Pediatric Neurogenetics

Pediatric Neurogenetics

Zheng (Jane) Fan, MD

Medical Genetics Fellow

UNC-CH

04/2006

What is Neurogenetics?

• Neurogenetics: the study of genetic factors that contribute to development of neurological disorders

• One third of known single gene defect cause diseases that affect the nervous system

• Not to intent to cover everything

• A field with rapid progress

Outlines

• Basics of human genetics

• Pediatric Neurogenetics– Classification– Common disorders

Human Genetics• Human Genome Project finished in

2003 (13 years effort)– Identified approximately 20,000-25,000

genes

• International HapMap Project (phase I) finished in the end of 2005– HapMap: Haplotype map– Haplotype: A set of closely linked genes

that tends to be inherited together as a unit (block of genes)

Human Genetics (continued)

• Human genome size: 2.85 Gb• Protein coding genes only consist of 1.5%

of genome• The vast majority of the rest genome:

repeats {transposon-derived repeats, pseudogenes, SSR (micro- and minisatellites), segmental duplication, blocks of tandem repeats} and non-coding genes (introns). Little is known about these regions.

Types of genetics conditions and commonly used studies

• Chromosomal aberrations: aneuploidy, deletion and duplication/multiplications.– Karyotype, subtelomere study (study of the ends of the

chromosomes), FISH (florescent in situ hybridization), CGH (comparative genomic hybridization, signature chip is one of them)

• Mutations– Mutation scanning for common mutations, sequencing

(commonly the coding region = exons), SNP (single nucleotide polymorphism) chip (Affimetrix etc)

• Others– Methylation study (commonly for imprinting

disorders), linkage analysis, parental testing (finger printing)

Inheritance Pattern• Mendalian inheritance:

– Autosomal resessive - AR– Autosomal dominant -AD – X-linked disorders (most recessive, can be dominant)

• Non-Mendalian inheritance:– Genomic imprinting– Trinucleotide repeat disorders– AD with incomplete penetrance– Mitochondrial inheritance– X-inactivation related disorders– Modifier Genes– Complex trait

Classification of Neurogenetics

• Localization based– 1. CNS: Cerebral cortical, basal ganglia

disorders and cerebellum– 2. Spinal cord and anterior horn cell disorders– 3. PNS: Peripheral nerve disorders– 4. Muscle disorders and neuromuscular

junction– 5. Many disorders affect more than one

localization sites– Others

1. CNS• A. Cerebral cortical disorders

– Cortical dysplasias/Neuronal migration disorder

– Developmental delay/Autism

– Epilepsy

– Dementia (adult)

• B. Basal ganglia disorders: movement dso– Pediatric:, Dystonia and Wilson dsz

– Adult: Huntington dsz, Parkinson dsz, and PKAN (Pantothenate Kinase-associated neurodegeneration) used to be called Hallervorden-Spatz disease

• C. Disorders mainly affect cerebellum– Ataxia syndromes

Neuronal migration

Six layers cortex

Normal Brain development

A. Cortical dysplasia

• Segmentation: Schizencephaly• Prosencephalon cleavage: holoproencephaly,

septo-optic dysplasia and agenesis of corpus callosum

• Neuronal and glial proliferation: microcephaly, megalencephaly and hemimegalencephaly

• Neuronal differentiation: Tuberous sclerosis• Neuronal migration: Lissencephaly,

polymicrogyria and heterotopia

Brain & Development ( 2004 ) Clark GD

Disorder of segmentation: Schizencephaly

• Types: the cleft can be open-lipped or close-lipped

• Unilateral or bilateral• When it is severe malformation,

almost always associate with epilepsy, mental retardation and spastic cerebral palsy.

• Severe familiar cases: mutation in EMX2, a transcriptional regulator

Open-lipped

Close-lipped

Disorders of prosencephalon (forebrain) cleavage

• Holoprosencephaly– Spectrum: alobar, semilobar and lobar– Genetically heterogeneous group

• Chromosomal aberration: trisomy 13, etc• Single gene: Sonic hedgehog, HPE1-4, PACHED, ZIC2, SIX3

– Maternal exposure: retinoic acid, diabetes, CMV

• Septo-optic dysplasia– Up to 60% pts with endocrine dysfunction (hypothalamic

dysfunction)– Minority: mutation in HESX1 gene, transcriptional regulator gene

• Agenesis of corpus callosum (ACC)– Single gene: SLC12A6 (AR) is responsible for ACC and

neuropathy– A/w syndromes: Miller-Dieker S., Walker-Warbrug S., and

Zellweger S

Holoprosencephaly (HPE)

• HPE: the developing forebrain fails to divide into two separate hemispheres and ventricles

• Wide spectrum of phenotypes: almost normal to severely impaired

• Single central incisor can be a clue

Disorders of cell proliferation• Microcephaly

– Microcephaly vera: term for genetic form– Mostly < 4SD, with MR, hypotonia, and seizures– Linked to multiple locations, no single gene identified

yet, can be AD, AR or X-linked

• Megalencephaly (big brain volume) and hemimegalencephaly– Hemimegalencephaly may be a/w linear sebaceous

nevus syndrome (50%) and hypomelanosis of Ito– No single gene identified

Disorders of differentiation

• Tuberous sclerosis– Clinically: hamartomas of the subependymal

layer (subependymal nodules), areas of cortical migration abnormalities (tubers) and the development of giant-cell astrocytomas (5% TS pts). Epilepsy is a prominent feature.

– Genes: TSC1 (encodes for Hamartin, on 9q34) and TSC2 (encodes for Tuberin, on 16p13.3)

– Both are AD

Neuronal migration disorders• Lissencephaly (smooth brain)

– Classic lissencephaly: LIS1 gene, a/w Miller-Dieker syndrome

– X-linked lissencephaly: DCX (doublecortin) – Lisencephaly with cerebellar hypoplasia: REELIN gene– Cobble stone lissencephaly, a/w Walker-Warburg

syndrome, muscle-eye-brain syndrome. Can also a/w Fukuyama muscular dystrophy (fukutin gene).

• Polymicrogyria (many small gyri), a/w genetic or chromosomal dso, such as Zellweger syndrome.

• Heterotopias (collections of normal-appearing neurons in abnormal location), DCX (doublecortin)

Spectrum of lissencephaly with LIS1 mutation

Lissencephaly and heterotopia with DCX mutation

Cobblestone lissencephaly

Heterotopia




– Epilepsy





Developmental Delay/Autism

• Heterogeneous groups

• Inborn errors of metabolism

• Chromosomal anomalies

• Genetic syndromes

• Others

Autism

• No single gene identified for autism

• Most syndromes are associated with atypical autistic features

• Chromosomal aberrations are associated with mental retardation.

• Submicroscopic chromosomal arrangements

• Can be associated with specific genetic syndromes.

Genetic disorders with autistic features

• Syndromes: Fragile X syndrome, tuberous sclerosis, Angelman syndrome, 15q duplication, Down syndrome, MECP2 related disorders (Rett syndrome), Smith-Magenis syndrome, 22q13 deletion, Cohen syndrome, and Smith-Lemli-Opitz syndrome, etc.

• Inborn errors of metabolism: PKU, adenylosuccinate lyase deficiency, Sanfilippo syndrome (MPS III), etc.

J Autism Dev Disorder (2005) Feb, Cohen D et al




– Epilepsy





Epilepsy - etiology

• Genetic epilepsy: next slide for details• Chromosomal abnormalities

– Angelman syndrome, 4p deletion syndrome, and ring chromosome 20

• Abnormal cortical development– Focal cortical dysplasia: heterotopia,

schizencephaly, hemimegalencephaly etc.– Neurocutaneous syndrome: tuberous sclerosis,

Sturge-Weber syndrome

Genetic epilepsy• Most are iron channel related single gene disorders.• Idiopathic generalized epilepsies

– Cl- channel: CLCN2, GABA receptors (GABRA1 and GABRG20 and Ca++ channel (EFHC1 gene) are reported

• Familiar autosomal dominant epilepsies– Benign familial neonatal-infantile convulsions: K+ channels

genes (KCNQ3 and KCNQ2) and Na+ channel gene (SCN2A)– Autosomal dominant nocturnal frontal lobe epilepsy is a/w

nicotinic acetylcholine receptor genes (CHRNA4 and CHRNB2)

– Autosomal dominant partial epilepsy with auditory features: LGI1-epitempin (leucine-rich glioma-inactivated 1 gene)

Lancet. (2006) Feb, Epilepsy in children, Guerrini R.




– Epilepsy





Hereditary ataxias

• Clinical: progressive incoordination of gait and often poor coordination of hands, speech, and eye movements.

• Pathology: dysfunction of cerebellum and its associated systems (spinal cord and peripheral nerves)

• Onset age: childhood (common) to adulthood

Genetests.org, Bird T, updated April 2006

Hereditary ataxias Classified by inheritance

• Autosomal dominant cerebellar ataxias (ADCA)– Most are SCAs (spinocerebellar ataxias). All are

trinucleotide repeat expansion disorders with anticipation.

– Genes: ATXN genes, SCA genes (at least 28 to date) and others

– DRPLA (also called Haw River syndrome)• Autosomal recessive hereditary ataxias

– Friedreich ataxia (FXN gene: Frataxin), Ataxia-telangiectasia (ATM gene) and others.

• X-linked hereditary ataxias:– single family is described

Prevalence of SCA subtypes around the world

2. Spinal cord and anterior horn cell disorders

• Spinal cord disorders– Hereditary spastic paraplegias (HSPs)

• Anterior horn cell disorders– Spinal muscular atrophies (SMAs)– Kennedy's disease (X-linked spinal-bulbar

muscular atrophy, adult onset)– Amyotrophic lateral sclerosis (ALS), adult

onset, familial subgroup: SOD1 mutation

Hereditary spastic paraplegias (HSPs)• Clinical: insidiously progressive lower extremity

weakness and spasticity. Onset varies from early childhood to adulthood.

• Neuropath: Axonal degeneration (corticospinal tracts)

• Classified as uncomplicated (pure) and complicated (complex). Complicated is a/w other neurological symptoms: seizures, MR, etc.

• Clinical presentation Can overlap with other hereditary syndromes

• Genetics: many genes (SPG1-29, SAX1, PLP1, etc) identified (up to 2004), list is expanding.

• Inheritance: AD (most common), AR and X-linked Genetests.org, updated Oct 2004

Anterior horn motor neuron disease: SMAs (Spinal muscular atrophies)

• Clinical: Motor weakness. Tongue fasciculation in an alert weak baby is highly suggestive.

• Classification is based on age of onset (spectrum of phenotype): – SMA 0 (proposed name) (prenatal onset) = Congenital

SMA with arthrogryposis – SMA I (0-6m) = Werdnig-Hoffmann syndrome– SMA II (after 6mo) and SMA III (after 10m, with

ability to walk) = Kugelberg-Weland syndrome– SMAIV (adult onset) = later onset SMA

• Pathology: Loss of the anterior horn motor neurons in the spinal cord and the brain stem nuclei

Genereviews.org, Prior T, April 2006 and www.neuro.wustl.edu/neuromuscular

Genetics of SMA• Genetics: AR• Two closely related genes,

SMN1 (= telomeric SMN) and SMN2 (= centromeric SMN)

• SMN1 and SMN2, adjacent to each other on 5q– SMN1 and SMN2 only differ

by 5 base pairs– SMN1 is the primary disease

causing gene– SMN2 is a modifier gene

Congenital SMA with arthrogryposis

3. Hereditary polyneuropathy-CMT• Charcot-Marie-Tooth disease (CMT) =

Hereditary sensory and motor neuropathy (HSMN)

• Incidence:Hereditary neuropathies: ~30 per 100,000

• Most common: CMT 1A: 10.5 per 100,000 • Heterogeneous inherited polyneuropathies• Classification: complex and changing

– CMT1: demyelinating neuropathy (AD or X-linked)– CMT2: axonal neuropathy (most AD, minority AR)– CMT3: severe demyelinating neuropathy {Dejerine-

Sottas disease (DSD)} (AD or AR)– CMT4: demyelinating neuropathy (AR)

-- Curr Opin Neurol. 2005 Apr, Ryan MM, Ouvrier R.

CMT1A and PMP22 gene

• Clinical: slow onset of weakness (ankle and knee), age of onset: 4-25yrs.

• CMT1A represents 70-80% CMT1• PMP22 duplication responsible for 98% CMT1A• PMP22 point mutation cause CMT1E• PMP22 deletion responsible for 80% Hereditary

Liability to Pressure Palsies (HNPP)

4. Muscles and neuromuscular junction

• Dystrophinopathies• Congenital muscular dystrophies • Congenital Myopathies • Congenital presentations of adult dystrophies

– Myotonic dystrophy

• Mitochondrial myopathies • Myasthenic syndromes (neuromuscular junction)

DystrophinopathiesDuchenne and Becker muscular dysphophies

• Diagnosis:– Progressive symmetric muscle weakness, proximal>distal– Normal at birth, occasional congenital form can present with

hypotonia at birth.– Gower maneuver: indication of proximal muscles weakness, most

common seen in DMD (Duchenne muscular dystrophy)– Molecular genetic diagnosis is preferred– Muscle biopsy only needed in case without molecular dx

• Treatment: – Supportive: PT and others– Surveillance for cardiomyopathy, respiratory failure and

orthopedic complications.– Steroids prolong walking, q weekly dosing is most commonly

used, with reduced side affects– Research: gene therapy

Gower maneuver

Genetics of Dystrophinopathies (DMD and BMD)

• Clinical features– It is the most common myopathy in children: ~ 1 in

every 3500 boys worldwide– DMD: delayed motor milestones, mean age of dx is

~4yo (no FH), wheelchair dependency <13yo, mean age of living 15-25yrs

– BMD: milder phenotype , alleic disorder to DMD• Molecular genetics

– Located at Xp21– Gene: DMD (the largest human gene, 79 exons),

protein: dystrophin (rod like protein)– Mutation types:

• Deletion: ~65% male with DMD, ~85% male with BMD• Duplication: ~ 6-10% DMD, ~6-10% BMD• Point mutation/small deletion, insertion/splicing mutation:

~25-30 DMD, ~5-10% BMD

Congenital muscular dystrophies (CMD)

• A group of inherited disorders• Muscle weakness is present at birth• Muscle weakness tends to be stable over time, but

complications of dystrophy become severe with time; in contrast, weakness from dystrophinopathies is progressive.

• Clinical features – Weakness: Diffuse – Contractures – CNS involvement: Common in severe forms of CMD – Disorders of myelin or neuronal migration

Congenital muscular dystrophies - continued

• Inheritance: Autosomal recessive (AR)

• Frequency: Common cause of AR neuromuscular disorders

• Diagnosis is based on muscle biopsy findings traditionally

• May overlap with other conditions: LGMD (limb girdle muscular dystrophy), congenital myopathies, etc.

Selected syndromes of congenital muscular dystrophies

• Fukuyama: Fukutin; 9q31, common in Japan, rare in western, severe, often death <11yo

• Integrin α-7 deficient, laminin receptor, on 12q13, most nl intelligence• Merosin (laminin α2-chain) deficient, spectrum of severity, nl congnition• Normal merosin: "Pure" formal: nl CNS, nl cognition, merosin present

– CMD with Rigid spine – CMD + Respiratory failure & Muscle hypertrophy (CMD1B; MDC1B) – Ulrich: Collagen 6A2 – CMD + Muscle hypertrophy

• Muscle-Eye-Brain Disorders– Santavuori (Finnish): POMGnT1(O-Mannosyltransferase 1); 1p32– Walker-Warburg: POMT1; 9q3l, Fukutin, FKRP(Fukutin related protein)

• Congenital muscular dystrophy with muscle hypertrophy– Normal CNS (MDC1C): FKRP; 19q13, allelic with LGMD 2I– Severe retardation (MDC1D): LARGE; 22q12

• Ullrich congenital myopathy, joint contractures are very common– COL6A1; 21q22– COL6A2; 21q22– COL6A3; 2q37

Congenital myopathiesSelected syndromes

• Centronuclear (myotubular) myopathy – X-linked, AD or AR– Myotubular family– Spectrum of severity, can present at birth

• Nemaline (rod) myopathy – Onset: congenital (90%) to adult– α-Actin; α-tropomyosin 3 (TPM3) – AD, AR or sporadic

• Central core disease +/- malignant hyperthermia– AD or AR– >20 mutations found, related to

Ryanodine receptor mutations (Calcium release channel)

Central core

Nemaline (rod)

Centronuclear

Myotonic dystrophy• Myotonic dystrophy (MD) is a trinucleotide repeat disease

with multi-systemic involvement: muscle (myotonia and weakness), nerve, CNS (MR), heart (conduction problems), eyes (cataract), etc.

• Myotonia refers to the slow/impaired relaxation of the muscles after voluntary contraction or electrical stimulation

• AD with anticipation• 3 Genetic loci :

– DM 1 : 98% of families l Myotonin protein kinase (DMPK) ; Chromosome 19q13.3; Dominant

– DM 2 (PROMM), l Zinc finger protein 9 (ZNF9) ; Chromosome 3q21; Dominant

– DM3l Chromosome 15q21-q24; Dominant

Congenital myotonic dystrophy -DM1

• Congenital MD, Largest # of triplet repeats of any MD syndrome (> 1,000), large expansion happens when it is transmitted maternally.

• Severe hypotonia/weakness at birth, respiratory failure is major cause of mortality, if infant survives infancy, weakness improve during early childhood. MR common.

Mitochondrial disorders

• Mitochondrial genome: 16.5 kb, circular, two complimentary strands

• Maternally inherited• Heteroplasmy: the wide type and mutant type co-

exist intracellularly• Mutation types: large-scale rearrangements

(deletion or duplications) and point mutations• Energy powerhouse

Clinical presentation

• Multisystemic with remarkable variability in the phenotypic presentation

• Neurological: myopathy, exercise intolerance, ophthalmoplegia, headache, seizures, dementia, ataxia, myoclonus, etc.

• Non-neurological: short stature, heart, endocrine, metabolic acidosis (lactic), etc.

Diagnosis

• Biochemical: lactate, CK

• Mutation analysis: large arrangement study for deletion/duplication, point mutation analysis

• Muscle bx: – Ragged red fibers: accumulated of abnormal

mitochondria under the sarcolemmal membrane. Absent does not rule out.

Childhood myasthenia gravis CHILDHOOD MYASTHENIA GRAVIS: TYPES

Neonatal Congenital Familial Infantile

Acquired Juvenile

Recurrent Arthrogryposis

Maternal MG

+ - - - +

Onset 0 to 3 days post natal

Birth Birth to 1 year

> 1 year More in Orientals

Congenital

Weakness Generalized Ocular ± Generalized

Respiratory Generalized

Ocular ± Generalized

Generalized

Time course

Remission 1 to 6 weeks

Fixed weakness

Fatal early, or Improvement > 2 years

Improvement over years

Static

Family history

± Other sibs Mother Untreated

Often Usual Rare Other sibs Mother Untreated

Anti-AChR antibodies

Most - - 50% More in Orientals

+ vs Fetal AChR

Neuroanatomy tools

Normal Anatomy in 3-D with MRI/PET

• Interactive website

• >150 slides• Modalities:

T1, T2, PET or combined

• Pointer shows structure

http://www.med.harvard.edu/AANLIB/cases/caseNA/pb9.htm

Documents

Pediatric Neurogenetics