Clinical GeneticsPediatric Residents

Deborah Lyn, Ph.DEmail: dlyn@msm.eduPhone: 404-752-1521

November 2009

Family History is the First Step in Constructing a Pedigree

Questions to consider:• Is the transmitted disease due to the inheritance

(predisposition) of a cancer-related gene or chromosomal abnormality?

• Does the pattern of transmission follow Mendelian mode of inheritance?

Parent does not manifest cancer, but risk is passed to next generation.

Single Gene Inheritance:(Monogenic)

Nuclear Genome

• Assumes a single gene defect.• Allelic frequency usually <0.1%.• Transmitted on autosomal, X or Y chromosome (follow

Mendelian pedigree pattern).• Disease likely to be severe, occur at early age of

onset.• Severity may depend on location of gene defect.• Complicated by environmental modifiers.

Dominant inheritance: heterozygotes display phenotypeRecessive inheritance: homozygotes display phenotype

http://genetics.gsk.com

Autosomal dominant diseases:Huntington’s, Myotonic Dystrophy, Marfan syndrome

Inheritance of an Autosomal Dominant Inheritance of an Autosomal Dominant DisorderDisorder

• Male to male transmission observed. • Ratios of affected offspring will give clues to mode of inheritance.• Heterozygotes will display the disease.

http://genetics.gsk.com

Recessive traits:Sickle cell anemiaCystic fibrosisPhenylketonuria- mutation in phenylalanine hydroxylase.

recessive

Autosomal RecessiveAutosomal Recessive

(a) What is the probability of aa from Aa heterozygote parents?

(b) All offspring will be Aa.

X-Linked Disorders

• X-linked disorders are notable for their expression in males.

• Males always display disease when they inherit mutant gene.

• X-linked dominant and recessive genes are only applicable in females.

• Absence of father to son transmission, but daughters of a male with an X-linked trait must inherit the mutant gene.

X-Linked Recessive Inheritance

Family with X-linked choroideremia, due to a mutation in the gene CHM, which is located on the X chromosome. The asterisk represents a mutation within that gene.

http://ordc.ohsu.edu/disease-information/inheritance

http://genetics.gsk.com

Menke’s disease [defect in copper transport]Hemolytic anemia (mutation in glucose 6-phosphate dehydrogenase).

Affected Father’sGenotype

Normal XA YMother’s X XXA XYGenotype X XXA XY

Punnett Square for X-Linked Recessive

Inheritance

Inheritance of an X-linked Dominant Inheritance of an X-linked Dominant DisorderDisorder

Fig. 7.4 ©Scion Publishing Ltd

X-Chromosome InactivationX-Chromosome Inactivation

[implantation of embryo]

Complication to X-Linked Inheritance

Lyon HypothesisOne X chromosome in each cell is randomly inactivated in the embryonic development of females. (Paternal and maternal derived x chromosome will be inactivation in about half of the embryos’ cells).

• Compensate for gene dosage on X-chromosome between males and females.

• Inactivation is permanent once it is determined.• In females, X chromosome exhibits somatic cell mosaics.• Barr Body in females is due to a condensed heterochromatin

structure (transcriptional inactive).

• X-chromosome inactivation will affect disease severity, and complicate pattern of recessive inheritance.

X-Chromosome InactivationX-Chromosome Inactivation

Turner’s SyndromeTurner’s Syndrome

• Monosomy X chromosome: Xm (maternal) or Xp (paternal).

• Barr body is not detectable.• No X-inactivation.• Intelligence usually normal, but impaired

social competence and adjustment.

Other Factors That Complicate Other Factors That Complicate Mendelian PatternsMendelian Patterns

New Mutation• A child will be born with a disease for which there is no

previous history of the disease in the family.

New autosomal dominant mutation, appears to mimic an autosomal or X-linked recessive pattern.

New MutationNew Mutation

• Chromosomal abnormality (meiosis).• Post-zygotic event (mitosis).• New dominant mutation (altered protein structure).• Sporadic change-tumor development - environmental

exposure.• Exposure to viral infection.

– Congenital rubella syndrome.

• Exposure to teratogenic agent during pregnancy.– Fetal alcohol syndrome.– Neural tube defect.– Cigarette smoking (embryonic hypoxia).

Exposure Within First Trimester Can Result in Exposure Within First Trimester Can Result in Craniofacial MalformationsCraniofacial Malformations

J Orofac Orthop (2007) 68: 266-277

Vitamin B6, B12 and folic acid:Vitamin B6, B12 and folic acid: Role in One-carbon Transfer Reactions. Role in One-carbon Transfer Reactions.

• Purine, pyrimidine synthesis.• Cytosine methylation.• Methylation of histones.

Germline Mosaicism

Centre for Genetics Education. Http://www.genetics.edu.au

• Severity of the disease can vary greatly - modified by environment, modifier genes (protein variants).

• Heteroplasmic mitochondrial mutation.

• Mosaicism (e.g. Due to X-chromosome inactivation).

Variable Expression

Uniparental Disomy (UPD)

Inheritance of both autosomal chromosomes (or a segment) from the same parent.

Impact observed in:• Genes regulated by genomic imprinting.• Mitochondrial inheritance.• Skew pattern of recessive inheritance.

Uniparental Disomy (UPD)

rr N N

X

Nr

Heterozygote:No display of

phenotype/disease

N N

X

Autosomal Recessive

rr

rr

Homozygosity arises as a result of UPD

Loss of Mendelian Principles-> even segregation and

independent assortment of chromosomes.

Trisomy rescue

Single Gene InheritanceMitochondrial Genome

• Assumes a single gene defect.

• Severity may depends on percentage of mitochondria bearing mutation (homoplasmy versus heteroplasmy).

• Genome more vulnerable to mutations: higher incidence of acquired diseases associated with aging.

• Diseases of cardiac and skeletal muscle are common-mitochondrial myopathies.

• Diseases of nervous system.

• Egg contributes most of the mitochondria to the zygote.

Egg Contributes Most of the

Mitochondria to the Zygote

Inheritance of Variable Severity

“Heteroplasmy” more than one population of mitochondrial DNA in the cell.

Individuals harbor multiple copies of mitochondrial genome.

Oogenesis

Characteristics of Mitochondrial Inheritance

• Mitochondrial disorders are inherited maternally. Both male and female offspring are affected, but affected males cannot transmit the disease.

• Mutations in genes involved with oxidative phosphorylation.

– Leber’s hereditary optic neuropathy (mutation in electron transport protein NADH-coenzyme Q).

– Myoclonus Epilepsy Associated with Ragged-Red Fibers (MERRF)

– Mitochondrial Encephalomyopathy with Lactic Acidosis and Stroke-like Episodes (MELAS).

• Homoplasmy and heteroplasmy mutations.

Note that transmission occurs only through females (circles).

Pedigree Showing Transmission and Expression of a Mitochondrial Trait

[probably due to heteroplasmy]

Multi-factorial Inheritance“One Gene One Phenotype” hypothesis

is not applicable.

Spectrum of human characters: from “Human Molecular Genetics by Strachan & Read.

[Figures from: www.uic.edu/classes/bms/bms655/gfx]

Figure 18: Recurrence risk to first degree relatives of affected individuals.

Figure 15: Below the threshold, the trait is not expressed. Individuals above the threshold have the disease.

healthy disease

The Threshold Model for Multi-factorial TraitsThe Threshold Model for Multi-factorial Traits

-Body mass index-Height-Blood pressure

-Phenotype is present or absent.-Cleft lip, cleft palate.

Phenotype is Combination of Genotypes + Epigenome +

Environment

Deviations From Expected Ratios Deviations From Expected Ratios May Be Due To:May Be Due To:

• Alleles show incomplete dominance or co-dominance.• Genes under investigation are closely linked on the

same chromosome.• X-linked inactivation may result in manifesting

heterozygote females.• Genetic interactions between different genes.• Trait is inherited on genetic material from only one

parent. e.g. mitochondrial DNA is only inherited from the mother.

• Gene is imprinted.

Genetic Variation is More than a SNPGenetic Variation is More than a SNP

Chromosomal Defects

DiGeorge Syndrome• Deletion at chromosome 22q11. Variation in symptoms is

related to the amount of genetic material lost.

• CHARGE association (coloboma, heart defect, choanal atresia, growth or developmental retardation, genital hypoplasia, and ear anomalies, including deafness).

• Cardiac and some speech impairments may be corrected surgically or therapeutically.

• DGS gene is required for the normal development of the thymus (reduced T-cell function) and parathyroid gland.

• “Complete DiGeorge anomaly" is used to describe infants who are athymic. Those with “typical” phenotype have low T-cell numbers.

• Some success with thymus transplantation.

Spontaneous Fetal Loss for Autosomal Trisomies

Fig 3.1, Medical Genetics (1999) G.H. Sack, Jr. McGraw-Hill.

Poor survival of trisomy 13 (Patau syndrome) and 18 (Edwards syndrome) conception, but no survival of trisomy 16.

http://www.bio.miami.edu/~cmallery/150/mendel/heredity.htm

Maternal Age is Associated with Rise Maternal Age is Associated with Rise in Incidence in Trisomyin Incidence in Trisomy

Robertsonian TranslocationRobertsonian TranslocationNon-Homologous RearrangementNon-Homologous Rearrangement

Long arms of acrocentric chromosome fuse at centromere, short arms are lost.

Down Syndrome due to Robertsonian Translocation

Robertsonian translocation arises from fusion of long arms of chromosome 21 and 14 [46 t(14;21)].

Translocation trisomy

Translocation Down syndromeTranslocation Down syndrome

• Accounts for 5% of cases.• Woman with balanced translocation have high

recurrent risk of producing a second child with Down syndrome.

• ROBs usually occur during meiosis and originate from a single parent.

• Post-zygotic model would predict random translocation events.

Patau Syndrome

• Trisomy 13; 1/12,000 births.• Short life-span, mean survival is 95 days. Longer

survival linked with heteromorphisms.• Clinical triad (microphthalmia/eye development

disorder, cleft lip/palate, and polydactyly). Other facial deformities - abnormally small ears, small jar.

• Non-cyanotic heart defects.• Intra-uterine growth retardation, single umbilical

artery (evident on sonogram).• Associated with advanced age of parents.

Cytogenetic mosaicism - longer survival, variable expression.

European Journal of Medical Genetics (2008) 51, July-AugustFig. 1. Phenotypic aspects of the patient. The infant at birth (A) and at 12 years (B); pigmentary mosaicism on the back (C); feet (D). Linear white streaks are evident on both feet.

Patau Syndrome - Trisomy 13

• Involves micro-deletions spanning multiple,

adjacent genes positioned along a chromosome.• Require FISH (fluorescent in situ hybridization) for

detection.• Imprinted genes usually located in cluster and

harbor long range cis-acting DNA elements (imprint control element).

• Outcome of a contiguous deletion of imprinted genes is dependent on the chromosomal parent of origin.

Contiguous Gene SyndromesContiguous Gene Syndromes

Normal Embryos Can ONLY Arise When Normal Embryos Can ONLY Arise When MaternalMaternal and and PaternalPaternal Genomes Are Genomes Are

CombinedCombined

www.mcb.ucdavis.edu/.../chedin/research.htmModified from Janine LaSalle

• Haploid genome is not equivalent in regards to gene transcription.

• Selective Expression/Transcription from either maternal or paternal allele.

• Selective gene silencing from either maternal or paternal allele. Does not occur on every chromosome.

• Occurs during gametogenesis.• Disease manifestation is dependent on allelic loss of

normally transcribed gene.• Usually involved with development and post-natal

growth.

Expression of Imprinted Genes

Beckwith-Wiedemann SyndromeBeckwith-Wiedemann Syndrome(overgrowth disorder)(overgrowth disorder)

Normal Function Depends on Coordinated IGF2 (insulin-like growth factor-2) and H19 (non-coding RNA) genes expression on chromosome 11p15.5.

IGF2 gene is normally silenced on maternal chromosome.In BWS, biallelic expression of IGF2 gene; escape of epigenetic silencing.

transcription

CH3

IGF2

Maternal

PaternalIGF2

H19

H19

CH3

Silver Russell Syndrome

CH3

IGF2

Maternal

PaternalIGF2

H19

H19

CH3

Epimutation is opposite to BWS.Biallelelic expression of H19 genes on both parental

chromosomes, due to loss of methylation at imprint control region of H19.

Reduced expression of IGF2.SRS-like condition is also due to loss imprint control on

chromosome 7p.

transcription

Chromosome 11p15

Silver Russell Syndrome

Poor intrauterine growth: low birth weight, small for gestational age.Cranofacial features, triangular shaped face (pointed small chin), broad forehead.Body asymmetry of arms and legs.Fifth finger clinodactyly.Normal mental development.

Expression of Paternal “A” Gene is Lost.

Fig. 7.18 ©Scion Publishing Ltd

Monoallelic Expression of Genes at 15q11-13Monoallelic Expression of Genes at 15q11-13

Deletion of paternally expressed genes -> Prader Willi syndrome.Deletion of maternally expressed genes -> Angelman syndrome

Prader-Willi and Angelman syndrome Prader-Willi and Angelman syndrome Neurogenetic Disorders Caused by the Lack of a Neurogenetic Disorders Caused by the Lack of a

Paternal or a Maternal Contribution from Paternal or a Maternal Contribution from Chromosome 15q11-q13.Chromosome 15q11-q13.

Fig 3.13, Medical Genetics (1999) G.H. Sack, Jr. McGraw-Hill.

Prader-Willi Angelman

UDP: Uniparental DisomyIC: Imprint Control regionID: Imprint Deletion/Mutation in Regulatory Sequences

Selective expression of genes from this chromosomeTranscriptional silencing

Genomic Imprint of PWS and AS Gene Clusters:Genomic Imprint of PWS and AS Gene Clusters:Monoallelic Expression at Chromosome 15qMonoallelic Expression at Chromosome 15q

15q

11-1

3

Prader-Willi Syndrome• 1 in 10,000-1,25,000.• Fetal phenotype: fetal movement is diminished, amniotic

fluid in lower range, abnormal fetal position, low birth weight.

• At birth: hypotonic with difficult feeding, facial dysmorphism; difficulty establishing respiration.

• Hypogonadism, may have undescended testes in males.

• Misdiagnosed as Down syndrome, due to marked obesity. • Genetic test can distinguish chromosome difference,

Methylation sensitive PCR, polymorphic markers can separate UPD.

Prader-Willi Syndrome• Childhood: Excessive sleeping, failure to thrive, delayed

milestones, speech delay.• Hyperphagia/ change in feeding from infancy.• Learning disability, but some cases of average intelligence.• Early intervention requires GH injections to support increased

muscle mass and reduce food re-occupation. • Adolescence: Delayed puberty (girls have premature

adrenarche), short stature, obesity, excessive eating.• Adult: Uncertain fertility outcome especially in boys. Prone to

diabetes.

• General: small hands and feet, high narrow forehead, excess fat, light skin and hair relative to other family members.

Angelman SyndromeAngelman Syndrome

• Severe mental retardation. • Delayed motor development, absence of speech, lack of

balance, jerky movements (“puppet”). • Excessive laughter.• Typical face: wide spread teeth, broad mouth, protruding

tongue, and prominent chin.• Caused by loss of function of the imprinted UBE3A

(ubiquitin ligase3A) gene.

Prader-Willi Syndrome

70% Paternal Deletion25% Maternal Uniparental Disomy5% Imprinting Defect

AngelMan Syndrome

70% Maternal Deletion4% Paternal Uniparental Disomy3-5% Sequence mutation in UBE3A gene1% Chromosomal rearrangement10% Unknown

del 15(q11-q13)del 15(q11-q13)

Summary

Website from Univ. Florida

http://www.peds.ufl.edu/divisions/genetics/teaching-resources.htm