Background
In recent years, there has been an increasing focus on the genetic basis for sleep/wake traits. Several genome wide association studies (GWAS) have identified numerous single nucleotide polymorphisms (SNPs) that influence sleep traits [
1]. For example, Hu and colleagues reported 15 SNPs, several of which were on circadian genes, that were significantly associated with being a “morning person” [
2] while a study by Gottlieb et al. focused on sleep duration identified seven SNPs in two circumscribed genetic loci. Caffeine induced insomnia was the object of another GWAS that identified several loci in melatonin and adenosine pathways [
3]. Using the same methodology, Byrne et al. identified several loci with plausible biological role influencing sleep quality and timing [
4]. Finally, two studies on the UK biobank sample reported more than a hundred novel SNPs associated to accelerometer registered sleep duration, efficiency and number of nocturnal sleep episodes [
5,
6] and reproduced three SNPs from previous studies [
6].
These sleep-related traits appear to occur with varying frequencies in individuals with insomnia, a condition characterized by decreased quality and/or quantity of sleep in absence of other organic disorders [
7]. A study by Vgontzas et al. reported that individuals with short sleep duration are almost five times more likely to suffer from persistent insomnia [
8]. Several studies have shown that individuals with evening chronotype are more likely to present insomnia symptoms [
2,
9,
10], in particular difficulties in falling asleep [
11]. Therefore, the co-occurrence of specific sleep traits and insomnia might be the product of a common genetic and biological background.
Several GWAS studies seems to support the hypothesis of pleiotropy. A GWAS study by Stein et al. reported an inverted correlation between genetic loci associated with both insomnia and morning chronotype [
12]. Several studies conducted on the UK biobank population presented also overlapping genes for insomnia and sleep duration [
13‐
15]. On the other hand, another GWAS study of the same population did not report any common genes for objective measure of several sleep phenotypes [
5].
Most studies on insomnia treat insomnia as a single entity and do not consider individual patterns of insomnia symptoms. Insomnia may presents itself as a combination of night-time symptoms but one of these symptom may prevail over the others: difficulties in falling asleep, trouble with staying asleep and waking up too early. Few studies have examined each individual nocturnal symptom of insomnia, despite evidence that the different symptoms may represent biologically distinct mechanisms. Stoffers et al. described decreased gray matter density in a part of the left orbitofrontal cortex in individuals reporting waking up too early, but not in those reporting trouble with falling asleep or sleep maintenance [
16]. Epidemiological studies showed that different symptoms are associated with different incidence of physical and psychiatric conditions [
17] and mortality [
18]. In one such study, males experiencing sleep onset insomnia or terminal insomnia had a risk three-fold higher than healthy sleepers to receive a disability pension due to a mental condition, while maintenance insomnia gave a considerably lower risk.
For these reasons, we argue that investigating individual insomnia symptoms may aid in the identification of genetic overlap with sleep-related traits. Elucidating the relationship between nocturnal insomnia symptoms and sleep-related traits might clarify the etiology and help diagnostic and therapeutic processes.
In order to investigate this relationship, we conducted an association study on individual symptoms of insomnia and SNPs previously reported to be associated with sleep-related phenotypes. The use of material from the Nord-Trøndelag Health Study (HUNT) gave us the opportunity to investigate this relationship in a large sample from a general population.
Discussion
Sixteen SNPs previously associated with sleep-related traits were significantly associated with at least one symptom or a combination of symptoms of insomnia. However, none of these variations stayed significant after correction for multiple statistical testing.
Among our highest hits, there was SNP rs10493596. This variation is close to the
AK5 (Adenylate Kinase 5) gene that encodes for an adenylate kinase expressed exclusively in the brain. This protein is involved in ATP homeostasis by catalyzing the transfer of phosphate groups among adenine nucleotides. Rs10493596 was associated with “morningness” in a study by Hu et al. [
2] while in our study it gave the lowest
p-value for difficulties falling asleep in combination with early morning awakenings.
Difficulties in falling asleep with early morning awakenings showed also an association with rs2302729 that was previously associated with sleep quality and latency [
4]. This SNP is located on
CACNA1C (Calcium Voltage-Gated Channel Subunit Alpha1 C) whose involvement in several psychiatric conditions is supported by epidemiological and animal studies [
26]. Knockout mice for
CACNA1C display traits that resemble symptoms of mental disorders and autism such as cognitive decline, anxiety, hyperactivity, decreased sociability, decreased synaptic plasticity [
27].
Of note is the presence of a polymorphism on gene
MEIS1 among our highest hits.
MEIS1 is involved in restless leg syndrome (RLS) [
28] but recently also reported to be associated with insomnia symptoms [
5,
13,
29]. In our study, combinations of symptoms sleep onset problems with maintenance insomnia and all symptoms together showed low
p-values (0.01 and 0.02 respectively) and discrete odds ratio (1.5 and 1.4) for the T allele of rs113851554, in agreement with previous studies. This finding strengthens the hypothesis that insomnia and RLS may be overlapping phenotypes not easy to discern [
13].
The A allele of rs12927162 on gene
TOX3 (TOX High Mobility Group Box Family Member 3), decreased the chances of maintenance insomnia. This SNP is reported in significant association with being a morning person [
2] but also with measure of circadian phase delay [
5].
Rs1823125 near gene
PAX8 was firstly reported as associated with sleep duration in the CHANGE consortium sample [
23], and successively in the UK Biobank sample [
6] in which it was associated also with sleep efficiency [
5]. In our study, it was associated with maintenance insomnia. PAX8 is a transcription factor with proven role in kidney and thyroid morphogenesis. Its role on sleep is yet to be investigated, and it is possible that rs1823125 is not in fact influencing
PAX8 but another gene nearby as it is located in a intragenic region.
Most of our results seems to have a plausible explanation in spite of ending up statistically non-significant after correction for multiple testing. The need for correction when testing multiple hypotheses may often be redundant especially in the context of biology or in K. J. Rothman words “… when scientists are studying biological relations rather than random numbers, the premise that type I errors are the major concern may be wrong” [
30]. Therefore overestimating the role of “chance” when analyzing biological data may lead to type II errors. In our case, we chose SNPs known to influence sleep-related traits, therefore their involvement in insomnia is plausible.
Strengths and limitations
The HUNT study collected data from Norwegians from the region of Nord-Trøndelag which gives the advantage of a relative-high genetic isolation and exposure to similar environmental factor that may influence sleep (natural light, cultural habits etc.). Also, strong welfare polices implemented in Norway lessen the effect of socioeconomic disparities that may affect the analyses.
Unfortunately, the HUNT study did not gather information about sleep length or satisfaction that could have helped determining the presence of an actual disorder. Inclusion of this information in future studies and the finding of endophenotypes may help the discovery of relevant genetic associations.
Several of our significant SNPs (before correction for multiple testing) were associated with the subgroup of symptoms “sleep onset with early morning awakenings”. This combination of symptoms was the rarest, with only 73 individuals reporting it. This further support the strength of the association.
Acknowledgements
The Nord-Trøndelag Health Study (The HUNT Study) is a collaboration between HUNT Research Centre (Faculty of Medicine, Norwegian University of Science and Technology NTNU), Nord-Trøndelag County Council, Central Norway Health Authority, and the Norwegian Institute of Public Health.
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