Chromosome Structure and Abnormalities - 21.01.26

Chromosomal Structure and Terminology

  • Chromosome Structure

    • Two arms on a chromosome

    • Short arm: Known by the convention letter p.

    • Long arm: Known by the convention letter q.

    • Centromere location can vary:

    • Offset position results in clearly defined short and long arms.

    • Central position may not yield clearly defined arms but is still referred to as such.

  • Ideograms

    • Chromosomal patterns represented visually in banded patterns known as genus banded (Giemsa staining)

    • Cytogeneticists analyze band patterns to identify chromosome types and detect abnormalities (missing pieces, etc.).

Chromosomal Analysis Methods

  • Specimen Sources

    • Commonly analyzed from:

      • Blood (venous blood preferred).

      • Skin cells.

      • Amniotic fluid containing fetal cells.

    • Cell Requirements:

      • Nucleated cells are necessary for chromosomal analysis (red blood cells lack nuclei and are removed).

  • Cell Culture Process

    • White blood cells cultured for 48-72 hours at 37°C to obtain dividing cells.

    • Colchicine: Used to arrest cells in metaphase during mitosis, allowing for visualization of condensed chromosomes.

  • Preparation and Staining

    • Cells lysed with saline and fixed on slides.

    • Stained with a proprietary dye (e.g., from a company called Gems) to enhance visibility of band patterns.

    • Historical methods involved photography and manual cutting of bands, whereas modern techniques utilize digital processing.

Chromosome Terminology

  • Chromosome Shapes

    • Metacentric: Centromere located centrally, resulting in arms of similar length.

    • Submetacentric: Centromere offset, creating one short arm (p) and one long arm (q).

    • Acrocentric: Minimal short arm and predominantly long arm; often has satellite regions which contain no coding DNA.

    • Notable acrocentric chromosomes: 13, 14, 15, 21, 22.

Chromosome Identification and Karyotyping

  • Chromosome Identification Rules

    • Essential for universal understanding among geneticists globally.

    • The identification format includes:

    • Chromosome number.

    • Arm (p or q).

    • Band number.

    • Sub-band number, typically in a format like 9q34.

  • Clinical Relevance of Karyotypes

    • Key example: ABO blood group gene located on chromosome 9, q arm, 3rd band, and 4th sub-band (written as 9q34).

    • Karyotypes convey genetic abnormalities based on numerical or structural findings.

Common Chromosomal Conditions

  • Numerical Abnormalities

    • Trisomy: Presence of an extra chromosome (e.g., Down syndrome, characterized by an extra chromosome 21).

    • Karyotype for Down syndrome: 47 XX or XY +21.

    • Monosomy: Loss of a chromosome; clinically relevant example includes Turner syndrome (45X).

    • Other trisomies: Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13).

  • Structural Abnormalities

    • Deletion: Loss of part of a chromosome (e.g., cri du chat syndrome).

    • Inversions: Segment of a chromosome is reversed.

    • Pericentric inversion: Includes the centromere.

    • Paracentric inversion: Does not include the centromere.

    • Translocations: Exchange of genetic material between chromosomes (reciprocal translocation).

    • Often does not affect health but may increase risks for offspring.

    • Reciprocal Translocation: A mutual exchange of segments between two chromosomes. Usually balanced (no genetic material lost), but carriers are at risk for producing unbalanced offspring with partial trisomy or monosomy.

    • Robertsonian Translocation: Occurs between two acrocentric chromosomes (e.g., 14 and 21). The long arms fuse at the centromere, and the short arms are lost. A carrier has 4545 chromosomes but a normal phenotype. However, if they pass the fused 14;2114;21 chromosome and a normal 2121, the child will have Trisomy 21 (Translocation Down Syndrome), which is independent of maternal age.

Specific Chromosomal Syndromes

  • Down Syndrome

    • Characteristic features include short stature, characteristic facial features, and developmental delays.

    • Heart defects: Congenital heart disease is common; some cases are surgically correctable.

    • Thyroid Problems: Increased frequency of thyroid dysfunction.

    • Characterized by hypotonia, up-slanting palpebral fissures, Simian crease (single palmar crease), Brushfield spots in the iris, and increased risk for Alzheimer’s disease and leukemia.

  • Edwards Syndrome (Trisomy 18)

    • Notable for micrognathia (small jaw), low-set malformed ears (elfin features), clenched fists with overlapping fingers (2nd/5th over 3rd/4th), and rocker-bottom feet.

    • A severe condition with high neonatal mortality rates

  • Patau Syndrome (Trisomy 13)

    • Severe structural anomalies: Cleft lip, polydactyly (extra number of digits), congenital heart disease, as well as holoprosencephaly (brain fails to divide) → leads to high rates of neonatal mortalities and stillbirths

  • Turner Syndrome (Monosomy X)

    • Females with short stature, webbed neck (cystic hygroma), shield chest with widely spaced nipples, absence of proper ovaries (streak ovaries => non-functional fibrous tissue), primary amenorrhea (no/late menstruation), and potential congenital heart defects (coarctation).

    • Mosaicism: Some patients may possess a mixture of normal (46 chromosomes) and Turner (45X) cells.

  • Klinefelter Syndrome (47 XXY)

    • Males exhibit symptoms such as tall stature, underdeveloped genitals (small and firm testes → can lead to low testosterone), and possible learning difficulties.

    • Often diagnosed incidentally during fertility evaluations due to azoospermia (lack of sperm in semen during ejaculation).

Chromosomal Analysis Techniques

  • Fluorescence In Situ Hybridization (FISH)

    • Utilized for identifying smaller deletions not detectable by standard karyotyping; applicable for conditions such as DiGeorge syndrome (22q11 deletion).

    • Involves attaching DNA probes to chromosomes to visualize specific genetic material.

    • Essential for diagnosing DiGeorge Syndrome (22q11.222q11.2 deletion), which presents with CATCH-22: Cardiac defects, Abnormal facies, Thymic hypoplasia, Cleft palate, and Hypocalcemia.

Important Considerations for Patients

  • Genetic counseling is vital for families with known chromosomal abnormalities to understand risks for future pregnancies.

  • Recognition of syndromic features is essential for timely diagnosis and management of affected children.

  • Healthcare providers should be aware of the developmental implications of diagnosed conditions, facilitating appropriate educational and medical support for affected individuals.