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Genetics Lecture 18Principles of Clinical Cytogenetics I

Thursday, October 27, 2011, 10:00-10:50 AM

Objectives:

Know the basic anatomy of human chromosomes. Introduce the concept that chromosome abnormalities can have clinical consequences by

disrupting genes or affecting gene dosage. Learn the clinical indications for a chromosome study. Be able to list the names of methodologies, scope, and limitations of current cytogenetic

and molecular cytogenetic techniques. Be able to define haploid, diploid, polyploid, euploid, aneuploid, trisomy, and monosomy. Understand and recognize numerical and structural chromosome abnormalities. Learn the clinical implications of balanced and unbalanced chromosome abnormalities. Understand the effects of inversions and translocations on chromosome pairing and

segregation during meiosis.

Learn the basic cytogenetic nomenclature.

Why Think Chromosomes?

Chromosome problems are suspected in cases of:Growth/developmental problemsMultiple birth defects (live born or ultrasound)Mental retardation (mild or severe, associated with birth defects)Stillbirth and neonatal deathInfertility (multiple miscarriages >3)

Family history of a known chromosome problemNeoplasia (tumor) analysisPregnant women 35 years old at deliveryAmbiguous genitalia

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Normal Human Karyotype

Common sources: T-Lymphocytes Fibroblasts Amniocytes Bone marrow

Usually done from metaphase cells

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Nomenclature

Classified by the position of the centromere:MetacentricSubmetacentricAcrocentric

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G-Banding is the Most Common Staining Method Dark bands (G+)

AT rich gene poor

Light bands (G-) GC rich gene rich

Numerical Abnormalities in Humans: Terminology

Haploid: chromosome number in gametes23 chromosomes (n)

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Euploid: multiple of haploid numberDiploid: 46 chromosomes (2n), normal countPolyploidy, abnormalTriploid: 69 chromosomes (3n)Tetraploid: 92 chromosomes (4n)

Aneuploid (heteroploid): any other number that is not a multiple of the haploid numberExamples:

Trisomy 21: 47 chromosomes, Down syndromeMonosomy X: 45 chromosomes, Turner syndrome

Chromosome Abnormalities

NumericalExtra or missing chromosomesAssociated with spontaneous abortion and birth defects

StructuralUnbalanced

Extra or missing chromosome portions

Associated with birth defectsBalanced

Misplaced chromosome portionsAssociated with no phenotype in carrier, but could lead to abnormalities in offspring

Numerical Abnormalities: Trisomy

47, XX, +21Female with Down syndrome

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Numerical Abnormalities: Monosomy

45, XFemale with Turner syndrome

Structural Abnormalities

Unbalanced extra or missing chromosome portions can lead to clinical phenotype

Deletions Duplications Marker and ring chromosomes Dicentric chromosomes Isochromosomes

Balanced misplaced chromosome portions not associated with a clinical phenotype Translocations

Robertsonian Reciprocal

Inversions Insertions

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Unbalanced Structural Abnormalities

Deletions and Duplications

Caused by breakage and unequal crossing overIn general, duplications are less harmful than deletions

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Balanced Structural AbnormalitiesTranslocations

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Balanced Translocation

46,XY,t(5;8)(q31.1;p23.1

Balanced Robertsonian Translocation

Note that carriers of Robertsonian translocations have 45 instead of 46 chromosomes

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Chromosome Pairing During Meiosis

Chromosomes pair during meiosis I to promote recombinationMeiosis I pairing occurs among matching segmentsWhen there are translocations and inversions, chromosomes must find a way so thatthis pairing can occur

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Inversions

ParacentricOutside of centromeric area

PericentricAround centromeric area

Pericentric Inversions During Meiosis

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Paracentric Inversions During Meiosis

Cytogenetic Nomenclature Examples

Normal: male, 46,XY; female, 46,XXTrisomy: 47,XY,+21Monosomy: 45,XDeletion: 46,XX,del(5)(p13)Duplication: 46,XY,dup(1)(q22q25)

Reciprocal balanced translocation: 46,XY,t(3;11)(q12;p15.5)Unbalanced translocation: 46,XX, der(11)t(11;12)(q23;q13)Robertsonian translocation carrier: 45,XX,der(14;21)(q10;q10)Inversion: 46,XX,inv(3)(p25q21).

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FISH Detection of Microdeletions

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Chromosome Painting to Define Structural Abnormalities

Array Comparative Genomic Hybridization

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Fig. 4.7 Scion Publishing Ltd

Comparative Genomic Hybridization (CGH)

Advantages:FasterHigher resolution than regular chromosome preparation.

Regions of net gain or loss can be identifiedUseful in tumor analysis, where it is difficult/impossible to obtain a metaphasepreparation

Will confirm apparently balanced translocations

Limitations:Will not detect balanced rearrangementsThe origin of dup/del is identified, but not the mechanism