Sids And Genetics The Untold Story

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U.S. Department of Health and Human Services

SIDS AND GENETICS The Untold Story Carl E. Hunt, MD

National Institutes of Health

2005 Annual International Conference September 17, 2005

National Heart, Lung, and Blood Institute

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Sudden Unexpected Death in Infancy (SUDI) • Explained (18-20%) – Natural (60%) – Unintentional (15%) – Traumatic child abuse

• Unexplained – SIDS – Intentional suffocation

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Sudden Unexpected Death in Infancy (SUDI) • Unexplained – SIDS – Intentional suffocation

• “Unexplained” is moving target – Original definition assumed there was no definable cause – Will still be “SIDS” even if later linked with a genetic polymorphism

• SUDI therefore is the categorical label

Take Home Message NCSDR

• SIDS, like other natural causes for SUDI and other human conditions and diseases: – Important genetic and environmental risk factors • Complex interactions

• No choice required! – Genetic and environmental determinants of risk

• Next Steps – Apply new technologies to identify additional genes – Delineate gene-gene and gene-environment interactions – For defined genotypes • Establish clinical phenotypes • Identify pathological correlates

Take Home Message NCSDR

• Next Steps – Apply new technologies to identify additional genes – Delineate gene-gene and gene-environment interactions – For defined genotypes • Establish clinical phenotypes • Identify pathological correlates

• Following this presentation – 10-minute presentation by Dr. Kinney – Illustrate combining genetic with antemortem physiological and with pathological information – Transition to 3:15 “Neuropathology Update”

Definitions NCSDR

(Health And Disease)

• GENOTYPE: – Genetic constitution of an individual

• PHENOTYPE: – Physical manifestation of a genotype – May not be identifiable on physical exam – May require perturbation to “unmask” Hypercarbic/hypoxic challenge Other tests of autonomic responsiveness

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Single Gene Disorders The Historical Model

GENOTYPE

PHENOTYPE

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Genes

Disease

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SIDS

Environment

Disease

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Standard Environmental Model SIDS Prone/Side Sleeping

Cigarette Smoke

SIDS Thermal Stress (Etc.)

Gene X Environment Model NCSDR

Genes

Applies to SUDI In Same Manner as Other Clinical Conditions

Environment

Disease

Summary

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Genetic & Environmental Risks for SUDI Behavioral, Sociocultural, & Environmental Factors PHENOTYPE

SUDI

Genetic Factors Adapted from NIH (OBSSR)

Genetics NCSDR

New Terms Additional Complexity • Genomics – 33,000 human genes • Estimated 90,000 functional gene products • 300,000 RNA’s – Fat metabolism: 305 genes (50/50)

• Proteomics – 3 million distinct proteins – Cell signaling: 2,400 proteins

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Genetic Complexity (Health & Disease) • Single gene – Polymorphisms

• Multiple genes (polygenic) • Gene-gene interactions – Redundancy – Modifier genes

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Single Gene Disorders Are Not Simple

• Knowing genotype rarely predicts or defines the phenotype • Homozygous (SS) for Sickle Cell Disease – Virtually asymptomatic versus severe phenotype with repeated strokes and early death

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Strokes in Homozygous SSA (HbSS)

Meschia, et al. Nature Genetics 2005;37:340

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Strokes in Homozygous SSA (HbSS) (cont.) (69 SNPs Associated with Stroke)

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Gene X Environment Interactions

• Glucose-6-phosphate dehydrogenase deficiency – Prototype of interaction of genotype with environment – Multiple polymorphisms – Hemolytic anemia precipitated by drugs, fava beans, or infection

• Cystic Fibrosis – Cystic fibrosis transmembrane conductance regulator (CFTR gene)  Polymorphisms  Identical genotypes can have marked differences in lung function  Modifier genes likely important

– Environment modifies clinical course

• Epigenetics (Environmental Genomics) – How environmental factors alter gene expression

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BEHAVIOR OF GENES DNA is both inherited and environmentally responsive Recent findings from animal studies help to resolve nature versus nurture by upsetting assumption that the two work differently Discoveries emphasize what genes do (produce proteins) rather than simply who they are (their fixed DNA sequence) Robinson GE. NY Times, 12-13-04

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Gene X Environment Model In Health and In Disease Genes

Health

Environment

Disease F. Collins, NIH, 2004

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Health And Disease • Multiple genes interacting in complex ways with multiple environments to both increase and decrease risk for clinical disease • Multiple influences on phenotype – Polymorphisms – Gene X gene interactions – Gene X environmental interactions – Illustrative examples • Many common clinical conditions

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Polygenic Conditions Asthma • Multiple environmental risk factors – Smoking – Allergen exposure

• Multiple independent segregating genes – Airway and vascular reactivity – Infection and immunity

Polygenic Conditions (con’t.)

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Cerebral Palsy (CP) in Preterm Infants Single nucleotide polymorphisms (SNPs) of multiple genes associated with CP – – – – –

Endothelial nitric oxide synthase (Enos) Coagulation Factor 7 (F7) Lymphotoxin (LTA) Plasminogen activator inhibitor 1 (PAI-1) Adrenergic receptor beta-2 (ADRB-2 )

Nelson, et al. Pediatr Res 2005;57:494

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Polygenic Conditions (con’t.) Retinopathy of Prematurity (ROP) Vascular Endothelial Growth Factor (VEGF)

• Association between VEGF haplotype and ROP probably independent of known risk factors – Gender – Supplemental O2 therapy – GA

• VEGF G+405 C genotype associated with risk for ROP requiring cryotherapy/photocoagulation • Carriers of -460 TT/+405 CC haplotype may be at increased risk for proliferating ROP • -460 TT/+405 GG haplotype seems to be protective against ROP Vannay, et al. Pediatr Res 2005;57:396

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Cystinuria • Autosomal disorder of renal tubular transport • Only 19-26% of people homozygous for cystinuria have urinary crystals • Polygenic, gene-gene and geneenvironmental explanations not delineated yet

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Cardiovascular Diseases, including Coronary Artery Disease Genetically complex, with major environmental risk factors – Multiple monogenic diseases  Increase or decrease LDL cholesterol  Increase or decrease blood pressure and renal salt reabsorption

– Genes regulating thrombosis/hemostasis and arrhythmias also contribute to individual risk for CAD – Inflammation  Gene encoding 5-lipoxygenase activating protein confers risk for myocardial infarction and stroke

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SUDI/SIDS Gene x Environment Model Environment 2

Gene 2

Gene 3

Risk Factor Environment 3

Environment 1

Gene 4

SUDI/SIDS

Gene 1

SUDI/SIDS

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Potential Gene X Environmental Interactions ENVIRONMENTAL

• Smoking • Unsafe Sleeping – – – –

• • • • •

Nonsupine sleeping Soft Bedding/blankets Rebreathing Thermal stress

Low SES Cultural/Ethnic Urban URI Prematurity

GENETIC

• Cardiorespiratory responses & arousal • Thermoregulation • Other autonomic dysregulation • Sleep/Circadian rhythmicity • Immune/Infection • Racial • Gender • Fatty Acid Disorders • QT Interval • Prematurity

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Evidence for Genetic Risk Factors For SUDI and SIDS Category of Data

Strength of Evidence

Comments

Genotyping studies

Strong

Environmental influences; clinical phenotypes

-Autopsy -Physiology

Inconclusive

Sibling recurrence

Inconclusive

-Causal/secondary -Unclear relevance to death Environmental factors not excluded

Hunt CE. Arch Dis Child 2005;90:48

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Genetic Causes Of SIDS Indirect Evidence

• Regulation of sleep state & arousal – Sleep-wake cycle under control of circadian clock – These circadian master genes likely influence sleeping behavior

• Pathologic studies in SIDS Victims • Physiology studies in future SIDS victims – Studies of autonomic control – Respiratory Pattern – Decreased ventilatory/arousal responses to hypercarbia and hypoxia

• Physiology Studies in siblings and parents – Studies of autonomic control – Respiratory Pattern – Decreased ventilatory/arousal responses Hunt CE. Am J Resp Crit Care Med 2001;164:346

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Genetic Causes Of SIDS Indirect Evidence (cont.) • Upper airway (UA) patency: – Retroposition of maxilla and larger tongue in SIDS

• Genetic epidemiology family studies: SIDS, ALTE, SDB – Families 1+ sleep apnea (SDB): increased SIDS/ALTE – Adult SDB with family history SIDS/ALTE:  Smaller pharyngeal airways than adult SDB without SIDS/ALTE Hunt CE. Am J Resp Crit Care Med 2001;164:346

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Genetic Causes of SIDS, cont. SIDS Siblings*

• Evidence indirect, incomplete, or inconclusive • Recurrent infant mortality in 1st-2nd born siblings: – Guntheroth et al (J Pediatrics 1990); Oyen et al (Am J Epidemiology 1996)

– Recurrent SIDS in 2nd sibling 5.4 & 5.8 – Recurrent non-SIDS 2nd sibling 6.0-7.2 – RR 9.1 for similar cause, 1.6 for dissimilar

• Cannot exclude shared environmental contributors to increased risk *Hunt CE. Am J Resp Crit Care Med 2001;164:346-357

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Schematic Illustration of Potential Interactions Between Representative Environmental and Genetic Risk Factors Gene X Polymorphism

Smoking

Race/Ethnicity

Impaired Autonomic Regulation

5-HTT Polymorphism

Prematurity

Gene Y Polymorphism

Prone Sleeping EPIDEMIOLOGICAL RISK FACTORS

ANS Polymorphism

SUDI/ SIDS

GENETIC RISK FACTORS

Hunt CE. Arch Dis Child 2005;90:48

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Barriers to Acceptance of Genetic Risk Factors for SIDS • BARRIERS – Well-established environmental risk factors • Overwhelm theoretical role for defined genetic risk factors

– Delineating genetic risk factors not meaningful since • Does not enhance potential for effective intervention or prevention

– SIDS is not distinct entity but rather appears to have multiple causal pathways • Cannot be A SIDS gene

Hunt CE. Arch Dis Child 2005;90:48

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Invalid Assumptions That Serve as Additional Barriers • Diseases are genetic OR environmental in origin – Existence of established environmental risk factors (RF) precludes concurrent existence of causal genetic RF

• SIDS does not follow same “rules” as human disorders with known causal genetic RF • Is 1:1 relation between genotype and disease – Knowing genotype defines/predicts destiny Hunt CE. Arch Dis Child 2005;90:48

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Invalid Assumptions That Serve as Additional Barriers, cont.

• If SIDS genetic disorder – Must be SIDS genotype – Will be substantial recurrence risk – Parental source • Some potential to infer blame or responsibility

Hunt CE. Arch Dis Child 2005;90:48

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Genetic Frontiers In SIDS Research Identified genes for which distribution of polymorphisms differ in SIDS – Cardiac channelopathies • Sodium channel (SCN5A) • Potassium channel

– Promoter region of serotonin transporter protein (5-HTT) – Autonomic nervous system (ANS) development  PHOX2, RET, ECE 1, TLX 3, EN 1

– Infection/Inflammation  Complement: C4A, C4B  Interleukin 10 Hunt CE. Arch Dis Child 2005;90:48

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Genetic Frontiers In SIDS Research, (cont.) Sodium Channel Gene

• SCN5A is leading candidate ion channel gene relevant to SIDS • Mutational analyses identify >100 distinct mutations, >30 of which associated with Long QT Syndrome (LQTS) – About 5% of LQTS subjects have SCN5A defect

• Among 93 SIDS victims, 2% had distinct SCN5A channel defect (exon 17, 28)* – Lower prevalence estimate of 3% LQT3 in white SIDS

• QT intervals longer in 18 SIDS than controls** *Ackerman et al. JAMA, 2001

**Franco et al. Sleep Abstract Supplement 2004

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Cardiac Potassium (K) Channel Defects • Vast majority of LQTS due to perturbed K channels via mutations in: – KCNQ1, KCNH2, KCNE1, KCNE2

• Spontaneous P117L missense mutation in KCNQ1-encoded K channel – 4 M female infant found supine (Schwartz PJ et al. Lancet 2001;358:1342)

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Cardiac Potassium (K) Channel Defects, cont. • Same population-based series of 93 SIDS cases (Tester DJ et al. Cardiovasc Res 2005;67:388)

– 51 males, mean age 3 M, 58 White – 2 cases with molecular and functional evidence for LQT3associated sodium (Na) channelopathy

• Molecular evidence for K channel defects in 2 more infants – G294V-KCN2 mutation in 6 M white infant – 5 different variants in 2 M AA infant

• Summary: estimated 5-10% of SIDS associated with defective cardiac ion channels – ? Higher if QT interval a continuous variable (Franco et al)

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Genetic Frontiers In SIDS Research, (cont.) Autonomic Nervous System (ANS)*

• Molecular genetic mutations pertinent to early embryologic development of ANS • Eleven protein-changing mutations in 14/92 SIDS cases: – PHOX 2a, RET, ECE 1, TLX 3, EN 1

• Only 1 mutation (TLX 3) in 2/92 controls • 10/11 mutations in SIDS occurred in African Americans *Weese-Mayer et al. Pediatric Research, in press

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Genetic Frontiers In SIDS Research, (cont.) Autonomic Nervous System (ANS)

• Phenotype defined only by sudden death • Candidate physiologic phenotypes – Ret proto-oncogene:  Muscarinic system (ANS) from neural crest  Ret knockout mice:  Decreased ventilatory responsiveness to CO2

– Mash 1 (molecular link with ret):  Involved in respiratory control via X-linked mechanisms  Involved with arousal responsiveness  Expressed in vagal neural crest derivatives & locus coeruleus neurons:

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Candidate Physiologic Phenotypes, cont. PHOX2b*.

• Breathing pattern in mice – Wild-type (normals) -- PHOX2b++ – Targeted deletion (heterozygous) • PHOX2b deficient -- PHOX2b+/-

• Wild-type and mutant mice – Apnea time and ventilation similar awake – Similar sleep-wake states • Longer apnea time during sleep (mutant mice) – Sleep apnea total time 6X longer (p<0.015)

• Lower ventilation during active sleep – 21% decrease (p<0.006) Durand E et al. Am J Resp Crit Care Med 2005;172:238

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Genetic Frontiers In SIDS Research, (cont.) Serotonin

Serotonin (5-HT) – Widespread neurotransmitter affecting breathing, C-V system, temperature, mood – Modulates activity of circadian clock – Neurotransmitter of non-REM sleep May be responsible for synthesis/accumulation of sleep-inducing factors

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Serotonin (5-HT), cont. • Many genes involved: – Synthesis, storage, membrane uptake, metabolism – Several receptors

• Serotonin Transporter gene (5-HTT) – Regulates membrane uptake – Chromosome 17q11.1-q12

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Genetics of Serotonin (cont.) (5-HTT)*

• 2 polymorphisms, with different effects on expression – 1 in promoter region – Short allele (S) and long allele (L)

• “L” increases effectiveness of promoter compared to “S” allele – Reduced serotonin concentrations at nerve endings

• L/L genotype – Increased serotonin transporters on neuroimaging – Increased binding in postmortem studies *Narita et al, Pediatrics 2001;

Weese-Mayer et al, Am J Med Genetics 2003

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Genetics of Serotonin (cont.) (5-HTT)

Gene*

• SIDS more likely than controls to have “L” allele in Japanese, Caucasian, and African American study groups • Positive correlation between SIDS and L/L genotype, and between SIDS and L allele • Lower incidence of “S/L” and “SS” genotypes in SIDS than matched controls • Association between SIDS and 5-HTT intron 2 polymorphism affecting 5-HTT expression* *Weese-Mayer et al. Am J Med Genetics 2003;122A:238-245

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5-HTT and SIDS Summary

• Genetic studies in SIDS victims consistent with neuropathology studies • Decreased serotonin receptor binding (and other receptors) in arcuate nucleus (Panigrahy et al, J Neuropath Exp Neurol 2000; Kinney et al, J Neuropath Exp Neurol 2003)

• Decreased 5-HT 1 A and 5-HT 2 receptor binding – Dorsal nucleus of vagus, solitary nucleus, ventrolateral medulla (Ozawa et al, Neuropediatrics 2002)

• Major Gap In Knowledge – Phenotype defined only by sudden death – Need to identify candidate phenotypes defined by physiology  Sleep, ventilatory/arousal responsiveness, other ANS

• A genetic model for SIDS

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Genetics & SIDS Infection & Inflammation

• SIDS victims with URI more likely have deletion of complement C4A or C4B gene than SIDS or nonSIDS controls (Opdal et al, Eur J Pediatrics 1999) • Family members of some SIDS victims and ALTE (Gold et al, J Pediatrics 2000)

– Increased mast cell hyper-releasability & degrannulation – Consistent with genetic immune deficit

SUMMARY

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Genes

Health

Environment

Disease F. Collins, NIH, 2004

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Summary, cont. Genetic & Environmental Risk Factors Gene X Polymorphism

Smoking

Race/ethnicity

Impaired Autonomic Regulation

5-HTT Polymorphism

Prematurity

Prone Sleeping

ANS Polymorphism

SUDI/ SIDS

Gene Y Polymorphism

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Future Opportunities for Research

Translational Research

Knowledge Base

Identify & Validate common variants (Genome-wide associations; cohort studies)

Research Translation

Diagnostic Chip

Clinical Medicine •Assess patient risk •Guidelines •Education

Discovery; New Hypotheses

Validation; (? Registry)

•Dissemination •Therapeutics

Summary, cont. SIDS

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• Apply perspectives from other health and disease conditions – Enhance our understanding of natural causes of SUDI

• Technologies now exist to expand our horizons of knowledge of genetic risk factors for SIDS • Genomic and proteomic research needed to identify: – Genes – Protein products

• Presentation: Hannah Kinney, MD – Illustrative of a future research focus • Integrating genetic, physiological and pathological data

NCSDR WebPage U.S. Department of Health and Human Services

National Institutes of Health

National Heart, Lung, and Blood Institute

www.nhlbi.nih.gov/sleep E-Mail: [email protected]

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