Known pathogenic gene variants and new candidates detected in sudden unexpected infant death using whole genome sequencing.

The purpose of this study is to gain insights into potential genetic factors contributing to the infant's vulnerability to Sudden Unexpected Infant Death (SUID). Whole Genome Sequencing (WGS) was performed on 144 infants that succumbed to SUID, and 573 healthy adults. Variants were filtered by gnomAD allele frequencies and predictions of functional consequences. Variants of interest were identified in 88 genes, in 64.6% of our cohort. Seventy-three of these have been previously associated with SIDS/SUID/SUDP. Forty-three can be characterized as cardiac genes and are related to cardiomyopathies, arrhythmias, and other conditions. Variants in 22 genes were associated with neurologic functions. Variants were also found in 13 genes reported to be pathogenic for various systemic disorders and in two genes associated with immunological function. Variants in eight genes are implicated in the response to hypoxia and the regulation of reactive oxygen species (ROS) and have not been previously described in SIDS/SUID/SUDP. Seventy-two infants met the triple risk hypothesis criteria. Our study confirms and further expands the list of genetic variants associated with SUID. The abundance of genes associated with heart disease and the discovery of variants associated with the redox metabolism have important mechanistic implications for the pathophysiology of SUID.

Dysregulation of platelet serotonin, 14-3-3, and GPIX in sudden infant death syndrome.

Sudden infant death syndrome (SIDS) is the leading cause of post-neonatal infant mortality, but the underlying cause(s) are unclear. A subset of SIDS infants has abnormalities in the neurotransmitter, serotonin (5-hydroxytryptamine [5-HT]) and the adaptor molecule, 14-3-3 pathways in regions of the brain involved in gasping, response to hypoxia, and arousal. To evaluate our hypothesis that SIDS is, at least in part, a multi-organ dysregulation of 5-HT, we examined whether blood platelets, which have 5-HT and 14-3-3 signaling pathways similar to brain neurons, are abnormal in SIDS. We also studied platelet surface glycoprotein IX (GPIX), a cell adhesion receptor which is physically linked to 14-3-3. In infants dying of SIDS compared to infants dying of known causes, we found significantly higher intra-platelet 5-HT and 14-3-3 and lower platelet surface GPIX. Serum and plasma 5-HT were also elevated in SIDS compared to controls. The presence in SIDS of both platelet and brainstem 5-HT and 14-3-3 abnormalities suggests a global dysregulation of these pathways and the potential for platelets to be used as a model system to study 5-HT and 14-3-3 interactions in SIDS. Platelet and serum biomarkers may aid in the forensic determination of SIDS and have the potential to be predictive of SIDS risk in living infants.

Altered 5-HT2A/C receptor binding in the medulla oblongata in the sudden infant death syndrome (SIDS): Part II. Age-associated alterations in serotonin receptor binding profiles within medullary nuclei supporting cardiorespiratory homeostasis.

The failure of chemoreflexes, arousal, and/or autoresuscitation to asphyxia may underlie some sudden infant death syndrome (SIDS) cases. In Part I, we showed that some SIDS infants had altered 5-hydroxytryptamine (5-HT)2A/C receptor binding in medullary nuclei supporting chemoreflexes, arousal, and autoresuscitation. Here, using the same dataset, we tested the hypotheses that the prevalence of low 5-HT1A and/or 5-HT2A/C receptor binding (defined as levels below the 95% confidence interval of controls-a new approach), and the percentages of nuclei affected are greater in SIDS versus controls, and that the distribution of low binding varied with age of death. The prevalence and percentage of nuclei with low 5-HT1A and 5-HT2A/C binding in SIDS were twice that of controls. The percentage of nuclei with low 5-HT2A/C binding was greater in older SIDS infants. In >80% of older SIDS infants, low 5-HT2A/C binding characterized the hypoglossal nucleus, vagal dorsal nucleus, nucleus of solitary tract, and nuclei of the olivocerebellar subnetwork (important for blood pressure regulation). Together, our findings from SIDS infants and from animal models of serotonergic dysfunction suggest that some SIDS cases represent a serotonopathy. We present new hypotheses, yet to be tested, about how defects within serotonergic subnetworks may lead to SIDS.

Multiomic Analysis of Neuroinflammation and Occult Infection in Sudden Infant Death Syndrome.

Importance: Antemortem infection is a risk factor for sudden infant death syndrome (SIDS)-the leading postneonatal cause of infant mortality in the developed world. Manifestations of infection and inflammation are not always apparent in clinical settings or by standard autopsy; thus, enhanced resolution approaches are needed. Objective: To ascertain whether a subset of SIDS cases is associated with neuroinflammation and occult infection. Design, Setting, and Participants: In this case-control study, postmortem fluids from SIDS cases and controls collected between July 2011 and November 2018 were screened for elevated inflammatory markers, specifically cerebrospinal fluid (CSF) neopterin and CSF and serum cytokines. CSF, liver, and brain tissue from SIDS cases with elevated CSF neopterin were subjected to metagenomic next-generation sequencing (mNGS) to probe for infectious pathogens. Brainstem tissue from a subset of these cases was analyzed by single-nucleus RNA sequencing (snRNAseq) to measure cell type-specific gene expression associated with neuroinflammation and infection. All tissue and fluid analyses were performed from April 2019 to January 2023 in a pathology research laboratory. Included was autopsy material from infants dying of SIDS and age-matched controls dying of known causes. Exposures: There were no interventions or exposures. Main Outcomes and Measures: CSF neopterin levels were measured by high-performance liquid chromatography. Cytokines were measured by multiplex fluorometric assay. mNGS was performed on liver, CSF, brain, and brainstem tissue. snRNAseq was performed on brainstem tissue. Results: A cohort of 71 SIDS cases (mean [SD] age, 55.2 [11.4] postconceptional weeks; 42 male [59.2%]) and 20 controls (mean [SD] age, 63.2 [16.9] postconceptional weeks; 11 male [55.0%]) had CSF and/or serum available. CSF neopterin was screened in 64 SIDS cases and 15 controls, with no exclusions. Tissues from 6 SIDS cases were further analyzed. For CSF neopterin measures, SIDS samples were from infants with mean (SD) age of 54.5 (11.3) postconceptional weeks (38 male [59.4%]) and control samples were from infants with mean (SD) age of 61.5 (17.4) postconceptional weeks (7 male [46.7%]). A total of 6 SIDS cases (9.3%) with high CSF neopterin were identified, suggestive of neuroinflammation. mNGS detected human parechovirus 3 (HPeV3) in tissue and CSF from 1 of these 6 cases. snRNAseq of HPeV3-positive brainstem tissue (medulla) revealed dramatic enrichment of transcripts for genes with predominately inflammatory functions compared with 3 age-matched SIDS cases with normal CSF neopterin levels. Conclusions and Relevance: Next-generation molecular tools in autopsy tissue provide novel insight into pathogens that go unrecognized by normal autopsy methodology, including in infants dying suddenly and unexpectedly.

Known pathogenic gene variants and new candidates detected in Sudden Unexpected Infant Death using Whole Genome Sequencing.

Purpose: To gain insights into potential genetic factors contributing to the infant's vulnerability to Sudden Unexpected Infant Death (SUID). Methods: Whole Genome Sequencing (WGS) was performed on 145 infants that succumbed to SUID, and 576 healthy adults. Variants were filtered by gnomAD allele frequencies and predictions of functional consequences. Results: Variants of interest were identified in 86 genes, 63.4% of our cohort. Seventy-one of these have been previously associated with SIDS/SUID/SUDP. Forty-three can be characterized as cardiac genes and are related to cardiomyopathies, arrhythmias, and other conditions. Variants in 22 genes were associated with neurologic functions. Variants were also found in 13 genes reported to be pathogenic for various systemic disorders. Variants in eight genes are implicated in the response to hypoxia and the regulation of reactive oxygen species (ROS) and have not been previously described in SIDS/SUID/SUDP. Seventy-two infants met the triple risk hypothesis criteria (Figure 1). Conclusion: Our study confirms and further expands the list of genetic variants associated with SUID. The abundance of genes associated with heart disease and the discovery of variants associated with the redox metabolism have important mechanistic implications for the pathophysiology of SUID.

Altered 5-HT2A/C receptor binding in the medulla oblongata in the sudden infant death syndrome (SIDS): Part I. Tissue-based evidence for serotonin receptor signaling abnormalities in cardiorespiratory- and arousal-related circuits.

The sudden infant death syndrome (SIDS), the leading cause of postneonatal infant mortality in the United States, is typically associated with a sleep period. Previously, we showed evidence of serotonergic abnormalities in the medulla (e.g. altered serotonin (5-HT)1A receptor binding), in SIDS cases. In rodents, 5-HT2A/C receptor signaling contributes to arousal and autoresuscitation, protecting brain oxygen status during sleep. Nonetheless, the role of 5-HT2A/C receptors in the pathophysiology of SIDS is unclear. We hypothesize that in SIDS, 5-HT2A/C receptor binding is altered in medullary nuclei that are key for arousal and autoresuscitation. Here, we report altered 5-HT2A/C binding in several key medullary nuclei in SIDS cases (n = 58) compared to controls (n = 12). In some nuclei the reduced 5-HT2A/C and 5-HT1A binding overlapped, suggesting abnormal 5-HT receptor interactions. The data presented here (Part 1) suggest that a subset of SIDS is due in part to abnormal 5-HT2A/C and 5-HT1A signaling across multiple medullary nuclei vital for arousal and autoresuscitation. In Part II to follow, we highlight 8 medullary subnetworks with altered 5-HT receptor binding in SIDS. We propose the existence of an integrative brainstem network that fails to facilitate arousal and/or autoresuscitation in SIDS cases.

Copy Number Variation and Structural Genomic Findings in 116 Cases of Sudden Unexplained Death between 1 and 28 Months of Age.

In sudden unexplained death in pediatrics (SUDP) the cause of death is unknown despite an autopsy and investigation. The role of copy number variations (CNVs) in SUDP has not been well-studied. Chromosomal microarray (CMA) data are generated for 116 SUDP cases with age at death between 1 and 28 months. CNVs are classified using the American College of Medical Genetics and Genomics guidelines and CNVs in our cohort are compared to an autism spectrum disorder (ASD) cohort, and to a control cohort. Pathogenic CNVs are identified in 5 of 116 cases (4.3%). Variants of uncertain significance (VUS) favoring pathogenic CNVs are identified in 9 cases (7.8%). Several CNVs are associated with neurodevelopmental phenotypes including seizures, ASD, developmental delay, and schizophrenia. The structural variant 47,XXY is identified in two cases (2/69 boys, 2.9%) not previously diagnosed with Klinefelter syndrome. Pathogenicity scores for deletions are significantly elevated in the SUDP cohort versus controls (p = 0.007) and are not significantly different from the ASD cohort. The finding of pathogenic or VUS favoring pathogenic CNVs, or structural variants, in 12.1% of cases, combined with the observation of higher pathogenicity scores for deletions in SUDP versus controls, suggests that CMA should be included in the genetic evaluation of SUDP.

Genetic Determinants of Sudden Unexpected Death in Pediatrics.

PURPOSE: This study aimed to evaluate genetic contributions to sudden unexpected death in pediatrics (SUDP). METHODS: We phenotyped and performed exome sequencing for 352 SUDP cases. We analyzed variants in 294 "SUDP genes" with mechanisms plausibly related to sudden death. In a subset of 73 cases with parental data (trios), we performed exome-wide analyses and conducted cohort-wide burden analyses. RESULTS: In total, we identified likely contributory variants in 37 of 352 probands (11%). Analysis of SUDP genes identified pathogenic/likely pathogenic variants in 12 of 352 cases (SCN1A, DEPDC5 [2], GABRG2, SCN5A [2], TTN [2], MYBPC3, PLN, TNNI3, and PDHA1) and variants of unknown significance-favor-pathogenic in 17 of 352 cases. Exome-wide analyses of the 73 cases with family data additionally identified 4 de novo pathogenic/likely pathogenic variants (SCN1A [2], ANKRD1, and BRPF1) and 4 de novo variants of unknown significance-favor-pathogenic. Comparing cases with controls, we demonstrated an excess burden of rare damaging SUDP gene variants (odds ratio, 2.94; 95% confidence interval, 2.37-4.21) and of exome-wide de novo variants in the subset of 73 with trio data (odds ratio, 3.13; 95% confidence interval, 1.91-5.16). CONCLUSION: We provide strong evidence for a role of genetic factors in SUDP, involving both candidate genes and novel genes for SUDP and expanding phenotypes of disease genes not previously associated with sudden death.

X-linked serotonin 2C receptor is associated with a non-canonical pathway for sudden unexpected death in epilepsy.

Sudden Unexpected Death in Epilepsy is a leading cause of epilepsy-related mortality, and the analysis of mouse Sudden Unexpected Death in Epilepsy models is steadily revealing a spectrum of inherited risk phenotypes based on distinct genetic mechanisms. Serotonin (5-HT) signalling enhances post-ictal cardiorespiratory drive and, when elevated in the brain, reduces death following evoked audiogenic brainstem seizures in inbred mouse models. However, no gene in this pathway has yet been linked to a spontaneous epilepsy phenotype, the defining criterion of Sudden Unexpected Death in Epilepsy. Most monogenic models of Sudden Unexpected Death in Epilepsy invoke a failure of inhibitory synaptic drive as a critical pathogenic step. Accordingly, the G protein-coupled, membrane serotonin receptor 5-HT2C inhibits forebrain and brainstem networks by exciting GABAergic interneurons, and deletion of this gene lowers the threshold for lethal evoked audiogenic seizures. Here, we characterize epileptogenesis throughout the lifespan of mice lacking X-linked, 5-HT2C receptors (loxTB Htr2c). We find that loss of Htr2c generates a complex, adult-onset spontaneous epileptic phenotype with a novel progressive hyperexcitability pattern of absences, non-convulsive, and convulsive behavioural seizures culminating in late onset sudden mortality predominantly in male mice. RNAscope localized Htr2c mRNA in subsets of Gad2+ GABAergic neurons in forebrain and brainstem regions. To evaluate the contribution of 5-HT2C receptor-mediated inhibitory drive, we selectively spared their deletion in GAD2+ GABAergic neurons of pan-deleted loxTB Htr2c mice, yet unexpectedly found no amelioration of survival or epileptic phenotype, indicating that expression of 5-HT2C receptors in GAD2+ inhibitory neurons was not sufficient to prevent hyperexcitability and lethal seizures. Analysis of human Sudden Unexpected Death in Epilepsy and epilepsy genetic databases identified an enrichment of HTR2C non-synonymous variants in Sudden Unexpected Death in Epilepsy cases. Interestingly, while early lethality is not reflected in the mouse model, we also identified variants mainly among male Sudden Infant Death Syndrome patients. Our findings validate HTR2C as a novel, sex-linked candidate gene modifying Sudden Unexpected Death in Epilepsy risk, and demonstrate that the complex epilepsy phenotype does not arise solely from 5-HT2C-mediated synaptic disinhibition. These results strengthen the evidence for the serotonin hypothesis of Sudden Unexpected Death in Epilepsy risk in humans, and advance current efforts to develop gene-guided interventions to mitigate premature mortality in epilepsy.

Airway basal stem cells generate distinct subpopulations of PNECs.

Pulmonary neuroendocrine cells (PNECs) have crucial roles in airway physiology and immunity by producing bioactive amines and neuropeptides (NPs). A variety of human diseases exhibit PNEC hyperplasia. Given accumulated evidence that PNECs represent a heterogenous population of cells, we investigate how PNECs differ, whether the heterogeneity is similarly present in mouse and human cells, and whether specific disease involves discrete PNECs. Herein, we identify three distinct types of PNECs in human and mouse airways based on single and double positivity for TUBB3 and the established NP markers. We show that the three PNEC types exhibit significant differences in NP expression, homeostatic turnover, and response to injury and disease. We provide evidence that these differences parallel their distinct cell of origin from basal stem cells (BSCs) or other airway epithelial progenitors.

Medullary Serotonergic Binding Deficits and Hippocampal Abnormalities in Sudden Infant Death Syndrome: One or Two Entities?

Sudden infant death syndrome (SIDS) is understood as a syndrome that presents with the common phenotype of sudden death but involves heterogenous biological causes. Many pathological findings have been consistently reported in SIDS, notably in areas of the brain known to play a role in autonomic control and arousal. Our laboratory has reported abnormalities in SIDS cases in medullary serotonin (5-HT) receptor 1A and within the dentate gyrus of the hippocampus. Unknown, however, is whether the medullary and hippocampal abnormalities coexist in the same SIDS cases, supporting a biological relationship of one abnormality with the other. In this study, we begin with an analysis of medullary 5-HT1A binding, as determined by receptor ligand autoradiography, in a combined cohort of published and unpublished SIDS (n = 86) and control (n = 22) cases. We report 5-HT1A binding abnormalities consistent with previously reported data, including lower age-adjusted mean binding in SIDS and age vs. diagnosis interactions. Utilizing this combined cohort of cases, we identified 41 SIDS cases with overlapping medullary 5-HT1A binding data and hippocampal assessment and statistically addressed the relationship between abnormalities at each site. Within this SIDS analytic cohort, we defined abnormal (low) medullary 5-HT1A binding as within the lowest quartile of binding adjusted for age and we examined three specific hippocampal findings previously identified as significantly more prevalent in SIDS compared to controls (granular cell bilamination, clusters of immature cells in the subgranular layer, and single ectopic cells in the molecular layer of the dentate gyrus). Our data did not find a strong statistical relationship between low medullary 5-HT1A binding and the presence of any of the hippocampal abnormalities examined. It did, however, identify a subset of SIDS (~25%) with both low medullary 5-HT1A binding and hippocampal abnormalities. The subset of SIDS cases with both low medullary 5-HT1A binding and single ectopic cells in the molecular layer was associated with prenatal smoking (p = 0.02), suggesting a role for the exposure in development of the two abnormalities. Overall, our data present novel information on the relationship between neuropathogical abnormalities in SIDS and support the heterogenous nature and overall complexity of SIDS pathogenesis.

SCN1A variants associated with sudden infant death syndrome.

We identified SCN1A variants in 2 infants who died of sudden infant death syndrome (SIDS) with hippocampal abnormalities from an exome sequencing study of 10 cases of SIDS but no history of seizures. One harbored SCN1A G682V, and the other had 2 SCN1A variants in cis: L1296M and E1308D, a variant previously associated with epilepsy. Functional evaluation in a heterologous expression system demonstrated partial loss of function for both G682V and the compound variant L1296M/E1308D. Our cases represent a novel association between SCN1A and SIDS, extending the SCN1A spectrum from epilepsy to SIDS. Our findings provide insights into SIDS and support genetic evaluation focused on epilepsy genes in SIDS.

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.

Hippocampal Formation Maldevelopment and Sudden Unexpected Death across the Pediatric Age Spectrum.

Sudden infant death syndrome (SIDS) and sudden unexplained death in childhood (SUDC) are defined as sudden death in a child remaining unexplained despite autopsy and death scene investigation. They are distinguished from each other by age criteria, i.e. with SIDS under 1 year and SUDC over 1 year. Our separate studies of SIDS and SUDC provide evidence of shared hippocampal abnormalities, specifically focal dentate bilamination, a lesion classically associated with temporal lobe epilepsy, across the 2 groups. In this study, we characterized the clinicopathologic features in a retrospective case series of 32 children with sudden death and hippocampal formation (HF) maldevelopment. The greatest frequency of deaths was between 3 weeks and 3 years (81%, 26/32). Dentate anomalies were found across the pediatric age spectrum, supporting a common vulnerability that defies the 1-year age cutoff between SIDS and SUDC. Twelve cases (38%) had seizures, including 7 only with febrile seizures. Subicular anomalies were found in cases over 1 year of age and were associated with increased risk of febrile seizures. Sudden death associated with HF maldevelopment reflects a complex interaction of intrinsic and extrinsic factors that lead to death at different pediatric ages, and may be analogous to sudden unexplained death in epilepsy.

Sudden unexpected death in early childhood: general observations in a series of 151 cases: Part 1 of the investigations of the San Diego SUDC Research Project.

PURPOSE: The purpose of this study was to determine the major subcategories and clinicopathologic features of sudden unexpected death in young children in a large retrospective cohort, and to confirm the association of sudden unexplained death in children (abbreviated by us for unexplained deaths as SUDC) with hippocampal pathology and/or febrile seizures. METHODS: We undertook analysis of a retrospective cohort of 151 cases, of which 80% (121/151) were subclassified as SUDC, 11% (16/151) as explained, 7% (10/151) as undetermined, and 3% (4/151) as seizure-related. RESULTS: There were no significant differences between SUDC and explained cases in postnatal, gestational, or postconceptional age, frequency of preterm birth, gender, race, or organ weights. In contrast, 96.7% (117/121) of the SUDC group were discovered during a sleep period compared to 53.3% (8/15) of the explained group (p < 0.001), and 48.8% (59/121) of the SUDC cases had a personal and/or family history of febrile seizures compared to 6.7% (1/15) of the explained group (p < 0.001). Of the explained deaths, 56% (9/16) were subclassified as infection, 31% (5/16) cardiac, 6% (1/16) accidental, and 6% (1/16) metabolic. Two of the three cases specifically tested for cardiac channelopathies at autopsy based upon clinical indications had genetic variants in cardiac genes, one of uncertain significance. Bacterial cultures at autopsy typically revealed organisms interpreted as contaminants. Two of the four seizure-related deaths were witnessed, with two of the brains from these cases showing generalized malformations. Hippocampal anomalies, including a specific combination we termed hippocampal maldevelopment associated with sudden death, were found in almost 50% (40/83) of the SUDC and undetermined cases in which hippocampal sections were available. CONCLUSIONS: This study highlights the key role for the hippocampus, febrile seizures, and sleep in SUDC pathophysiology. It also demonstrates the role of known predisposing conditions such as cardiac channelopathies and infections in causing sudden unexpected death in childhood, and the need for improved ancillary testing and protective strategies in these cases, even when the cause of death is established at autopsy.

Hippocampal malformation associated with sudden death in early childhood: a neuropathologic study: Part 2 of the investigations of The San Diego SUDC Research Project.

PURPOSE: Sudden unexplained death in childhood (SUDC), while rare, accounts for an important fraction of unexpected deaths in children >1 year of age. Previously we reported an association between febrile seizures, hippocampal maldevelopment, and sudden, unexpected deaths in young children (1-6 years), termed "hippocampal maldevelopment associated with sudden death (HMASD)." Here, we characterize in greater detail the hippocampal pathology in a large cohort of cases (n = 42) of this entity, and attempt to define possible new entities responsible for sudden, unexplained death in young children without HMASD/febrile seizure phenotypes. METHODS: We performed comparative analysis on cases, which we classified in a cohort of 89 sudden and unexpected deaths as HMASD, explained deaths, SUDC with febrile seizure phenotype (SUDC-FS) but without hippocampal pathology, and SUDC (without hippocampal pathology or febrile seizure phenotype). RESULTS: The frequency of each subgroup was: HMASD 48% (40/83); SUDC 27% (22/83); SUDC-FS 18% (15/83); explained 7% (6/83). HMASD was characterized clinically by sudden, sleep-related death, term birth, and discovery in the prone position. Key morphologic features of HMASD were focal granule cell bilamination of the dentate gyrus with or without asymmetry and/or malrotation of the hippocampus, associated with significantly increased frequencies of 11 other developmental abnormalities. We identified no other distinct phenotype in the unexplained categories, except for an association of febrile seizures without hippocampal maldevelopment. CONCLUSIONS: HMASD is a distinct clinicopathologic entity characterized by a likely developmental failure of neuronal migration in the dentate gyrus. Future research is needed to determine the causal role of HMASD in sudden death in early childhood.

Dentate gyrus abnormalities in sudden unexplained death in infants: morphological marker of underlying brain vulnerability.

Sudden unexplained death in infants, including the sudden infant death syndrome, is likely due to heterogeneous causes that involve different intrinsic vulnerabilities and/or environmental factors. Neuropathologic research focuses upon the role of brain regions, particularly the brainstem, that regulate or modulate autonomic and respiratory control during sleep or transitions to waking. The hippocampus is a key component of the forebrain-limbic network that modulates autonomic/respiratory control via brainstem connections, but its role in sudden infant death has received little attention. We tested the hypothesis that a well-established marker of hippocampal pathology in temporal lobe epilepsy-focal granule cell bilamination in the dentate, a variant of granule cell dispersion-is associated with sudden unexplained death in infants. In a blinded study of hippocampal morphology in 153 infants with sudden and unexpected death autopsied in the San Diego County medical examiner's office, deaths were classified as unexplained or explained based upon autopsy and scene investigation. Focal granule cell bilamination was present in 41.2% (47/114) of the unexplained group compared to 7.7% (3/39) of the explained (control) group (p < 0.001). It was associated with a cluster of other dentate developmental abnormalities that reflect defective neuronal proliferation, migration, and/or survival. Dentate lesions in a large subset of infants with sudden unexplained death may represent a developmental vulnerability that leads to autonomic/respiratory instability or autonomic seizures, and sleep-related death when the infants are challenged with homeostatic stressors. Importantly, these lesions can be recognized in microscopic sections prepared in current forensic practice. Future research is needed to determine the relationship between hippocampal and previously reported brainstem pathology in sudden infant death.

Serotonin metabolites in the cerebrospinal fluid in sudden infant death syndrome.

Forensic biomarkers are needed in sudden infant death syndrome (SIDS) to help identify this group among other sudden unexpected deaths in infancy. Previously, we reported multiple serotonergic (5-HT) abnormalities in nuclei of the medulla oblongata that help mediate protective responses to homeostatic stressors. As a first step toward their assessment as forensic biomarkers of medullary pathology, here we test the hypothesis that 5-HT-related measures are abnormal in the cerebrospinal fluid (CSF) of SIDS infants compared with those of autopsy controls. Levels of CSF 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA), the degradative products of 5-HT and dopamine, respectively, were measured by high-performance liquid chromatography in 52 SIDS and 29 non-SIDS autopsy cases. Tryptophan (Trp) and tyrosine (Tyr), the substrates of 5-HT and dopamine, respectively, were also measured. There were no significant differences in 5-HIAA, Trp, HVA, or Tyr levels between the SIDS and non-SIDS groups. These data preclude the use of 5-HIAA, HVA, Trp, or Tyr measurements as CSF autopsy biomarkers of 5-HT medullary pathology in infants who have died suddenly and unexpectedly. They do, however, provide important information about monoaminergic measurements in human CSF at autopsy and their developmental profile in infancy that is applicable to multiple pediatric disorders beyond SIDS.

Potential asphyxia and brainstem abnormalities in sudden and unexpected death in infants.

OBJECTIVE: Sudden and unexplained death is a leading cause of infant mortality. Certain characteristics of the sleep environment increase the risk for sleep-related sudden and unexplained infant death. These characteristics have the potential to generate asphyxial conditions. We tested the hypothesis that infants may be exposed to differing degrees of asphyxia in sleep environments, such that vulnerable infants with a severe underlying brainstem deficiency in serotonergic, γ-aminobutyric acid-ergic, or 14-3-3 transduction proteins succumb even without asphyxial triggers (e.g., supine), whereas infants with intermediate or borderline brainstem deficiencies require asphyxial stressors to precipitate death. METHODS: We classified cases of sudden infant death into categories relative to a "potential asphyxia" schema in a cohort autopsied at the San Diego County Medical Examiner's Office. Controls were infants who died with known causes of death established at autopsy. Analysis of covariance tested for differences between groups. RESULTS: Medullary neurochemical abnormalities were present in both infants dying suddenly in circumstances consistent with asphyxia and infants dying suddenly without obvious asphyxia-generating circumstances. There were no differences in the mean neurochemical measures between these 2 groups, although mean measures were both significantly lower (P < .05) than those of controls dying of known causes. CONCLUSIONS: We found no direct relationship between the presence of potentially asphyxia conditions in the sleep environment and brainstem abnormalities in infants dying suddenly and unexpectedly. Brainstem abnormalities were associated with both asphyxia-generating and non-asphyxia generating conditions. Heeding safe sleep messages is essential for all infants, especially given our current inability to detect underlying vulnerabilities.

Risk factor changes for sudden infant death syndrome after initiation of Back-to-Sleep campaign.

OBJECTIVE: To test the hypothesis that the profile of sudden infant death syndrome (SIDS) changed after the Back-to-Sleep (BTS) campaign initiation, document prevalence and patterns of multiple risks, and determine the age profile of risk factors. METHODS: The San Diego SIDS/Sudden Unexplained Death in Childhood Research Project recorded risk factors for 568 SIDS deaths from 1991 to 2008 based upon standardized death scene investigations and autopsies. Risks were divided into intrinsic (eg, male gender) and extrinsic (eg, prone sleep). RESULTS: Between 1991-1993 and 1996-2008, the percentage of SIDS infants found prone decreased from 84.0% to 48.5% (P < .001), bed-sharing increased from 19.2% to 37.9% (P < .001), especially among infants <2 months (29.0% vs 63.8%), prematurity rate increased from 20.0% to 29.0% (P = .05), whereas symptoms of upper respiratory tract infection decreased from 46.6% to 24.8% (P < .001). Ninety-nine percent of SIDS infants had at least 1 risk factor, 57% had at least 2 extrinsic and 1 intrinsic risk factor, and only 5% had no extrinsic risk. The average number of risks per SIDS infant did not change after initiation of the BTS campaign. CONCLUSIONS: SIDS infants in the BTS era show more variation in risk factors. There was a consistently high prevalence of both intrinsic and especially extrinsic risks both before and during the Back-to-Sleep era. Risk reduction campaigns emphasizing the importance of avoiding multiple and simultaneous SIDS risks are essential to prevent SIDS, including among infants who may already be vulnerable.