chromosomes Abnormalities

Chapter Overview

  • Content relevant to chapters five and six.
  • Agenda for the session includes:
    • Categorizing chromosome abnormalities.
    • Discussing chromosomal microarray diagnostics.
    • Exploring deletions and duplications, particularly recurrent ones through non-allelic homologous recombination.
    • Addressing structural disorders of chromosomes.

Chromosome Abnormalities

  • Structural Abnormalities

    • Focus on green bars representing structural abnormalities of chromosomes.
    • Dark green bar statistics:
    • Approximately 1 in 375 newborns (from a sample of over 68,000) have a structural chromosome abnormality.
    • Majority are balanced forms of structural abnormalities (all correct amounts of genetic material but with changes in structure).
    • About 1 in 1,600 newborns have an unbalanced structural chromosome abnormality (involves deletions and duplications).

  • Balanced vs. Unbalanced Abnormalities

    • Balanced Abnormalities:
    • Imply correct amounts of genetic material, but with possible location changes.
    • Generally not as concerning for health outcomes.
    • Unbalanced Abnormalities:
    • Involves missing or additional copies of genetic material (deletions or duplications).
    • More likely to result in health issues due to the dosage effect of having the wrong number of gene copies.

Chromosomal Microarray Diagnostics

  • Overview of Chromosomal Microarray

    • A first-line genetic test for diagnosing deletions and duplications.
    • Expected to potentially be replaced by whole genome sequencing in the future.
    • Not ideal for single-gene conditions but excellent for assessing overall genetic information.

  • Mechanism of Action

    • Utilizes a glass slide with DNA probes in a grid format.
    • Patient and control DNA hybridize – competition for binding sites allows for detection of copy number variants.
    • If patient DNA has deletions or duplications, the binding will differ from controls.

  • Single Nucleotide Polymorphism (SNP) Arrays

    • Inclusion of common SNP loci provides an additional layer of information.
    • Comparison of SNP distribution can indicate uniparental disomy or suggest consanguinity (relatedness of parents).
    • Consanguinity identified by reduced heterozygosity across chromosomes.

Miscarriage Rates and Genetic Variants

  • Statistical Overview

    • Majority of balanced rearrangements have a <16% miscarriage rate.
    • Higher miscarriage rates associated with unbalanced structural chromosome abnormalities due to gene dosage issues.

  • Mechanisms of Changes

    • Previous lectures covered aneuploidy and polyploidy; current focus is on structural rearrangements.
    • Example of a duplication demonstrates structural anomalies (e.g., adjacent sequences on a chromosome).

Chromosome Locations and Definitions

  • Chromosome Notation

    • Defined using chromosome number, arm (p or q), region, and band.
    • Example: 1q24 indicates chromosome 1, long arm (q), region 2, band 4.
    • Importance of clear nomenclature for genetic communication in medicine and genetics.

  • Karyotype Overview

    • Total of 23 chromosome pairs showcased, with emphasis on acrocentric chromosomes (13, 14, 15, 21, 22) having unique banding patterns.

Technology and Techniques in Genetics

  • FISH (Fluorescence In Situ Hybridization)

    • Can visualize specific chromosomal alterations and abnormalities, such as deletions.

  • Clinical Implications of Microarray Testing

    • Works best to reveal genetic differences across the genome rather than just specific mutations.
    • Outputs provide clear karyotypic visualizations for further analysis.

Types of Chromosome Aberrations

  • Deletions vs. Duplications

    • Missing sections in deletions commonly lead to more severe phenotypes than duplications.
    • Examples such as Charcot-Marie-Tooth disease (duplication) and Cri-du-chat syndrome (deletion).

  • Inheritance Patterns

    • Many deletion and duplication syndromes often show autosomal dominant inheritance.

Mechanism of Recurrent Copy Number Variants

  • Recombination Mechanisms

    • Segmental duplications can lead to misalignment and slippage during recombination.
    • Mechanical breakdowns during meiosis or through specific chromosome features can lead to recurrent abnormalities.

  • Examples of Recurrent Variants & Syndromes

    • Specific chromosome regions known to frequently exhibit deletions or duplications due to structural characteristics.

Structural Rearrangements: Inversions and Translocations

  • Inversions

    • Require 3 breaks in DNA.
    • Generally well-tolerated with clear mechanisms for inheritance.
    • May be paracentric (without centromere) or pericentric (includes centromere).

  • Reciprocal Translocations

    • Balanced usually tolerated well without issues, but problems arise when gametes formed from a carrier.
    • Important to monitor balanced individuals who may unknowingly carry risks of passing on unbalanced chromosomes to offspring.
    • Example: Philadelphia chromosome associated with leukemia.

Case Studies and Examples

  • Cri-du-chat Syndrome

    • Characterized by distinct physical features, identifiable early by unique crying pattern.
    • Linked to chromosomal deletion on 5p.

  • Visualizing Chromosomal Differences

    • Discussion of karyotypes and identifying common chromosomal alterations, such as the Philadelphia chromosome.

Conclusion

  • Reviewed mechanisms and implications of various chromosomal abnormalities.
  • Importance of understanding genetic backgrounds in relation to clinical diagnostics and associated conditions.