptidase; more commonly known as neprilysin). Neprilysin degrades amyloid beta-protein, which is the main constituent of amyloid plaques seen in patients with Alzheimer's disease (AD) [45]. A candidate gene-based association study suggested neprilysin was a susceptibility gene for AD [46]. Melatonin levels are altered in AD patients, possibly due to a decrease in suprachiasmatic nucleus cell number and functional pineal gland volume [47]. Our present unbiased genetic study, revealing the MME variant associated with melatonin secretion from pineal gland, provides additional evidence for potential mechanistic explanation in AD patients with altered melatonin levels.
rs11625484 is an intergenic SNP. Whether long non-coding RNAs or microRNAs encoded in this intergenic region might affect melatonin levels remains to be elucidated.
rs76204167 is located near NBPF22P (neuroblastoma breakpoint family, member 22, pseudogene). Members of NBPF are likely to be involved in neuronal development besides cancer occurrence [48]. A previous GWAS regarding to actigraphy-based sleep parameters has shown NBPF22P related to sleep quality [49]. The exact mechanism of relation between NBPF22P and melatonin is to be explored.
This study also showed borderline significance regarding to the positive correlation between age and log UMCR. Since aging causes sarcopenia, subsequent decreased creatinine excretion from urine tends to increase the substance-to-creatinine ratio. Our result support the current concept that aging itself will not cause a decrease in melatonin secretion or excretion.
There was a concern that aMT6s excretion may be altered when renal function declines. A previous study enrolling 20 elderlies demonstrated that 24-hour urine aMT6s was a reliable surrogate for plasma melatonin level, at least among individuals with GFR 24.6 ml/min or above [50]. Our study confirmed that morning UMCR is not significantly correlated with eGFR, and adjusting eGFR in GWAS analysis essentially did not have influence on the results.
There are limitations of our study. First, it lacks replication of the result in another cohort. We searched in the UK Biobank, but melatonin as phenotype does not exist in the database. Moreover, the sample size is relatively small, thus for the time being these SNPs can be only seen as suggestive signals, and true loci remain to be identified and validated.
In summary, we have performed the first GWAS regarding to melatonin secretion to date. This GWAS identified 6 highly suggested genetic loci encompassing genes that had been demonstrated potential functional connectivity between the genes-associated melatonin level and clinical documented disorders such as AD and sleep disorders. Replication and functional studies of these genetic variations are warranted to better understand the regulation of melatonin secretion and related clinical disorders.