Chromosomal abnormalities

Cytogenetics

Study of structure and function of chromosomes

Karyotype

A preparation of chromosomes arranged in size order

Making a karyotype

1) Culture cells

• Add blood sample to a culture flask with growth medium

• Add phytohaemagglutinin to stimulate mitosis

• Incubate to allow cell division

• Arrest mitosis in metaphase

  • Add colcemid (1–2 hours) to stop cells in metaphase

• Prepare cells

  • Centrifuge to pellet cells

  • Add hypotonic solution to burst cells (spread chromosomes)

  • Fix cells with fixative (methanol/acetic acid)

• Slide preparation

  • Drop cells on microscope slide

  • Stain with Giemsa to reveal G-bands

  • View under microscope and photograph metaphases

• Chromosome arrangement

  • Pair chromosomes by:

    • Length

    • Banding pattern

    • Centromere position

  • Types of centromere positions:

    • Metacentric: centromere central

    • Sub-metacentric: off-centre

    • Acrocentric: near one end

1. Culture cells

• Add blood sample to a culture flask with growth medium

• Add phytohaemagglutinin to stimulate mitosis

• Incubate to allow cell division

• Arrest mitosis in metaphase

  • Add colcemid (1–2 hours) to stop cells in metaphase

2. Arrest mitosis in metaphase

Add colcemid (1–2 hours) to stop cells in metaphase

3. Prepare

• Centrifuge to pellet cells

• Add hypotonic solution to burst cells (spread chromosomes)

• Fix cells with fixative (methanol/acetic acid)

4. Slide preparation

• Drop cells on microscope slide

• Stain with Giemsa to reveal G-bands

• View under microscope and photograph metaphases

5. Chromosome arrangement

• Paired chromosomes

• Types of centromere positions

Paired chromosomes

• Length

• Banding pattern

• Centromere position

Centromere positions

• Metacentric

• Sub-metacentric

• Acrocentric

Metacentric

Centromere central

Sub-metacentric

Off centre

Acrocentric

Near one end

Uses of karyotyping

• Detect numerical changes in chromosomes (e.g. trisomy, monosomy)

• Detect structural changes (e.g. deletions, translocations)

• Prenatal screening, cancer diagnosis, fertility issues

Changes in chromosome number

• Polyploidy

• Aneuploidy

Polyploidy

• Extra sets of chromosomes (e.g. 3n, 4n)

• Common in plants, rare in animals

Aneuploidy

• Missing or extra single chromosomes

  • Monosomy

  • Trisomy

• Caused by non-disjunction during meiosis

• Very common in conceptions; most result in miscarriage

Monosomy

one missing (e.g., Turner's Syndrome, 45,X)

Trisomy

one extra (e.g., Down's Syndrome, 47,XX,+21)

Down’s (Trisomy 21)

• 47,XX,+21

• Facial features, learning disability, heart issues, ↑ risk of leukaemia & Alzheimer's

Turner’s (Monosomy X)

• 45,XO

• Female, short stature, sterile, needs estrogen therapy

Aneuploidies

• Down’s (Trisomy 21)

• Turner’s (Monosomy X)

• Klinefelter’s (XXY)

Klinefelter’s (XXY)

• 47,XXY

• Male, tall, sterile, mild feminisation, testosterone therapy

Deletion chromosome mutations

• Cri-du-chat (5p deletion)

• Prader-Willi (15q deletion - paternal)

• Angelman (15q deletion)

Cri-du-chat (5p deletion)

Cat-like cry, facial features, developmental delay

Prader-Willi (15q deletion - paternal)

Weak suckling, obesity, low fertility

Angelman (15q deletion - maternal)

Happy affect, speech/language delay, seizures

Genomic imprinting

Different expression depending on maternal vs paternal origin due to gene silencing (e.g., by methylation)

Translocation chromosome mutations

• Familial Down’s

• CML (Chronic Myeloid Leukaemia)

CML (Chronic Myeloid Leukaemia)

• t(9;22) – Philadelphia chromosome

• BCR-ABL fusion gene leads to excess white blood cells

• CML is spontaneous, not inherited

• Philadelphia chromosome found in 95% of CML cases

Familial Down’s

• t(14;21)

• Inherited, 5% of Down's cases

Chromosomal mapping and banding

• Each chromosome arm labeled:

  • p = short arm ("petite")

  • q = long arm

• Each G-band assigned an address (e.g., 15q11.2)

Screening for Chromosomal Abnormalities

• Blood tests – Detect fetal proteins

• Ultrasound – Look for nuchal fold thickening

• Amniocentesis + karyotyping – Accurate diagnosis; slight miscarriage risk