Table 3 Biological studies on melatonin in ASD

Rossignol et al. [36]

Initial studies review: 35 reviewed independently by two reviewers

Five of them were investigated through meta-analysis

Database used: PubMed, Google Scholar, CINAHL, EMBASE, Scopus, ERIC

Quality of studies was assessed through Downs and Black checklist

Nine studies measured melatonin or its metabolites in ASD: all reported at least one alteration (four studies: abnormal melatonin circadian rhythm; seven studies: below average physiological levels; four studies: positive correlation between melatonin/derivatives levels and autistic behaviors). Five studies reported gene abnormalities that could decrease melatonin production or impair melatonin receptor function in a small percentage of ASD children

Limitations of the review: small sample size; variation in protocols for measuring changes in sleep parameters; five studies contained a mixture of individuals with ASD and other developmental disabilities

Wang et al. [40]

ASD children (N = 398, M = 367, F = 31, range 2–17 years) and healthy controls (N = 437, M = 406, F = 31)

Genotyping sequences in ASMT, DNA analysis and prediction the effects of coding non-synonymous variants on protein function

Four rare ASMT mutations were found only in ASD group (p.R115W, p.V166I, p.V179G, and p.W257X)

Limitations: low sample size for a rare genetic mutations investigation; important disproportion between F and M composition (367/31 in ASD group vs. 406/31 in control one); lack of information on the clinical and biochemical impacts of the ASMT deleterious variants and/or SNPs; other genes in melatonin pathway were not sequenced

Strengths: controlled study

Veatch et al. [41]

ASD individuals (N = 29, M = 24 and F = 5, 15 of them underwent analysis for ASMT sequences, while 14 of the total for CY1A2 genotypes)

Examination of variation in two melatonin pathway genes, ASMT and CYP1A2

Higher frequencies than currently reported for variants evidenced to decrease ASMT expression and related to decreased CYP1A2 enzyme activity; a relationship between genotypes in ASMT and CYP1A2 was revealed; expression of sleep onset delay relates to melatonin pathway genes

Limitations: lack of a well-defined control group; all 11 individuals who participated in the melatonin trial were responsive to treatment; minimalization of the environmental effect of poor sleep habits through parent sleep education

Pagan et al. [32]

Unrelated patients with ASD (N = 278), first-degree relatives (129 unaffected siblings, 377 patients) and controls (N = 416)

Serotonin, melatonin and the intermediate NAS measured through whole-blood serotonin, platelet NAS and plasma melatonin

In patients the melatonin deficit was only significantly associated with insomnia. Impairments of melatonin synthesis in ASD may be linked with decreased 14-3-3 proteins. Disruption of the serotonin-NAS-melatonin pathway is a very frequent trait in ASD patients and may be a useful biomarker for a large subgroup of these individuals

Limitations: not equally subdivision between three groups

Strengths: two groups of possible controls (unaffected siblings and other controls)

Pagan et al. [44]

ASD patients (N = 239), ASD parents (N = 303), unaffected siblings (N = 78), controls (N = 278)

Examination of melatonin synthesis in post-mortem pineal gland, serotonin synthesis in gut samples, blood platelets

Melatonin deficits in ASD depends on reduction activity of both enzymes implicated in melatonin synthesis (AANAT and ASMT)

Limitations: small number of patient samples

Strengths: three different groups

Braam et al. [33]

Mothers of an ASD child (N = 60, mean age: 42.9 ± 5.7 years), control group of mothers (N = 15, mean age = 44.3 ± 9.7 years)

6-SM concentration

6-SM levels were significantly lower in mothers with an ASD child than in controls, and so, low parental melatonin levels could be one of the contributors to ASD and possibly ID etiology

Limitations: differences between number composition of both groups and small sample size; CYP1A2 activity not measured; different children ASD etiologies

Strength: presence of a control group

Maruani et al. [45]

ASD individuals (N = 81), unaffected relatives (N = 90), control participants (N = 48)

PGV estimation based on magnetic resonance imaging; blood sampling and plasma melatonin measurement

Patients had both morning melatonin levels and PGV lower than controls; plasma melatonin was correlated to the group of the participant, but also to the PGV melatonin; variations in ASD could be mainly driven by melatonin pathway dysregulation

Limitations: melatonin was detected only in mornings; difficulties in exactly detecting PGV; numeric differences between the first two groups and the third

Strengths: three experimental groups