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Genes associated with serotonin synthesis and receptivity have some of the strongest genetic association with SIDS. Principle among these genes is the serotonin transporter SLC6A4 and the serotonin receptor HTR1A. SLC6A4 exhibits decreased expression in the raphe nucleus of the medulla oblongata and polymorphisms specifically associated with SIDS (PMID:19342987). In 75% of infants with SIDS, there is decreased HTR1A expression relative to controls along with an increase in the number of raphe serotonin neurons (PMID:19342987). Over-expression of the mouse orthologue of the HTR1A gene in the juvenile mouse medulla produces an analogous phenotype to SIDS with death due to bradycardia and hypothermia (PMID:18599790). These genes as well as those involved in serotonin synthesis are predicted to be transcriptionally regulated by a common factor, FEV (human orthologue of PET-1). PET-1 knock-out results in up to a 90% loss of serotonin neurons (PMID:12546819), while polymorphisms in FEV are over-represented in African American infants with SIDS. In addition to FEV, other transcription factors implicated in the regulation of these genes (Putative transcriptional regulators (TRs)) and FEV are also listed (see associated references). In addition to serotonin, vasopressin signaling and its regulation by serotonin appear to be important in a common pathway of cardiopulmonary regulation (PMID:2058745). A protein that associates with vasopressin signaling, named pituitary adenylate cyclase-activating polypeptide (ADCYAP1), results in a SIDS like phenotype, characterized by a high increase in spontaneous neonatal death, exacerbated by hypothermia and hypoxia (PMID:14608012), when disrupted in mice. Protein for this gene is widely distributed throughout the central nervous system (CNS), including autonomic control centers (PMID:12389210). ADCYAP1 and HTR1A are both predicted to be transcriptionally regulated by REST promoter binding. Regulation of G-protein coupled signaling pathways is illustrated for these genes, however, it is not clear whether ADCYAP1 acts directly upon raphe serotonin neurons.
Another potentially important class of receptors in SIDS is nicotine. Receptors for nicotine are expressed in serotonin neurons of the raphe throughout development (PMID:18986852). Application of nicotine or cigarette smoke is sufficient to inhibit electrical activity of raphe serotonin neurons (PMID:17515803) and chronic nicotine infusion in rats decreases expression of SLC6A4 (PMID:18778441). Furthermore, nicotine exposure reduces both HTR1A and HTR2A immunoreactivity in several nuclei of the brainstem (PMID:17451658).
In addition to CNS abnormalities, several studies have identified a critical link between cardiac arrhythmia (long QT syndrome) and SIDS (PMID:18928334). A number of genetic association studies identified functionally modifying mutations in critical cardiac channels in as many as 10% of all SIDS cases (PMID:18928334). These mutations have been predicted to predispose infants for long QT syndrome and sudden death. The highest proportion of SIDS associated mutations (both inherited and sporadic) is found in the sodium channel gene SCN5A. Examination of putative transcriptional regulators for these genes, highlights a diverse set of factors as well as a relatively common one (SP1).
Finally, several miscellaneous mutations have been identified in genes associated with inflammatory response and thermoregulation. Infection is considered a significant risk factor for SIDS (PMID:19114412). For inflammatory associated genes, such as TNF alpha, interleukin 10 and complement component 4, many of these mutations are only significant in the presence of infection and SIDS. In addition to these mutations, cerebrospinal fluid levels of IL6 are increased in SIDS cases as well as IL6R levels in the arcuate nucleus of the brain, another major site of serotonin synthesis (PMID:19396608). Genes such as ILR6 and ADCYAP1 are also associated with autoimmune disorders, thus SIDS may also be associated with autoinflammation of autonomic centers in the brain. Regulation of thermogenesis by brown adipose tissue has been proposed be an important component of SIDS, given that SIDS incidence is highest in the winter time and that animal models of SIDS demonstrate variation in body temperature. Interestingly, activation of raphe HTR1A decreases both shivering and peripheral vasoconstriction in piglets (18094064). Although a putative significant polymorphism was identified in the thermoregulator gene HSP60, this only occurred in one SIDS case. It is important to note that in the large majority of all these studies, sleeping position and smoking were among the most significant risk factors for SIDS.
In loving memory of Milo Salomonis (http://www.milosalomonis.org).
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