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The initial search for attention-deficit hyperactivity disorder (ADHD) genes was hypothesis driven and focused on genes involved in neurotransmission, based on evidence from effective pharmacotherapeutic agents, animal model, and neuroimaging studies. Candidate genes, linkage, and genome-wide analyses studies (GWAS) have identified several gene variants involved in neurotransmission which confer a modest risk for ADHD. It is also possible that few common variants conferring risk for ADHD exist in the European population and that very large samples will be necessary to identify them. Structural variant studies are indicating that ADHD genetic risk is likely to be transmitted largely by rare variants that collectively disrupt a sizable constellation of genes, presumably with related functions. This is supported by animal genetic models which also implicate numerous genes involved in complex interactions between neural pathways. Epigenetic factors, environmental factors, and gene regulatory elements most likely also play a role in ADHD genetics.
Few mutations have been found in the human leptin gene and the relationship between leptin gene
sequence variation and human overweight is uncertain. To determine whether sequence variation
within the leptin gene and its regulatory elements contribute to extreme obesity, we screened ∼3 kb
of the 5′ flanking region and the three exons in 125 unrelated extremely obese
(BMI [ges ] 40 kg/m2) and 86 average weight women (BMI < 27 kg/m2). Within the protein coding regions only one
heterozygous silent mutation was found (codon 102; AAC/AAT). Within the 5′ flanking region, six
frequent sequence variants were detected (q > 0.10), and the allele frequencies of three of these
variants differed between obese and average weight Caucasian women (+19, χ2 = 4.46, p = 0.035;
−1823, χ2 = 4.36, p = 0.037; −2548, χ2 = 5.73, p = 0.017). Nine infrequent sequence variants were
detected (q < 0.05) but they did not occur more often among obese women compared with those of
average-weight. For extremely obese women, three polymorphisms (+19, −188, and −633)
predicted the degree of obesity. Allelic variants may influence the regulation of the leptin gene and
thereby influence body weight, particularly among extremely obese women. However, given the low
variability in coding regions and the high variability in the 5′ flanking region, discerning the
functional significance of each variant is likely to be difficult.
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