Serotonin transporter gene variation is a risk factor for sudden infant death syndrome in the Japanese population

Molecular autopsy for sudden death in Japan

Japan has various death investigation systems; however, external examinations, postmortem computed tomography, macroscopic examinations, and microscopic examinations are performed regardless of the system used. These examinations can reveal morphological abnormalities, whereas the cause of death in cases with non-morphological abnormalities can be detected through additional examinations. Molecular autopsy and postmortem genetic analyses are important additional examinations. They are capable of detecting inherited arrhythmias or inherited metabolic diseases, which are representative non-morphological disorders that cause sudden death, especially in infants and young people. In this review, we introduce molecular autopsy reports from Japan and describe our experience with representative cases. The relationships between drug-related deaths and genetic variants are also reviewed. Based on the presented information, molecular autopsy is expected to be used as routine examinations in death investigations because they can provide information to save new lives.

Home Cardiorespiratory Monitoring in Infants at Risk for Sudden Infant Death Syndrome (SIDS), Apparent Life-Threatening Event (ALTE) or Brief Resolved Unexplained Event (BRUE)

Sudden infant death syndrome (SIDS) is defined as the sudden death of an infant younger than one year of age which remains unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene, and review of the clinical history. About 90% of SIDS occur before six months of age, the peak incidence is between two and four months, and the median age for death is elven weeks. The clinical, social, and economic relevance of SIDS, together with the evidence that prevention of this syndrome was possible, has significantly stimulated research into risk factors for the development of SIDS in the hope of being able to introduce new effective preventive measures. This narrative review discusses the potential relationships between apparent life-threatening events (ALTE) or brief resolved unexplained events (BRUE) and SIDS development, and when a home cardiorespiratory monitor is useful for prevention of these conditions. A literature analysis showed that home cardiorespiratory monitoring has been considered a potential method to identify not only ALTE and BRUE but SIDS also. ALTE and BRUE are generally due to underlying conditions that are not detectable in SIDS infants. A true relationship between these conditions has never been demonstrated. Use of home cardiorespiratory monitor is not recommended for SIDS, whereas it could be suggested for children with previous ALTE or severe BRUE or who are at risk of the development of these conditions. However, use of home cardiorespiratory monitors assumes that family members know the advantages and limitations of these devices after adequate education and instruction in their use.

Sudden infant death syndrome revisited: serotonin transporter gene, polymorphisms and promoter methylation

BACKGROUND

Based on findings in the brain stems of SIDS victims, the serotonin transporter (5-HTT) gene has been discussed to be associated with SIDS.

METHODS

In the largest study to date, we investigated the promoter length (5-HTTLPR) and intron 2 VNTR polymorphisms in 274 cases and 264 controls and the Ile425Val polymorphism in 65 cases and 64 controls. Moreover, the methylation of the internal promoter region was investigated in 35 cases and 14 controls.

RESULTS

For 5-HTTLPR, we observed a trend towards an association of allele L (58.8% vs. 53.4%) with SIDS and significant results were observed after stratifying for age, season at death, and prone position. Nevertheless, when pooling all published data, a significant association of allele L with SIDS is confirmed (p: 0.001). For the intron 2 VNTR polymorphism, no significant differences were observed. After pooling, a significant accumulation of the rare allele 9 was observed in SIDS (2.1% vs. 0.6%; p: 0.018). For the Ile425Val polymorphism, no differences were observed.

CONCLUSION

We conclude that genetic variation at this gene might be of some importance in SIDS. Epigenetic analysis of the internal promoter, however, revealed no influence on the relative risk to succumb to SIDS.

IMPACT

This is the largest study published up to now on 5-HTT gene polymorphisms and SIDS. Polymorphisms in the 5-HTT gene appear to contribute (although to a small degree) to the risk to die from SIDS. There is no evidence that a methylation of the promoter region is of impact for the etiology of SIDS.

Identification and functional characterization of the extremely long allele of the serotonin transporter-linked polymorphic region

SLC6A4, which encodes the serotonin transporter, has a functional polymorphism called the serotonin transporter-linked polymorphic region (5-HTTLPR). The 5-HTTLPR consists of short (S) and long (L) alleles, each of which has 14 or 16 tandem repeats. In addition, the extralong (XL) and other rare alleles have been reported in 5-HTTLPR. Although they are more frequent in Asian and African than in other populations, the extent of variations and allele frequencies (AFs) were not addressed in a large population. Here, we report the AFs of the rare alleles in a large number of Japanese subjects (N = 2894) consisting of two cohorts. The first cohort (case-control study set, CCSS) consisted of 1366 subjects, including 485 controls and 881 patients with psychosis (bipolar disorder or schizophrenia). The second cohort (the Arao cohort study set, ACSS) consisted of 1528 elderly subjects. During genotyping, we identified 11 novel 5-HTTLPR alleles, including 3 XL alleles. One novel allele had the longest subunit ever reported, consisting of 28 tandem repeats. We named this XL_28-A. An in vitro luciferase assay revealed that XL_28-A has no transcriptional activity. XL_28-A was found in two unrelated patients with bipolar disorder in the CCSS and one healthy subject in the ACSS who did not show depressive symptoms or a decline in cognitive function. Therefore, it is unlikely that XL_28-A is associated with psychiatric disorders, despite its apparent functional deficit. Our results suggest that unraveling the complex genetic variations of 5-HTTLPR will be important for further understanding its role in psychiatric disorders.

The Genetics of Sudden Infant Death Syndrome-Towards a Gene Reference Resource

Sudden infant death syndrome (SIDS) is the unexpected death of an infant under one year of age that remains unexplained after a thorough investigation. Despite SIDS remaining a diagnosis of exclusion with an unexplained etiology, it is widely accepted that SIDS can be caused by environmental and/or biological factors, with multiple underlying candidate genes. However, the lack of biomarkers raises questions as to why genetic studies on SIDS to date are unable to provide a clearer understanding of the disease etiology. We sought to improve the identification of SIDS-associated genes by reviewing the SIDS genetic literature and objectively categorizing and scoring the reported genes based on the strength of evidence (from C1 (high) to C5 (low)). This was followed by analyses of function, associations between genes, the enrichment of gene ontology (GO) terms, and pathways and gender difference in tissue gene expression. We constructed a curated database for SIDS gene candidates consisting of 109 genes, 14 of which received a category 4 (C4) and 95 genes received the lowest category of C5. That none of the genes was classified into the higher categories indicates the low level of supporting evidence. We found that genes of both scoring categories show distinct networks and are highly diverse in function and involved in many GO terms and pathways, in agreement with the perception of SIDS as a heterogeneous syndrome. Genes of both scoring categories are part of the cardiac system, muscle, and ion channels, whereas immune-related functions showed enrichment for C4 genes. A limited association was found with neural development. Overall, inconsistent reports and missing metadata contribute to the ambiguity of genetic studies. Considering those parameters could help improve the identification of at-risk SIDS genes. However, the field is still far from offering a full-pledged genetic test to identify at-risk infants and is still hampered with methodological challenges and misunderstandings of the vulnerabilities of vital biological mechanisms.

Comprehensive Analysis of Genes Associated With Sudden Infant Death Syndrome

Background: Sudden infant death syndrome (SIDS) is a tragic incident which remains a mystery even after post-mortem investigation and thorough researches. Methods: This comprehensive review is based on the genes reported in the molecular autopsy studies conducted on SIDS so far. A total of 20 original studies and 7 case reports were identified and included in this analysis. The genes identified in children or adults were not included. Most of the genes reported in these studies belonged to cardiac channel and cardiomyopathy. Cardiac channel genes in SIDS were scrutinized for further analysis. Results: After screening and removing the duplicates, 42 unique genes were extracted. When the location of these genes was assessed, it was observed that most of these belonged to Chromosomes 11, 1 and 3 in sequential manner. The pathway analysis shows that these genes are involved in the regulation of heart rate, action potential, cardiac muscle cell contraction and heart contraction. The protein-protein interaction network was also very big and highly interactive. SCN5A, CAV3, ALG10B, AKAP9 and many more were mainly found in these cases and were regulated by many transcription factors such as MYOG C2C1 and CBX3 HCT11. Micro RNA, "hsa-miR-133a-3p" was found to be prevalent in the targeted genes. Conclusions: Molecular and computational approaches are a step forward toward exploration of these sad demises. It is so far a new arena but seems promising to dig out the genetic cause of SIDS in the years to come.

Perinatal Nicotine Reduces Chemosensitivity of Medullary 5-HT Neurons after Maturation in Culture

Perinatal exposure to nicotine produces ventilatory and chemoreflex deficits in neonatal mammals. Medullary 5-HT neurons are putative central chemoreceptors that innervate respiratory nuclei and promote ventilation, receive cholinergic input and express nicotinic acetylcholine receptors (nAChRs). Perforated patch clamp recordings were made from cultured 5-HT neurons dissociated from the medullary raphé of 0-3 day old mice expressing enhanced yellow fluorescent protein driven by the enhancer region for PET1 (ePet-EYFP). The effect of exposure to low (6 mg kg^-1day^-1) or high (60 mg kg^-1day^-1) doses of nicotine in utero (prenatal), in culture (postnatal), or both and the effect of acute nicotine exposure (10 μM), were examined on baseline firing rate (FR at 5% CO₂, pH = 7.4) and the change in FR with acidosis (9% CO₂, pH 7.2) in young (12-21 days in vitro, DIV) and older (≥22 DIV) acidosis stimulated 5-HT neurons. Nicotine exposed neurons exhibited ∼67% of the response to acidosis recorded in neurons given vehicle (p = 0.005), with older neurons exposed to high dose prenatal and postnatal nicotine, exhibiting only 28% of that recorded in the vehicle neurons (p < 0.01). In neurons exposed to low or high dose prenatal and postnatal nicotine, acute nicotine exposure led to a smaller increase in FR (∼+51% vs +168%, p = 0.026) and response to acidosis (+6% vs +67%, p = 0.014) compared to vehicle. These data show that exposure to nicotine during development reduces chemosensitivity of 5-HT neurons as they mature, an effect that may be related to the abnormal chemoreflexes reported in rodents exposed to nicotine in utero, and may cause a greater risk for sudden infant death syndrome (SIDS).

Specific localization of manserin peptide in the rat carotid body

The carotid body, located at the bifurcation of the common carotid artery, is a small sensory organ that detects changes in oxygen concentration and plays a vital role in controlling respiration. Although several molecules, such as neurotransmitters and neuropeptides, are involved in the regulation of the respiratory system, their detailed mechanisms have not been established yet. This study identifies that the presence of manserin, a neuropeptide, in the carotid body may play a crucial role in regulating respiration. The carotid bodies of adult Wistar rats were perfused with paraformaldehyde, and the frozen sections were subjected to immunohistochemical analyses. The carotid body comprises two distinct types of cells, neuron-like glomus cells and glial-like sustentacular cells. We used specific antibodies to distinguish the specific location of manserin in the carotid body, which included a tyrosine hydroxylase-positive antibody for glomus cells and an S100 protein antibody for sustentacular cells. Immunofluorescence analysis revealed that while tiny, round signals were exclusively observed in the cytoplasm of glomus cells, no signals were observed in sustentacular cells. Because manserin is believed to be secreted from precursor proteins by the endoproteolytic processing of a large precursor protein called secretogranin II, manserin secretion systems may exist in the carotid body, and thus, behave as potential regulators of respiration in the carotid body.

High serum serotonin in sudden infant death syndrome

Sudden infant death syndrome (SIDS), the leading cause of postneonatal infant mortality, likely comprises heterogeneous disorders with the common phenotype of sudden death without explanation upon postmortem investigation. Previously, we reported that ∼40% of SIDS deaths are associated with abnormalities in serotonin (5-hydroxytryptamine, 5-HT) in regions of the brainstem critical in homeostatic regulation. Here we tested the hypothesis that SIDS is associated with an alteration in serum 5-HT levels. Serum 5-HT, adjusted for postconceptional age, was significantly elevated (95%) in SIDS infants (n = 61) compared with autopsied controls (n = 15) [SIDS, 177.2 ± 15.1 (mean ± SE) ng/mL versus controls, 91.1 ± 30.6 ng/mL] (P = 0.014), as determined by ELISA. This increase was validated using high-performance liquid chromatography. Thirty-one percent (19/61) of SIDS cases had 5-HT levels greater than 2 SDs above the mean of the controls, thus defining a subset of SIDS cases with elevated 5-HT. There was no association between genotypes of the serotonin transporter promoter region polymorphism and serum 5-HT level. This study demonstrates that SIDS is associated with peripheral abnormalities in the 5-HT pathway. High serum 5-HT may serve as a potential forensic biomarker in autopsied infants with SIDS with serotonergic defects.

Sudden Infant Death Syndrome - Role of Trigeminocardiac Reflex: A Review

Sudden infant death syndrome (SIDS) is an unexplained death in infants, which usually occurs during sleep. The cause of SIDS remains unknown and multifactorial. In this regard, the diving reflex (DR), a peripheral subtype of trigeminocardiac reflex (TCR), is also hypothesized as one of the possible mechanisms for this condition. The TCR is a well-established neurogenic reflex that manifests as bradycardia, hypotension, apnea, and gastric hypermotility. The TCR shares many similarities with the DR, which is a significant physiological adaptation to withstand hypoxia during apnea in many animal species including humans in clinical manifestation and mechanism of action. The DR is characterized by breath holding (apnea), bradycardia, and vasoconstriction, leading to increase in blood pressure. Several studies have described congenital anomalies of autonomic nervous system in the pathogenesis of SIDS such as hypoplasia, delayed neuronal maturation, or decreased neuronal density of arcuate nucleus, hypoplasia, and neuronal immaturity of the hypoglossal nucleus. The abnormalities of autonomic nervous system in SIDS may explain the role of TCR in this syndrome involving sympathetic and parasympathetic nervous system. We reviewed the available literature to identify the role of TCR in the etiopathogenesis of SIDS and the pathways and cellular mechanism involved in it. This synthesis will help to update our knowledge and improve our understanding about this mysterious, yet common condition and will open the door for further research in this field.

Synaptogenesis and Myelination in the Nucleus/Tractus Solitarius: Potential Role in Apnea of Prematurity, Congenital Central Hypoventilation, and Sudden Infant Death Syndrome

Fetuses as early as 15 weeks' gestation exhibit rhythmical respiratory movements shown by real-time ultrasonography. The nucleus/tractus solitarius is the principal brainstem respiratory center; other medullary nuclei also participate. The purpose was to determine temporal maturation of synaptogenesis. Delayed synaptic maturation may explain neurogenic apnea or hypoventilation of prematurity and some cases of sudden infant death syndrome. Sections of medulla oblongata were studied from 30 human fetal and neonatal brains 9 to 41 weeks' gestation. Synaptophysin demonstrated the immunocytochemical sequence of synaptogenesis. Other neuronal markers and myelin stain also were applied. The nucleus/tractus solitarius was similarly studied in fetuses with chromosomopathies, metabolic encephalopathies, and brain malformations. Synapse formation in the nucleus solitarius begins at about 12 weeks' gestation and matures by 15 weeks; myelination initiated at 33 weeks. Synaptogenesis was delayed in 3 fetuses with different conditions, but was not specific for only nucleus solitarius. Delayed synaptogenesis or myelination in the nucleus solitarius may play a role in neonatal hypoventilation, especially in preterm infants and in some sudden infant death syndrome cases.

Hyperplasia of pulmonary neuroendocrine cells in infancy and childhood

Pulmonary neuroendocrine cells (PNEC) are widely distributed throughout the airway mucosa of mammalian lung as solitary cells and as distinctive innervated clusters, neuroepithelial bodies (NEB). These cells differentiate early during lung development and are more prominent in fetal/neonatal lungs compared to adults. PNEC/NEB cells produce biogenic amine (serotonin) and a variety of peptides (i.e., bombesin) involved in regulation of lung function. During the perinatal period, NEB are thought to function as airway O(2)/CO(2) sensors. Increased numbers of PNEC/NEBs have been observed in a variety of perinatal and postnatal lung disorders. Recent advances in cellular and molecular biology of these cells, as they relate to perinatal and postnatal lung disorders associated with PNEC/NEB cell hyperplasia are reviewed and their possible role in pulmonary pathobiology discussed (WC 125).

Systems-level perspective of sudden infant death syndrome

Sudden infant death syndrome (SIDS) remains one of the primary causes of infant mortality in developed countries. Although the causes of SIDS remain largely inconclusive, some of the most informative associations implicate molecular, genetic, anatomical, physiological, and environmental (i.e., infant sleep) factors. Thus, a comprehensive and evolving systems-level model is required to understand SIDS susceptibility. Such models, by being powerful enough to uncover indirect associations, could be used to expand our list of candidate targets for in-depth analysis. We present an integrated WikiPathways model for SIDS susceptibility that includes associated cell systems, signaling pathways, genetics, and animal phenotypes. Experimental and literature-based gene-regulatory data have been integrated into this model to identify intersecting upstream control elements and associated interactions. To expand this pathway model, we performed a comprehensive analysis of existing proteomics data from brainstem samples of infants with SIDS. From this analysis, we discovered changes in the expression of several proteins linked to known SIDS pathologies, including factors involved in glial cell production, hypoxia regulation, and synaptic vesicle release, in addition to interactions with annotated SIDS markers. Our results highlight new targets for further consideration that further enrich this pathway model, which, over time, can improve as a wiki-based, community curation project.

Carnitine palmitoyltransferase 2 gene polymorphism is a genetic risk factor for sudden unexpected death in infancy

RATIONALE

Carnitine palmitoyltransferase (CPT) II is one of a pivotal enzyme in mitochondrial fatty acid oxidation, which is essential for energy production during simultaneous glucose sparing and a requirement for major energy supply, such as prolonged fasting or exercise. When infants require more energy than provided by the glycolytic system, they rely on the mitochondrial fatty acid oxidation pathway. Mutations of the CPT2 gene have been reported to cause sudden unexpected death in infancy (SUDI). A thermolabile phenotype of a CPT2 polymorphism (F352C) has been recently reported to reduce CPT II enzyme activity. The F352C variant results in energy crisis at high temperature and is suspected as a risk factor for acute encephalopathy. However, a relationship between CPT2 gene polymorphism and SUDI has not been described.

METHODS

Single nucleotide polymorphisms of the CPT2 gene were investigated among 54 SUDI cases and 200 healthy volunteers.

RESULTS

The frequency of the C allele was significantly higher in the SUDI group than in the control group [25.0% vs 16.0%, odds ratio (OR)=1.75, 95% confidence interval (CI)=1.05-2.92, p=0.030). The frequency of the F352C homozygote was significantly higher in the SUDI group than in control group (11.1% vs 3.5%, OR=3.45, 95% CI=1.11-10.73, p=0.036).

CONCLUSION

The F352C CPT2 variant might be a genetic risk factor for SUDI.

Genetic variation in the monoamine oxidase A and serotonin transporter genes in sudden infant death syndrome

AIM

The purpose of this study was to investigate common polymorphisms in the genes encoding monoamine oxidase A (MAOA) and serotonin transporter (5-HTT) in Norwegian cases of sudden infant death syndrome (SIDS). This was done to further elucidate the role of genetic variation in these genes and SIDS.

METHODS

A variable number of tandem repeat area in the promoter of the MAOA gene and rs25531 in the promoter region of the gene encoding 5-HTT were investigated in 193 SIDS cases and 335 controls. The methods used were polymerase chain reaction, restriction fragment analysis and gel electrophoresis.

RESULTS

There were no differences between SIDS cases and controls for any of the investigated polymorphisms. This was also true when male and female SIDS cases were analysed separately.

CONCLUSION

This article indicates that neither the VNTR in the promoter of the MAOA gene, nor rs25531 in the gene encoding 5-HTT, is involved in SIDS. However, as medullary serotonergic abnormalities most likely contribute to the death in at least some SIDS cases, it is important to investigate these genes, as well as other genes involved in the serotonergic network, in more detail.

Sodium/proton exchanger 3 (NHE3) and sudden infant death syndrome (SIDS)

The sodium/proton exchanger protein 3 (NHE3) is located in chemosensitive areas of the medulla oblongata and plays an important role in the central control of respiration. Overexpression of NHE3 is correlated with lower respiration and might therefore contribute to the vulnerability of infants dying suddenly and unexpected (sudden infant death syndrome, SIDS). Our aim in this study was to verify already reported genetic variations in the NHE3 gene in an independent SIDS cohort from Switzerland. Two single nucleotide polymorphisms (SNPs) in the promoter region (G1131A and C1197T) and one variation in the coding sequence of exon 16 (C2405T) in the NHE3 gene were analyzed in 160 Caucasian SIDS infants and 192 Swiss adult controls by using a single base extension method (SNaPshot multiplex). No significant differences were detected in the allelic frequencies of the three NHE3 polymorphisms between SIDS cases and controls. We conclude that the three investigated NHE3 SNPs are unlikely to play a major role in the pathogenesis of SIDS in Caucasian infants. However, further genetic investigations in different ethnicities are required to determine whether variations in NHE3 are associated with an increased SIDS risk.

Gender-specific association of the SLC6A4 and DRD2 gene variants in bipolar disorder

Findings on the association between the risk for developing bipolar disorder and the functions of the serotonin transporter-linked polymorphic region gene (5-HTTLPR) and dopamine D2 receptor gene (DRD2) variants are contradictory. One explanation for this is that a gender difference may exist for genetic contributions. We compared the gender-related main effects and the gene-to-gene interaction between serotonin transporter gene (SLC6A4) and DRD2 in adult male and female patients with bipolar I (BP-I) and bipolar II (BP-II) disorder. Patients with BP-I (n = 400) and BP-II (n = 493), and healthy controls (n = 442) were recruited from Taiwan's Han Chinese population. The genotypes of the 5-HTTLPR and DRD2 Taq-IA polymorphisms were determined using polymerase chain reaction-restriction fragment length polymorphism analysis. Logistic regression analysis showed a significant gender-specific association of the DRD2 A1/A1 and the 5-HTTLPR S/S, S/LG , and LG/LG (S+) (p = 0.01) genotypes in men with BP-I (p = 0.002 and 0.01, respectively) and BP-II (p = 0.001 and 0.007, respectively), but not in women. A significant interaction for the DRD2 A1/A1 and 5-HTTLPR S+ polymorphisms was also found only in men with BP-I and BP-II (p = 0.003 and 0.001, respectively). We provide preliminary evidence for a gender-specific effect of the SLC6A4 and DRD2 gene variants for the risk of BP-I and of BP-II. We also found gender-specific interaction between 5-HTTLPR and DRD2 Taq-IA polymorphisms in patients with bipolar disorder.

PHOX2B polyalanine repeat length is associated with sudden infant death syndrome and unclassified sudden infant death in the Dutch population

Unclassified sudden infant death (USID) is the sudden and unexpected death of an infant that remains unexplained after thorough case investigation including performance of a complete autopsy and review of the circumstances of death and the clinical history. When the infant is below 1 year of age and with onset of the fatal episode apparently occurring during sleep, this is referred to as sudden infant death syndrome (SIDS). USID and SIDS remain poorly understood despite the identification of several environmental and some genetic risk factors. In this study, we investigated genetic risk factors involved in the autonomous nervous system in 195 Dutch USID/SIDS cases and 846 Dutch, age-matched healthy controls. Twenty-five DNA variants from 11 genes previously implicated in the serotonin household or in the congenital central hypoventilation syndrome, of which some have been associated with SIDS before, were tested. Of all DNA variants considered, only the length variation of the polyalanine repeat in exon 3 of the PHOX2B gene was found to be statistically significantly associated with USID/SIDS in the Dutch population after multiple test correction. Interestingly, our data suggest that contraction of the PHOX2B exon 3 polyalanine repeat that we found in six of 160 SIDS and USID cases and in six of 814 controls serves as a probable genetic risk factor for USID/SIDS at least in the Dutch population. Future studies are needed to confirm this finding and to understand the functional effect of the polyalanine repeat length variation, in particular contraction, in exon 3 of the PHOX2B gene.

Role of the serotonin transporter in pulmonary arterial hypertension

Pulmonary arterial hypertension is a disease in which pulmonary arterial pressure is raised, leading to right heart failure. Survival is poor despite current therapeutic strategies. The 'serotonin hypothesis of pulmonary arterial hypertension' arose in the 1960s following an 'epidemic' of pulmonary arterial hypertension in women taking the indirect serotinergic agonist aminorex as an anorexigen. In the 1980s, the hypothesis was revisited following the occurrence of pulmonary arterial hypertension associated with the use of fenfluramines as anorexigens; these are also indirect serotinergic agents. Research has identified changes in serotonin synthesis, serotonin receptor activation and serotonin uptake via the serotonin transporter in experimental and clinical pulmonary arterial hypertension. This review will discuss our current understanding of this serotonin hypothesis with particular reference to the role of the serotonin transporter.

Effects of calcium (Ca(2+)) extrusion mechanisms on electrophysiological properties in a hypoglossal motoneuron: insight from a mathematical model

Spike-frequency dynamics and spike shape can provide insight into the types of ion channels present in any given neuron and give a sense for the precise response any neuron may have to a given input stimulus. Motoneuron firing frequency over time is especially important due to its direct effect on motor output. Of particular interest is intracellular Ca(2+), which exerts a powerful influence on both firing properties over time and spike shape. In order to better understand the cellular mechanisms for the regulation of intracellular Ca(2+) and their effect on spiking behavior, we have modified a computational model of an HM to include a variety of Ca(2+) handling processes. For the current study, a series of HM models that include Ca(2+) pumps, Na(+)/Ca(2+) exchangers, and a generic exponential decay of excess Ca(2+) were generated. Simulations from these models indicate that although each extrusion mechanism exerts a similar effect on voltage, the firing properties change distinctly with the inclusion of additional Ca(2+)-related mechanisms: BK channels, Ca(2+) buffering, and diffusion of [Ca(2+)]i modeled via a linear diffusion partial differential equation. While an exponential decay of Ca(2+) seems to adequately capture short-term changes in firing frequency seen in biological data, internal diffusion of Ca(2+) appears to be necessary for capturing longer term frequency changes.

Serotonin gene variants are unlikely to play a significant role in the pathogenesis of the sudden infant death syndrome

Sudden infant death syndrome (SIDS) is defined as the sudden and unexpected death of an infant less than 12 months of age that is related to a sleep period and remains unexplained after a complete autopsy, death scene investigation, and review of the clinical history. The cause of SIDS is unknown, but a major subset of SIDS is proposed to result from abnormalities in serotonin (5-HT) and related neurotransmitters in regions of the lower brainstem that result in failure of protective homeostatic responses to life-threatening challenges during sleep. Multiple studies have implicated gene variants that affect different elements of 5-HT neurotransmission in the pathogenesis of these abnormalities in SIDS. In this review I discuss the data from these studies together with some new data correlating genotype with brainstem 5-HT neurochemistry in the same SIDS cases and conclude that these gene variants are unlikely to play a major role in the pathogenesis of the medullary 5-HT abnormalities observed in SIDS.

Congenital central hypoventilation syndrome and sudden infant death syndrome: disorders of autonomic regulation

Long considered a rare and unique disorder of respiratory control, congenital central hypoventilation syndrome has recently been further distinguished as a disorder of autonomic regulation. Similarly, more recent evidence suggests that sudden infant death syndrome is also a disorder of autonomic regulation. Congenital central hypoventilation syndrome typically presents in the newborn period with alveolar hypoventilation, symptoms of autonomic dysregulation and, in a subset of cases, Hirschsprung disease or tumors of neural crest origin or both. Genetic investigation identified PHOX2B, a crucial gene during early autonomic development, as disease defining for congenital central hypoventilation syndrome. Although sudden infant death syndrome is most likely defined by complex multifactorial genetic and environmental interactions, it is also thought to result from central deficits in the control of breathing and autonomic regulation. The purpose of this article is to review the current understanding of these autonomic disorders and discuss the influence of this information on clinical practice and future research directions.

Sudden infant death syndrome: an update and new perspectives of etiology

Sudden infant death syndrome (SIDS) is a condition in which an infant, usually in the early postnatal period and nearly always before 6 months of age, dies during sleep for unexplained reasons and the standard autopsy fails to disclose an etiology. Various physiological explanations of risk factors include the prone sleeping position, overheating by excessive bundling, viral upper respiratory tract infections, parental smoking at home, and birthing injury resulting in an insult to the inner ear and central chemoreceptor zone, an immaturity that involves CO2 chemoreceptors that regulate respiratory control. Neuropathological studies and theories implicate: (1) hypoplasia or defective transmitter function in the medullary arcuate nucleus, a derivative of the rhombencephalic lip of His; (2) synaptic or receptor immaturity of the nucleus of the fasciculus solitarius, the "pneumotaxic center"; and (3) functional impairment of the serotonergic raphé nuclei of the pontine and medullary ventral median septum and other serotonergic neurons of the brainstem. Additional neurological risk factors for SIDS include perinatal neuromuscular diseases, infantile epilepsies or status epilepticus, and genetic metabolic encephalopathies.

The evolutionary significance of depression in Pathogen Host Defense (PATHOS-D)

Given the manifold ways that depression impairs Darwinian fitness, the persistence in the human genome of risk alleles for the disorder remains a much debated mystery. Evolutionary theories that view depressive symptoms as adaptive fail to provide parsimonious explanations for why even mild depressive symptoms impair fitness-relevant social functioning, whereas theories that suggest that depression is maladaptive fail to account for the high prevalence of depression risk alleles in human populations. These limitations warrant novel explanations for the origin and persistence of depression risk alleles. Accordingly, studies on risk alleles for depression were identified using PubMed and Ovid MEDLINE to examine data supporting the hypothesis that risk alleles for depression originated and have been retained in the human genome because these alleles promote pathogen host defense, which includes an integrated suite of immunological and behavioral responses to infection. Depression risk alleles identified by both candidate gene and genome-wide association study (GWAS) methodologies were found to be regularly associated with immune responses to infection that were likely to enhance survival in the ancestral environment. Moreover, data support the role of specific depressive symptoms in pathogen host defense including hyperthermia, reduced bodily iron stores, conservation/withdrawal behavior, hypervigilance and anorexia. By shifting the adaptive context of depression risk alleles from relations with conspecifics to relations with the microbial world, the Pathogen Host Defense (PATHOS-D) hypothesis provides a novel explanation for how depression can be nonadaptive in the social realm, whereas its risk alleles are nonetheless represented at prevalence rates that bespeak an adaptive function.

Star-crossed? The association of the 5-HTTLPR s allele with season of birth in a healthy female population, and possible consequences for temperament, depression and suicide

BACKGROUND

Birth season has well-known effects on neuropsychiatric disorders, and may also influence genotype distribution by possibly influencing chance of conception via parental idiosyncratic conception patterns or survival of foetuses or infants. The 5-HTTLPR is associated with phenomena including affective temperaments or suicide which are also associated with birth season. Our aim was to investigate the association of 5-HTTLPR genotype and birth season in a healthy female population.

METHODS

Birth date and 5-HTTLPR genotype was determined for 327 psychiatrically healthy women. The association between presence of s allele and time of birth was analysed using generalized linear models.

RESULTS

A significant association between s allele frequency and time of birth was detected. S allele carrier frequency was marginally significantly higher in July borns and significantly lower in autumn borns.

LIMITATIONS

We investigated an adult sample so genotype frequency data do not reflect birth frequencies. Our sample consisted exclusively of females.

CONCLUSIONS

There is no clear explanation for the observed association, although idiosyncratic parental conception patterns, the association of 5-HTTLPR with sudden infant/intrauterine death, or other s allele-mediated behaviours may play a role. Our results are strikingly parallel with earlier data reporting a higher risk of completed suicide in July borns, and higher scores of July borns and lower scores of autumn borns on certain affective temperament scales, both of which are also associated with the s allele of 5-HTTLPR. Thus our results may add to the growing body of evidence regarding the etiological background of affective disorders.

Cardiac ion channelopathies and the sudden infant death syndrome

The sudden infant death syndrome (SIDS) causes the sudden death of an apparently healthy infant, which remains unexplained despite a thorough investigation, including the performance of a complete autopsy. The triple risk model for the pathogenesis of SIDS points to the coincidence of a vulnerable infant, a critical developmental period, and an exogenous stressor. Primary electrical diseases of the heart, which may cause lethal arrhythmias as a result of dysfunctioning cardiac ion channels (“cardiac ion channelopathies”) and are not detectable during a standard postmortem examination, may create the vulnerable infant and thus contribute to SIDS. Evidence comes from clinical correlations between the long QT syndrome and SIDS as well as genetic analyses in cohorts of SIDS victims (“molecular autopsy”), which have revealed a large number of mutations in ion channel-related genes linked to inheritable arrhythmogenic syndromes, in particular the long QT syndrome, the short QT syndrome, the Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. Combining data from population-based cohort studies, it can be concluded that at least one out of five SIDS victims carries a mutation in a cardiac ion channel-related gene and that the majority of these mutations are of a known malignant phenotype.

Impaired chemosensitivity of mouse dorsal raphe serotonergic neurons overexpressing serotonin 1A (Htr1a) receptors

Background

Serotonergic system participates in a wide range of physiological processes and behaviors, but its role is generally considered as modulatory and noncrucial, especially concerning life-sustaining functions. We recently created a transgenic mouse line in which a functional deficit in serotonin homeostasis due to excessive serotonin autoinhibition was produced by inducing serotonin 1A receptor (Htr1a) overexpression selectively in serotonergic neurons (Htr1a raphe-overexpressing or Htr1a^RO mice). Htr1a^RO mice exhibit episodes of autonomic dysregulation, cardiovascular crises and death, resembling those of sudden infant death syndrome (SIDS) and revealing a life-supporting role of serotonergic system in autonomic control. Since midbrain serotonergic neurons are chemosensitive and are implicated in arousal we hypothesized that their chemosensitivity might be impaired in Htr1a^RO mice.

Principal findings

Loose-seal cell-attached recordings in brainstem slices revealed that serotonergic neurons in dorsal raphe nucleus of Htr1a^RO mice have dramatically reduced responses to hypercapnic challenge as compared with control littermates. In control mice, application of 9% CO₂ produced an increase in firing rate of serotonergic neurons (0.260±0.041 Hz, n = 20, p = 0.0001) and application of 3% CO₂ decreased their firing rate (−0.142±0.025 Hz, n = 17, p = 0.0008). In contrast, in Htr1a^RO mice, firing rate of serotonergic neurons was not significantly changed by 9% CO₂ (0.021±0.034 Hz, n = 16, p = 0.49) and by 3% CO₂ (0.012±0.046 Hz, n = 12, p = 0.97).

Conclusions

Our findings support the hypothesis that chemosensitivity of midbrain serotonergic neurons provides a physiological mechanism for arousal responses to life-threatening episodes of hypercapnia and that functional impairment, such as excessive autoinhibition, of midbrain serotonergic neuron responses to hypercapnia may contribute to sudden death.

Interaction between serotonin transporter and serotonin receptor 1 B genes polymorphisms may be associated with antisocial alcoholism

Background

Several studies have hypothesized that genes regulating the components of the serotonin system, including serotonin transporter (5-HTTLPR) and serotonin 1 B receptor (5-HT1B), may be associated with alcoholism, but their results are contradictory because of alcoholism’s heterogeneity. Therefore, we examined whether the 5-HTTLPR gene and 5-HT1B gene G861C polymorphism are susceptibility factors for a specific subtype of alcoholism, antisocial alcoholism in Han Chinese in Taiwan.

Methods

We recruited 273 Han Chinese male inmates with antisocial personality disorder (ASPD) [antisocial alcoholism (AS-ALC) group (n = 120) and antisocial non-alcoholism (AS-N-ALC) group (n = 153)] and 191 healthy male controls from the community. Genotyping was done using PCR-RFLP.

Results

There were no significant differences in the genotypic frequency of the 5-HT1B G861C polymorphism between the 3 groups. Although AS-ALC group members more frequently carried the 5-HTTLPR S/S, S/L_G, and L_G/L_G genotypes than controls, the difference became non-significant after controlling for the covarying effects of age. However, the 5-HTTLPR S/S, S/L_G, and L_G/L_G genotypes may have interacted with the 5-HT1B G861C C/C polymorphism and increased the risk of becoming antisocial alcoholism.

Conclusion

Our study suggests that neither the 5-HTTLPR gene nor the 5-HT1B G861C polymorphism alone is a risk factor for antisocial alcoholism in Taiwan’s Han Chinese population, but that the interaction between both genes may increase susceptibility to antisocial alcoholism.

Serotonin transporter role in identifying similarities between SIDS and idiopathic ALTE

OBJECTIVE

Considering previous genetic studies on sudden infant death syndrome (SIDS) and the role of L/L serotonin transporter (5HTT) genotype and correlated genes monoamine oxidase A (MAOA) and dopamine transporter (DAT) in unexpected death, an investigation was carried out verifying their involvement in apparent life-threatening events (ALTE and idiopathic form [IALTE]), also assessing common molecular basis with SIDS.

METHODS

Differential diagnoses in 76 ALTE infants, distinguishing ALTE from IALTE was elaborated by using clinical-diagnostic data. Genotypes/allelic frequencies of DAT, MAOA, and 5HTT were determined in ALTE and IALTE infants and compared with data obtained from 20 SIDS and 150 controls.

RESULTS

No association was found between DAT polymorphisms and ALTE/IALTE groups either at the genotype or allelic level (P range .11-.94). MAOA genotypes and allele data comparison between ALTE and controls was not significant; IALTE data showed a tendency for genotypes (P = .09) and were statistically significant for alleles (P = .036); however, MAOA significance disappeared once the Bonferroni correction was applied. 5HTT polymorphisms in IALTE remarked the role of L/L genotype (P < .00001) and L (P < .00001), as previously demonstrated in SIDS.

CONCLUSIONS

Considering correspondence between 5HTT and MAOA in IALTE and SIDS, we hypothesize that the 2 syndromes are different expressions of a common ethiopathogenesis. In particular, genetic data suggest SIDS events could derive from IALTE episodes occurred during sleep, and therefore out of parental control. Despite its functional role, results highlight the usefulness of 5HTT as a valuable tracer of SIDS risk in IALTE infants. Owing to the small sample size, the results are to be considered preliminary and should be reevaluated in an independent sample.

5-HT7R/G12 signaling regulates neuronal morphology and function in an age-dependent manner

The common neurotransmitter serotonin controls different aspects of early neuronal differentiation, although the underlying mechanisms are poorly understood. Here we report that activation of the serotonin 5-HT(7) receptor promotes synaptogenesis and enhances synaptic activity in hippocampal neurons at early postnatal stages. An analysis of Gα(12)-deficient mice reveals a critical role of G(12)-protein for 5-HT(7) receptor-mediated effects in neurons. In organotypic preparations from the hippocampus of juvenile mice, stimulation of 5-HT(7)R/G(12) signaling potentiates formation of dendritic spines, increases neuronal excitability, and modulates synaptic plasticity. In contrast, in older neuronal preparations, morphogenetic and synaptogenic effects of 5-HT(7)/G(12) signaling are abolished. Moreover, inhibition of 5-HT(7) receptor had no effect on synaptic plasticity in hippocampus of adult animals. Expression analysis reveals that the production of 5-HT(7) and Gα(12)-proteins in the hippocampus undergoes strong regulation with a pronounced transient increase during early postnatal stages. Thus, regulated expression of 5-HT(7) receptor and Gα(12)-protein may represent a molecular mechanism by which serotonin specifically modulates formation of initial neuronal networks during early postnatal development.

Association between a functional polymorphism in the MAOA gene and sudden infant death syndrome

OBJECTIVES

Abnormalities in the serotonergic as well as the noradrenergic neuronal systems are believed to contribute to sudden infant death syndrome (SIDS). The X-chromosomal monoamine oxidase A (MAOA) gene is of importance for both systems and up to now no systematic study on a functional polymorphism in this gene has been performed in a sufficiently large group.

METHODS

We investigated a functional MAOA promoter length polymorphism in 156 white SIDS cases and 260 gender- and age-matched control subjects by using capillary electrophoresis and fluorescence dye labeled primers.

RESULTS

The pooled low-expressing alleles *2 and *3 were more frequent in the 99 male SIDS cases than in 161 male control subjects (44.4% vs 25.5%). However, there were no differences in female cases. The frequency of low expression alleles varied significantly with the age at death and were significantly more frequent in children who died between an age of 46 and 154 days than at an older age (54.9% vs 22.6%).

CONCLUSIONS

Our results indicate a relationship between SIDS and the MAOA genotype in boys via influencing serotonergic and noradrenergic neurons in the brainstem. This locus is the first X-chromosomal locus associated with SIDS. Our results support the theory that abnormalities in the brainstem contribute to a subset of SIDS, at least in boys. Moreover, we argue that not only the serotonergic system but also other neuronal systems, among those the noradrenergic one, are involved.

An Association Between the Serotonin Transporter Gene Promoter Polymorphism and Smoking Cessation Among Japanese Males

The objective of this study was to replicate the associations between the serotonin transporter genotypes and smoking cessation and between these genotypes and long-term smoking cessation success (for 2 years or more). Two case–control designs were used in this study. The first case–control design consisted of 47 ex-smokers (cases) and 94 smokers (controls). The second case–control design consisted of the 12 ex-smokers who had succeeded in long-term smoking cessation (cases) and the 24 smokers (controls). In the first design, there was no significant difference in allele frequency and genotype distribution between cases and controls. In the second design, the S/S genotype frequency was higher in cases than in controls, but the difference did not reach slightly statistical significance. The present study suggests that individuals with the S/S genotype are more inclined to achieve long-term smoking cessation than others.

5HT1A receptors inhibit glutamate inputs to cardiac vagal neurons post-hypoxia/hypercapnia

Synaptic inputs to cardiac vagal neurons (CVNs) regulate parasympathetic activity to the heart. Previous work has shown insults such as hypoxia and hypercapnia (H/H) alter CVN activity by activating post-synaptic serotonergic, purinergic, and glutamatergic receptors in CVNs. This study examines the role of serotonergic 5HT1A receptors in modulating these excitatory neurotransmissions to CVNs during control conditions, H/H and recovery from H/H. Excitatory post-synaptic currents (EPSCs) were recorded from identified CVNs in vitro before, during and post H/H. The 5HT1A receptor antagonist WAY 100635 had no effect on EPSCs in CVNs before, and during H/H. However, during recovery from H/H inspiratory-related excitatory serotonergic and purinergic pathways were recruited to excite CVNs. However, when these serotonergic and purinergic pathways are blocked, the 5HT1A receptor antagonist WAY 100635 restores an excitatory glutamatergic neurotransmission to CVNs. This study indicates endogenous activation of serotonergic 5HT1A receptors diminishes glutamatergic neurotransmission to CVNs following H/H, likely via a presynaptic site of action.

Molecular Investigations of Sudden Unexplained Deaths

Sudden unexplained deaths in apparently healthy individuals (newborn through adult) pose a vexing challenge to medical examiners, law enforcement and society as a whole. Recent advances in “molecular autopsies” have begun to uncover the mystery surrounding sudden unexplained deaths by identifying mutations that can result in or predispose an apparently healthy individual to sudden death. Genetic risks of sudden unexplained deaths have been studied from several different perspectives, and categorized generally by systems, including: cardiac, nervous, immune, and metabolic. This article reviews the genetic risks in sudden unexplained deaths, presents the current state and challenges of molecular investigations, and sheds light on future directions in sudden unexplained death investigations.

Simultaneous genotyping of multiple polymorphisms in human serotonin transporter gene and detection of novel allelic variants

The serotonin transporter, called SLC6A4, SERT or 5-HTT, modulates neurotransmission by removal of serotonin from the synapse of serotonergic neurons, facilitating serotonin reuptake into the presynaptic terminus. Selective serotonin reuptake inhibitors block the action of the serotonin transporter and are used to treat depression and other neuropsychiatric disorders. Three polymorphisms in the 5-HTT gene have been implicated in treatment response and neuropsychiatric disorders. A 44-bp promoter ins/del polymorphism (5-HTTLPR) produces primarily long and/or short alleles due to either 14 (short) or 16 (long) repeats of variably conserved 20-23 bp units. Also implicated, a 17-18 bp variable number tandem repeat found in intron2 (StIn2) is expressed as triallelic content with 9, 10, or 12 repeats (StIn2.9, StIn2.10 or StIn2.12). Finally, a single nucleotide polymorphism rs25531 located within the promoter polymorphic-linked region alters the function of the long promoter allele. We developed a PCR-based fragment analysis assay, which is analyzed on an ABI sequencer, whereby we are able to detect all three genotypes simultaneously. Using this technique, we identified novel sequences, which demonstrate promoter repeat regions containing (1) a 17 repeat with rs25531 A/G polymorphism, (2) two with 18-repeat units, (3) one with 20-repeat units and (4) a 24-repeat sequence. The novel repeats were confirmed by direct sequencing of gel-purified amplicons.

The serotonergic anatomy of the developing human medulla oblongata: implications for pediatric disorders of homeostasis

The caudal serotonergic (5-HT) system is a critical component of a medullary "homeostatic network" that regulates protective responses to metabolic stressors such as hypoxia, hypercapnia, and hyperthermia. We define anatomically the caudal 5-HT system in the human medulla as 5-HT neuronal cell bodies located in the raphé (raphé obscurus, raphé magnus, and raphé pallidus), extra-raphé (gigantocellularis, paragigantocellularis lateralis, intermediate reticular zone, lateral reticular nucleus, and nucleus subtrigeminalis), and ventral surface (arcuate nucleus). These 5-HT neurons are adjacent to all of the respiratory- and autonomic-related nuclei in the medulla where they are positioned to modulate directly the responses of these effector nuclei. In the following review, we highlight the topography and development of the caudal 5-HT system in the human fetus and infant, and its inter-relationships with nicotinic, GABAergic, and cytokine receptors. We also summarize pediatric disorders in early life which we term "developmental serotonopathies" of the caudal (as well as rostral) 5-HT domain and which are associated with homeostatic imbalances. The delineation of the development and organization of the human caudal 5-HT system provides the critical foundation for the neuropathologic elucidation of its disorders directly in the human brain.

The role of serotonin in respiratory function and dysfunction

Serotonin (5-HT) is a neuromodulator-transmitter influencing global brain function. Past and present findings illustrate a prominent role for 5-HT in the modulation of ponto-medullary autonomic circuits. 5-HT is also involved in the control of neurotrophic processes during pre- and postnatal development of neural circuits. The functional implications of 5-HT are particularly illustrated in the alterations to the serotonergic system, as seen in a wide range of neurological disorders. This article reviews the role of 5-HT in the development and control of respiratory networks in the ponto-medullary brainstem. The review further examines the role of 5-HT in breathing disorders occurring at different stages of life, in particular, the neonatal neurodevelopmental diseases such as Rett, sudden infant death and Prader-Willi syndromes, adult diseases such as sleep apnoea and mental illness linked to neurodegeneration.

Genomic risk factors in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is a major contributor to postneonatal infant death, and is the third leading cause of infant mortality in the USA. While public health efforts have reduced these deaths in recent years, the pathogenesis of SIDS remains unclear. Epidemiological data on SIDS-related deaths have suggested genetic factors, and many studies have attempted to identify SIDS-associated genes. This has resulted in a large body of literature implicating various genes and their encoded proteins and signaling pathways in numerous cohorts of various sizes and ethnicities. This review has undertaken a systematic evaluation of these studies, identifying the pathways that have been implicated in these studies, including central nervous system pathways, cardiac channelopathies, immune dysfunction, metabolism/energy pathways, and nicotine response. This review also explores how new genomic techniques will aid in advancing our knowledge of the genomic risk factors associated with SIDS, including SNPs and copy number variation. Last, this review explores how the current information can be applied to aid in our assessment of the at risk infant population.

Lack of association of the serotonin transporter polymorphism with the sudden infant death syndrome in the San Diego Dataset

Dysfunction of medullary serotonin (5-HT)-mediated respiratory and autonomic function is postulated to underlie the pathogenesis of the majority of sudden infant death syndrome (SIDS) cases. Several studies have reported an increased frequency of the LL genotype and L allele of the 5-HT transporter (5-HTT) gene promoter polymorphism (5-HTTLPR), which is associated with increased transcriptional activity and 5-HT transport in vitro, in SIDS cases compared with controls. These findings raise the possibility that this polymorphism contributes to or exacerbates existing medullary 5-HT dysfunction in SIDS. In this study, we tested the hypothesis that the frequency of LL genotype and L allele are higher in 179 SIDS cases compared with 139 controls of multiple ethnicities in the San Diego SIDS Dataset. We observed no significant association of genotype or allele with SIDS cases either in the total cohort or on stratification for ethnicity. These observations do not support previous findings that the L allele and/or LL genotype of the 5-HTTLPR are associated with SIDS.

Genetics of the sudden infant death syndrome

The sudden infant death syndrome (SIDS) is currently defined as "the sudden unexpected death of an infant less than 1 year of age with onset of the fatal episode apparently occurring during sleep, that remains unexplained after a thorough investigation". SIDS, whose etiology remains rather vague, is still the major cause of death among infants between 1 month and 1 year of age in industrialized countries with varying incidences in different populations. Herein, after touching on definitory approaches and several current hypotheses concerning SIDS etiology, we focus on the triple risk model of SIDS and discuss two large classes of genetic factors potentially contributing to or predisposing for the generation of a vulnerable infant that, when encountering an environmental trigger, may succumb to SIDS. We conclude by acknowledging that for the integration of the vast and complex genetic evidence concerning SIDS, a lot more research will be required and we briefly discuss the potential use of recently presented animal models for functional studies of SIDS pathology.

Gene variants predisposing to SIDS: current knowledge

Genetic risk factors play a role in sudden unexpected infant death; either as a cause of death, such as in cases with medium-chain acyl-coenzyme A dehydrogenase deficiency and cardiac arrest due to long QT syndrome, or as predisposing factors for sudden infant death syndrome (SIDS). Most likely genetic predisposition to SIDS represent a polygenic inheritance pattern leading to sudden death when combined with other risk factors, such as a vulnerable developmental stage of the central nervous system and/or the immune system, in addition to environmental risk factors, such as a common cold or prone sleeping position. Genes involved in the regulation of the immune system, cardiac function, the serotonergic network and brain function and development have so far emerged as the most important with respect to SIDS. The purpose of the present paper is to survey current knowledge on SIDS and possible genetic contributions.

Aquaporin-4 gene variation and sudden infant death syndrome

The purpose of this study was to investigate the aquaporin-4 (AQP4) gene in cases of sudden infant death syndrome (SIDS) and controls and to elucidate the hypothesis that a genetically determined disturbed water homeostasis in the brain is involved as a predisposing factor in SIDS. The single nucleotide polymorphisms (SNPs) rs2075575, rs4800773, rs162004, and rs3763043 in the AQP4 gene were investigated in 141 SIDS cases and 179 controls. For each SIDS case, a brain/body weight ratio was calculated. The study revealed an association between the T allele and the CT/TT genotypes of rs2075575 and SIDS (C versus T, p < 0.01; CC versus CT/TT, p = 0.03). For the other three investigated SNPs, there were no differences in genotype frequencies between SIDS cases and controls. For the SNP rs2075575, it was also found an association between brain/body weight ratio and genotype in the SIDS cases aged 0.3-12 wk (p = 0.014, median ratio CC 10.6, CT/TT 12.1). In conclusion, this study indicates that rs2075575 may be of significance as a predisposing factor for SIDS, and that the CT/TT genotypes are associated with an increased brain/body weight ratio in infants dying from SIDS during the vulnerable period from birth up to 3 mo of age.

Postnatal changes in tryptophan hydroxylase and serotonin transporter immunoreactivity in multiple brainstem nuclei of the rat: implications for a sensitive period

Previously, we found that the brainstem neuronal network in normal rats undergoes abrupt neurochemical, metabolic, and physiological changes around postnatal days (P) 12-13, a critical period when the animal's response to hypoxia is also the weakest. This has special implications for sudden infant death syndrome (SIDS), insofar as seemingly normal infants succumb to SIDS when exposed to respiratory stressors (e.g., hypoxia) during a narrow postnatal window. Because an abnormal serotonergic system has recently been implicated in SIDS, we conducted a large-scale investigation of the 5-HT-synthesizing enzyme tryptophan hydroxylase (TPH) and serotonin transporter (SERT) with semiquantitative immunohistochemistry in multiple brainstem nuclei of normal rats aged P2-21. We found that 1) TPH and SERT immunoreactivity in neurons of raphé magnus, obscurus, and pallidus and SERT in the neuropil of the pre-Bötzinger complex, nucleus ambiguus, and retrotrapezoid nucleus were high at P2-11 but decreased markedly at P12 and plateaued thereafter until P21; 2) SERT labeling in neurons of the lateral paragigantocellular nucleus (LPGi) and parapyramidal region (pPy) was high at P2-9 but fell significantly at P10, followed by a gradual decline until P21; 3) TPH labeling in neurons of the ventrolateral medullary surface was stable except for a significant fall at P12; and 4) TPH and SERT immunoreactivity in a number of other nuclei was relatively stable from P2 to P21. Thus, multiple brainstem nuclei exhibited a significant decline in TPH and SERT immunoreactivity during the critical period, suggesting that such normal development can contribute to a narrow window of vulnerability in postnatal animals.

The serotonin transporter polymorphism (5-HTTLPR): allelic variation and links with depressive symptoms

BACKGROUND

We compare the genotype distribution for the serotonin transporter polymorphism (5-HTTLPR) in a sample of older Taiwanese adults with samples of various racial and ethnic groups collected in other studies. We also explore interactions among sex, stressors, and 5-HTTLPR genotype on depressive symptoms in our sample.

METHODS

Using a nationally representative sample of 984 Taiwanese aged 53 and older, we model depressive symptoms as a function of 5-HTTLPR genotype and two classes of stressors: lifetime trauma and recent major life events. We test two- and three-way interactions among stressors, 5-HTTLPR, and sex.

RESULTS

This sample exhibits higher frequency of S/S and lower frequency of L/L genotype than Western samples, but the distribution is comparable to those in East Asian populations. Nearly 9% carry an allele (XL) that has rarely been reported in the literature. Although the gene-environment (GxE) interaction with recent major life events is not significant, our results suggest that trauma has a worse effect on depressive symptoms for those with S/S or S/L genotype than for those who do not carry the S allele (P<0.05). We find no evidence that this GxE interaction varies by sex.

CONCLUSIONS

Previous studies of this GxE interaction have been inconclusive, perhaps because interactions between genotype and stressful events are more prominent under extreme stressors. Our findings underscore the need to move beyond a bi-allelic parameterization of the 5-HTTLPR polymorphism and raise questions about why East Asian populations exhibit low rates of depression despite a high frequency of the S allele.