Genetics · MU Sofia

43. Chromosomal Diseases Associated with Structural Autosomal Aberrations

Covers structural autosomal aberrations, starting with how breakage and rearrangement of chromosome segments cause disease through gene loss, dosage excess or disrupted regulation. It then goes through deletions with cri-du-chat and Wolf–Hirschhorn syndromes, microdeletion syndromes including DiGeorge and Williams, duplications, inversions, reciprocal and Robertsonian translocations, ring chromosomes and isochromosomes. A classification table compares each aberration's effect and whether it is balanced.

Inside: 11 sections · 12 labelled pictures · 1 table · 13 flashcards · 5 min

Topic 43 of 65 in the MU Sofia syllabus for Genetics (Medicine, Year 3). The free AceMed note covers it in 11 sections, about 621 words (5 min of reading), with 13 flashcards.

In this note

  1. Chromosomal Diseases Associated with Structural Autosomal Aberrations
  2. General Mechanisms of Structural Aberrations
  3. General Pathogenesis
  4. Classification of Structural Autosomal Aberrations
  5. I. CHROMOSOMAL DELETIONS
  6. II. MICRODELETION SYNDROMES
  7. III. CHROMOSOMAL DUPLICATIONS
  8. IV. INVERSIONS
  9. V. TRANSLOCATIONS
  10. VI. RING AUTOSOMES
  11. VII. ISOCHROMOSOMES (AUTOSOMAL)

Chromosomal Diseases Associated with Structural Autosomal Aberrations

Structural autosomal aberrations are chromosomal disorders caused by breakage and abnormal rearrangement of chromosome segments. In these conditions, the total chromosome number is usually normal (46), but the structure of one or more autosomes is altered, leading to loss, gain, or misplacement of genetic material.

These abnormalities cause disease mainly through:

  • gene loss (haploinsufficiency),
  • gene dosage excess,
  • disruption of gene structure or regulation.

General Mechanisms of Structural Aberrations

Structural aberrations arise due to:

  • errors in meiotic recombination,
  • faulty DNA repair after double-strand breaks,
  • abnormal chromosomal segregation.

General Pathogenesis

  1. 1. Chromosome breakage occurs at specific sites.
  2. 2. Incorrect rejoining of fragments alters chromosome structure.
  3. 3. Essential genes may be deleted, duplicated, or disrupted.
  4. 4. Gene dosage imbalance interferes with normal development.
  5. 5. Balanced carriers are often phenotypically normal but produce abnormal gametes.

I. CHROMOSOMAL DELETIONS

A chromosomal deletion is the loss of a segment of an autosome, leading to partial monosomy. Deletions are always pathogenic because genetic material is lost.

Types of Deletions

  • Terminal deletion – loss of the end of a chromosome
  • Interstitial deletion – loss of an internal segment

Pathogenesis of Deletions

  1. 1. Loss of one copy of multiple genes.
  2. 2. Reduced gene dosage → haploinsufficiency.
  3. 3. Disruption of developmental pathways.
  4. 4. Severity depends on size and gene content of deleted region.

Cri-du-chat Syndrome (5p deletion)

  • Chromosome: 5p15
  • Main mechanism: deletion of genes involved in brain and laryngeal development

Clinical features:

  • High-pitched, cat-like cry
  • Microcephaly
  • Severe intellectual disability
  • Hypotonia
  • Facial dysmorphism (round face, hypertelorism)

Wolf–Hirschhorn Syndrome (4p deletion)

  • Chromosome: 4p16.3
  • Main mechanism: loss of genes critical for craniofacial and brain development

Clinical features:

  • “Greek warrior helmet” face
  • Severe growth retardation
  • Seizures
  • Congenital heart defects
  • Profound intellectual disability

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