By:By:Majid MojarradMajid Mojarrad

Why do we study humanWhy do we study humanchromosomes?chromosomes?

Chromosome disorders Chromosome disorders -- major category of major category of genetic diseasegenetic disease Responsible for >Responsible for >100100 identifiable syndromesidentifiable syndromes

More common than all mendelian single gene More common than all mendelian single gene disorders!disorders!

11% % livebirthslivebirths

22% % pregnanciespregnancies

HistoryHistory 18901890--19201920: : techniques of plant and insect cell staining techniques of plant and insect cell staining

applied to human cellsapplied to human cells 19231923: : XX/XY was postulated to be the sexXX/XY was postulated to be the sex--determining determining

mechanismmechanism 19561956: : 4646 chromosomes in human cellschromosomes in human cells

First karyotypeFirst karyotype 19591959: : Lejeune:Lejeune: Down Syndrome (extra small chr.), Down Syndrome (extra small chr.),

Turner Syndrome, Kleinfelter Syndrome other trisomies Turner Syndrome, Kleinfelter Syndrome other trisomies visualizedvisualized

19601960: : phytohemoglutanin (PHA)phytohemoglutanin (PHA)——stimulates blood cells stimulates blood cells to divideto divide When they divide, they condense to we can see them When they divide, they condense to we can see them

betterbetter

HistoryHistory

19701970: : chromosome bandingchromosome banding

19771977: : high resolution bandinghigh resolution banding

19861986: : fluorescence in situ hybridization fluorescence in situ hybridization (FISH)(FISH)

Clinical Indications forClinical Indications forChromosome AnalysisChromosome Analysis

Problems of early growth and developmentProblems of early growth and development

Stillbirth/neonatal deathStillbirth/neonatal death

Fertility problemsFertility problems

Family history of chromosome Family history of chromosome rearrangementrearrangement

Pregnancy indications Pregnancy indications –– LMA, U/S abn etcLMA, U/S abn etc

NeoplasiaNeoplasia

CytogeneticsCytogenetics

About About 400400 metaphase bands (haploid)metaphase bands (haploid) About About 66 million bp per bandmillion bp per band About About 8080--100100 genes per bandgenes per band Can see only gross changes (about Can see only gross changes (about 1010 genes or genes or

more), not a single gene deletionmore), not a single gene deletion GG--banding: dark bands are AT rich, light bands banding: dark bands are AT rich, light bands

are GC richare GC rich Centromeres are AT rich (stain darkly)Centromeres are AT rich (stain darkly) Light regions are generally more rich with coding Light regions are generally more rich with coding

sequences (promoters are GC rich)sequences (promoters are GC rich)

Chromosome nomenclatureChromosome nomenclature

Two armsTwo arms p (petite)p (petite) and q (follows p in alphabet)and q (follows p in alphabet)

11--2222 = = autosome numbersautosome numbers

X, Y = sex chromosomesX, Y = sex chromosomes

Karyotype Karyotype -- the chromosome constitution of an the chromosome constitution of an individualindividual

When chromosomes are preparing to divide the DNA When chromosomes are preparing to divide the DNA replicates itself into two strands called chromatidsreplicates itself into two strands called chromatids

Replicating chromosome The same chromosome under normal conditions

Centromere

Telomere

Telomere

The two chromatids

4646,,XX = femaleXX = female 4646,,XY = maleXY = male

Giemsa banding (G-banding)

Three classes of chromosomeThree classes of chromosome

Metacentric Metacentric -- centromere centromere in middlein middle

Submetacentric Submetacentric --centromere distant from centromere distant from middlemiddle

Acrocentric Acrocentric -- centromere centromere at endat end

Structural abnormalities ofStructural abnormalities ofchromosomeschromosomes

Six main typesSix main types

DeletionDeletion Ring chromosomeRing chromosome DuplicationDuplication IsochromosomeIsochromosome InversionInversion

paracentric &paracentric & pericentricpericentric

TranslocationTranslocation Robertsonian &Robertsonian & reciprocalreciprocal

Involves loss of part of a Involves loss of part of a chromosomechromosome

Results in Results in monosomymonosomy of that of that chromosomal segmentchromosomal segment

Clinical effects due toClinical effects due to Insufficient gene productsInsufficient gene products

Unmasking of mutant alleles Unmasking of mutant alleles on normal chromosomeon normal chromosome

DeletionDeletion

Beforedeletion

Afterdeletion

Two types of deletionTwo types of deletion

InterstitialTerminal

WolfWolf--Hirschhorn syndromeHirschhorn syndrome

• deletion of distal arm of chromosome 4p

• growth and mental retardation

• seizures

Ring chromosomeRing chromosomeBreaks occur in both arms of a chromosome. Breaks occur in both arms of a chromosome.

The two broken ends anneal; the two acentric fragments are The two broken ends anneal; the two acentric fragments are lost. lost.

Results in double deletion (in p and in q).Results in double deletion (in p and in q).

Epilepsy, mental retardation and craniofacial abnormalities

DuplicationDuplication

Direct Inverted

Trisomy of duplicated segment

IsochromosomeIsochromosomeMirror image chromosome

Loss of one arm with duplication of other

Loss of p-arm Duplication of q-arm

InversionInversionTwo breaks in one chromosomeTwo breaks in one chromosome

The fragment generated rotates The fragment generated rotates 180180oo and reinserts into and reinserts into the chromosomethe chromosome

Pericentric Pericentric -- involves p and q arminvolves p and q arm Paracentric Paracentric -- involves only one arminvolves only one arm

Chromosome structure

balanced rearrangements

inversions – 2 breaks, piece reattached in inverted order

paracentric – centromere not included

crossing over

Chromosome structure

balanced rearrangements

inversions – 2 breaks, piece reattached in inverted order

pericentric – centromere included

crossing over

TranslocationTranslocation -- exchange of exchange of chromosomal material between two or chromosomal material between two or

more chromosomesmore chromosomes ReciprocalReciprocal

RobertsonianRobertsonian

If no essential chromosome material lost or If no essential chromosome material lost or genes damaged then the individual is clinically genes damaged then the individual is clinically normalnormal

However, there is an increased chance of However, there is an increased chance of chromosomally unbalanced offspring chromosomally unbalanced offspring

Reciprocal TranslocationReciprocal Translocation

Involves two Involves two chromosomes chromosomes

One break in each One break in each chromosomechromosome

The two chromosomes The two chromosomes exchangeexchange broken segmentsbroken segments

Meiotic segregation in a reciprocal translocation

Chr. 3 Chr. 21

Reciprocal translocation

SEGREGATION

normal balanced

Alternate Adjacent 1 Adjacent 2

Pairing at meiosis

2

1

--unbalanced-- --unbalanced--

GAMETES

OFFSPRING normal balancedtranslocation

trisomy 3qmonosomy 21q

monosomy 3qtrisomy 21q

trisomy 3monosomy 21

monosomy 3trisomy 21

Robertsonian translocationRobertsonian translocation Named after W. R. B. Robertson who first identified Named after W. R. B. Robertson who first identified

them in grasshoppers in them in grasshoppers in 19161916 Most common structural chromosome abnormality in Most common structural chromosome abnormality in

humanshumans Frequency =Frequency = 11//10001000 livebirthslivebirths

Involves two acrocentric chromosomesInvolves two acrocentric chromosomes Two typesTwo types

Homologous acrocentrics involvedHomologous acrocentrics involved NonNon--Homologous acrocentrics involvedHomologous acrocentrics involved

Homologous acrocentric, i.e. chromosome 14

+ =

lost

Non-homologous acrocentric, i.e. chromosomes 14 & 21

+ =

lost

14 21

Robertsonian translocation

SEGREGATION

Meiotic segregation in a Robertsonian translocation

Alternate Adjacent

GAMETES

OFFSPRING

Pairing at meiosis

AB

A

C

B C

normal balanced ----------------unbalanced ----------------

----------A---------- ----------B---------- ----------C----------normal balanced

translocationunbalanced translocation

Down syndrome

monosomy 21 trisomy 14 monosomy 14

A balanced chromosome A balanced chromosome 1414 & & 2121Robertsonian translocationRobertsonian translocation

An unbalanced chromosome An unbalanced chromosome 1414 & & 2121Robertsonian translocation Robertsonian translocation --trisomy trisomy 2121,, Down syndromeDown syndrome

Numerical nomenclature & abnormalitiesNumerical nomenclature & abnormalities

Haploid Haploid -- the number of chromosomes in a gamete, N the number of chromosomes in a gamete, N (i.e. (i.e. 2323 chromosomes)chromosomes)

Diploid Diploid -- the number of chromosomes in a somatic the number of chromosomes in a somatic cell, cell, 22N (i.e. N (i.e. 4646 chromosomes)chromosomes)

Euploid Euploid -- any exact multiple of Nany exact multiple of N i.e. triploidy (i.e. triploidy (33N, N, 6969 chromosomes) chromosomes) -- spontaneous spontaneous

abortionsabortions

Aneuploid Aneuploid -- extra copy or absence of a chromosomeextra copy or absence of a chromosome i.e. trisomy i.e. trisomy 2121, Down syndrome, Down syndrome

Aneuploidy results from the failureAneuploidy results from the failureof chromosomes to separateof chromosomes to separatenormally during cell divisionnormally during cell division

NondisjunctionNondisjunction

NORMAL SEPARATION

NORMAL ZYGOTE

First meiotic division

Second meiotic division

Gametes

Fertilization

Zygotes

4N

2N

N

2N

NONDISJUNCTION

TRISOMIC ZYGOTE MONOSOMIC ZYGOTE

First meiotic division

Second meiotic division

Gametes

Fertilization

Zygotes

Distribution of nonDistribution of non--disjunctiondisjunction

Meiosis I Meiosis II

Mitosis

Maternal 21, 15, 16 18 15, 18, 21, 8

Paternal - 18, 21 18, 21

Parental origin ofParental origin ofaneuploidyaneuploidy

Paternal %Paternal % Maternal %Maternal %

Trisomy Trisomy 1313 1515 8585

Trisomy Trisomy 1818 1010 9090

Trisomy Trisomy 2121 55 9595

4545,,XX 8080 2020

4747,,XXXXXX 55 9595

4747,,XXYXXY 4545 5555

4747,,XYYXYY 100100 00

Clinically Important Autosomal Clinically Important Autosomal Chromosomal AbnormalitiesChromosomal Abnormalities

TrisomyTrisomy: : 2121,, 1818, , 1313, , 88, , 2222, , 99, mosaicism for trisomic and , mosaicism for trisomic and normal cellsnormal cells

Partial Trisomy (duplication)Partial Trisomy (duplication): : 11q,q, 1313q, q, 99p, p, 44p, p, 1010q, q, 1111q, q, 77q, q, 1414q, q, 11q, q, 33p, p, 44q, q, 88q, q, 1010p, p, 1111p, p, 1515q, q, 2020pp

MonosomyMonosomy: : 2121,,2222 Partial Monosomy (deletion of part)Partial Monosomy (deletion of part): : 55p,p, 1313q, q, 44p, p, 1818p, p,

1818q, q, 1111q, q, 77p, p, 99p, p, 1212pp DuplicationDuplication--deficiencydeficiency: : 33,,44,,22 TriploidyTriploidy Chromosome breakageChromosome breakage: : Fanconi anemia, Fanconi anemia,

Bloom syndrome, ataxia telangiectasia, Bloom syndrome, ataxia telangiectasia, glutathione reductase deficiencyglutathione reductase deficiency

Down Syndrome (Trisomy 21)Clinical Features (Clinical Features (11 in in 800800 births)births)•• Mental retardationMental retardation

•• Flat facial profileFlat facial profile

•• Oblique palpabral fissureOblique palpabral fissure

•• Muscle hypotoniaMuscle hypotonia

•• HyperflexibilityHyperflexibility

•• Lack of Moro reflexLack of Moro reflex

•• Abundant neck skinAbundant neck skin

•• Broad and short trunkBroad and short trunk

•• Dysplastic earsDysplastic ears

•• Horizontal palmar creaseHorizontal palmar crease

•• Dysplastic pelvisDysplastic pelvis

•• Dysplastic middle pharynxDysplastic middle pharynx

•• Epicanthic foldsEpicanthic folds

•• Acute leukemiasAcute leukemias

Down syndrome with downslanting palpebral fissures and a slightly protruding tongue.

Epicanthal fold Light smudgy opaque pupil

Simian crease

karyotypeExamples of Incidence MaternalExamples of Incidence MaternalKaryotypeKaryotype age age

Trisomy Trisomy 2121 typetype 4747,,XX,+XX,+2121 >>9090%% IncreasedIncreased Translocation typeTranslocation type

4646,,XX,der(XX,der(1414;;2121)(q)(q1010;q;q1010),+),+2121 33--44% % NormalNormal4646,XX,+,XX,+2121,der(,der(2121;;2222)(q)(q1010;q;q1010))

Mosaic typeMosaic type 4646,,XX/XX/4747,XX,+,XX,+2121 22--33%% NormalNormal Partial trisomyPartial trisomy

47,XY,+21

47,XX,+21

4646,,XX,+XX,+2121,der(,der(2121;;2121)(q)(q1010;q;q1010))

Edwards Syndrome (Trisomy 18)

KaryotypeKaryotype IncidenceIncidence

Trisomy Trisomy 1818 typetype 4747,,XX,+XX,+1818 9090%%

Translocation typeTranslocation type 4646,,XX,der(Dq;XX,der(Dq;1818q),+q),+1818 rarerare

Mosaic typeMosaic type 4646,,XX/XX/4747,XX,+,XX,+1818 1010%%

Clinical feature (Clinical feature (11 in in 75007500 births)births)Mental retardation, Failure to thrive, Prominent occiput, Mental retardation, Failure to thrive, Prominent occiput, Micrognathia and lowMicrognathia and low--set ears, Hypertonicity, Flection of fingersset ears, Hypertonicity, Flection of fingers

Cardiac,Cardiac, renal and intestinal defects, Short sternum and small renal and intestinal defects, Short sternum and small pelvis, Abduction deformity of hippelvis, Abduction deformity of hip

Mental retardation, Failure to thrive, Prominent occiput, Micrognathia and low-set ears, Hypertonicity, Overlapping fingers

(clenched fist)

Cardiac, renal and intestinal defects, Short sternum and Small pelvis, Abduction deformity of hipRocker bottom feet

A child with multiple congenital anomalies withSmall face, micrognathia Small chest & Low-set ear

9595% % of embyoes have been lostof embyoes have been lost

8080% % of livebirths are femaleof livebirths are female

Karyotype Karyotype

8080% % are trisomy are trisomy 1818

2020% % resultes from translocationresultes from translocation

Patau Syndrome (Trisomy 13)

KaryotypeKaryotype IncidenceIncidence

Trisomy Trisomy 1313 typetype 4747,,XX,+XX,+1313 >>8080%% Translocation Translocation 4646,,XX,der(Dq;XX,der(Dq;1313q),+q),+13 1013 10%% Mosaic typeMosaic type 4646,,XX/XX/4747,XX,+,XX,+1313 55%%

Clinical Features (Clinical Features (11 in in 2000020000--2500025000 births)births)Microcephaly and mental retardation, Scalp defect, Microcephaly and mental retardation, Scalp defect, Microphthalmia, Cleft palate, Polydactyly, RockerMicrophthalmia, Cleft palate, Polydactyly, Rocker--bottom bottom feet, Abnormal ears, Apneic spells and myotonic seizures, feet, Abnormal ears, Apneic spells and myotonic seizures, Extensive visceral defectsExtensive visceral defects

cleft palate, atrial septal defect, inguinal hernia, and postaxial polydactyly of the left hand.

Polydactyly, particularly of all extremities, strongly suggests trisomy 13.

Autosome deletion syndromesAutosome deletion syndromes

CRI DU CHAT syndromeCRI DU CHAT syndrome

Small deletion syndromesSmall deletion syndromes

Cri du Chat (Cat-cry) Syndrome

Karyotype : Karyotype : 4646,XX,,XX,55pp-- 4646,XY,,XY,55pp--1515 Incidence :Incidence : 11 in in 5050,,000000 birthsbirths Maternal age : NormalMaternal age : Normal

Clinical featuresClinical featuresMental retardationMental retardationMicrocephaly and round faciesMicrocephaly and round faciesMewing cryMewing cryEpicanthic foldsEpicanthic folds

Wolf-Hirschhorn Syndrome

Karyotype : Karyotype : 4646,XX,,XX,44pp--4646,XY,,XY,44pp--

Incidence :Incidence : 11 in in 5050,,000000 birthsbirths Maternal age : NormalMaternal age : Normal

Clinical featuresClinical featuresMental retardationMental retardationMicrocephaly and Microcephaly and abnormal facies. Cardiac, renal, and genital abnormalities. Most are stillborn or die in infancy

wide-spaced eyes and repaired cleft lip

46,XY,del(4)(p16)

DiGeorge Syndrome (DGS)22q11 Deletion Syndrome Velocardiofacial Syndrome (VCFS)

Disease characteristics: • Congenital heart disease (74%)• Palatal abnormalities (69%) • Characteristic facial features • Learning difficulties (70 - 90%)

Diagnosis: 22q11 submicroscopic deletion

Genetic counseling. 94% of probands have a de novo deletion of 22q11