Clinical Cytogenetics: Autosomal Chromosome Abnormalities – Study Notes

Clinical Cytogenetics: Autosomal Chromosome Abnormalities

Introduction to Chromosome Abnormalities

Chromosome abnormalities are a major cause of genetic diseases and syndromes, affecting both autosomes and sex chromosomes. They can involve changes in chromosome number or structure, and are a leading cause of pregnancy loss and developmental disorders.

• Prevalence: Only about 30% of human conceptions result in a live birth due to chromosomal abnormalities.

• Spontaneous Abortions: Chromosome abnormalities are seen in ~50% of first trimester and ~20% of second trimester spontaneous abortions.

• Live Births: Present in approximately 1 in every 150 live births.

• Clinical Impact: Leading cause of mental retardation and pregnancy loss.

Classification of Genetic Disorders

Genetic disorders are classified into three main categories:

• Single Gene Disorders: Caused by mutations in individual genes.

• Chromosome Disorders: Result from abnormalities in chromosome number or structure.

• Multifactorial Disorders: Involve multiple genes and environmental factors.

Methods for Identifying Chromosomes

Cytogenetic Technology & Nomenclature

Chromosomes are identified and analyzed using various cytogenetic techniques. The karyotype is a display of chromosomes arranged by length and centromere position.

• Chromosome Arms: The longer arm is designated as "q" and the shorter arm as "p".

• Centromere Position: Chromosomes are classified as metacentric, submetacentric, or acrocentric based on centromere location.

Types of Chromosomes

• Metacentric: Centromere is in the middle; arms are of equal length.

• Submetacentric: Centromere is off-center; arms are unequal.

• Acrocentric: Centromere is near one end; one very short arm (chromosomes 13, 14, 15, 21, 22).

Techniques for Identifying Individual Chromosomes

• Giemsa (G) Banding: Chromosomes are treated with trypsin and stained with Giemsa. Heterochromatic regions (AT-rich, gene-poor) stain darkly; euchromatic regions (GC-rich, gene-rich) stain lightly.

• FISH (Fluorescence In Situ Hybridization): Uses fluorescent probes to detect specific DNA sequences on chromosomes, allowing identification of structural abnormalities and aneuploidies.

Method for Identifying Chromosomes

Standard laboratory procedures involve cell culture, harvesting, fixation, and staining to prepare chromosomes for analysis.

Chromosome Nomenclature

Standard Nomenclature for Chromosomal Karyotypes

Chromosomal abnormalities are described using a standardized nomenclature indicating chromosome number, sex chromosomes, and specific structural changes.

Abbreviation	Meaning	Example
XX	Female	46,XX
XY	Male	46,XY
del	Deletion	46,XY,del(4)(p14)
dup	Duplication	46,XX,dup(5p)
t	Translocation	46,XY,t(11;22)(q23;q22)
+21	Trisomy 21	47,XX,+21
mosaic	Mosaicism	47,XY,+21/46,XY

Types of Autosomal Chromosome Abnormalities

Numerical Chromosome Abnormalities

Numerical abnormalities involve changes in the number of chromosomes and are classified as polyploidy, aneuploidy, or mixoploidy.

• Polyploidy: Any multiple of the basic haploid chromosome number other than diploid (e.g., 3n, 4n).

• Aneuploidy: Chromosome number is not an exact multiple of the haploid number. Includes monosomy (one copy) and trisomy (three copies).

• Mixoploidy: Presence of two or more genetically different cell lines (mosaicism or chimerism).

Aneuploidy

• Monosomy: Only one copy of a particular chromosome. Lethal for autosomes; compatible with life for some sex chromosome abnormalities.

• Trisomy: Three copies of a particular chromosome. Only trisomies 13, 18, and 21 are compatible with survival until birth.

Mechanisms of Aneuploidy

• Nondisjunction: Failure of paired chromosomes or sister chromatids to separate during cell division, leading to aneuploid cells.

• Anaphase Lag: Chromosome or chromatid fails to be incorporated into a daughter nucleus, resulting in loss.

Nearly all autosomal trisomies increase with maternal age due to increased nondisjunction in older oocytes.

Incidence of Chromosome Abnormalities

Stage	Total Incidence	Numerical (%)	Structural: Balanced (%)	Structural: Unbalanced (%)
First-Trimester Abortuses	1/2	96	0	4
Fetuses of Mothers >35 yrs	1/50	85	10	5
Live Births	1/160	60	30	10

Common Autosomal Aneuploidies

Trisomy 21 (Down Syndrome)

Down syndrome is the most common autosomal aneuploidy among live births, with a prevalence of 1/800. Most cases are not inherited and result from nondisjunction during maternal meiosis.

• Karyotype: 47,XY,+21 or 47,XX,+21

• Physical Signs: Flattened nose, upward slanting eyes, small mouth, wide short hands, single palmar crease, white spots on iris.

• Clinical Problems: Heart defects (40%), intellectual disability, visual/hearing problems, increased risk of leukemia, early-onset Alzheimer disease, cervical spine instability.

• Genetic Mechanism: Extra chromosome 21 is contributed by the mother in ~95% of cases (75% meiosis I, 25% meiosis II).

• Mosaicism: Seen in ~24% of live births; some cells are normal, others have trisomy 21.

• Gene Dosage Hypothesis: Clinical symptoms are due to overexpression of genes on the extra chromosome 21.

Trisomy 13 (Patau Syndrome)

• Incidence: 1/10,000 live births.

• Clinical Features: Congenital heart defects (80%), dextrocardia, omphalocele, holoprosencephaly, feeding difficulties, seizures, kidney defects, developmental disability.

• Prognosis: Most affected babies die within the first year; survivors are often mosaic.

Trisomy 18 (Edwards Syndrome)

• Incidence: 1/6,000 live births.

• Clinical Features: Growth retardation, multiple congenital anomalies, high mortality in embryonic/fetal life.

• Risk Factors: Advanced maternal age; 90% due to maternal nondisjunction.

Mixoploidy

• Mosaicism: Individual has two or more genetically different cell lines from a single zygote.

• Chimerism: Individual has cell lines from different zygotes.

• Clinical Impact: Abnormalities lethal in all cells may be compatible with life in mosaic or chimeric individuals.

Abnormalities in Chromosome Structure

Structural Rearrangements

Structural abnormalities occur due to improper alignment during meiosis or errors in DNA repair, leading to rearrangement of chromosome material.

• Balanced Rearrangements: No net gain or loss of genetic material; includes inversions and translocations. May not affect the individual but can cause disease in offspring.

• Unbalanced Rearrangements: Gain or loss of genetic material; includes duplications, deletions, isochromosomes, ring chromosomes, dicentric chromosomes. Can cause serious disease.

Types of Translocations

• Reciprocal Translocation: Exchange of segments between non-homologous chromosomes.

• Robertsonian Translocation: Fusion of two acrocentric chromosomes near the centromere, with loss of short arms. Involves chromosomes 13, 14, 15, 21, 22. Can cause Down syndrome.

Other Structural Abnormalities

• Deletions: Loss of chromosome segments.

• Duplications: Gain of chromosome segments.

• Isochromosomes: Chromosome with identical arms.

• Ring Chromosomes: Chromosome forms a ring due to deletions at both ends.

• Dicentric Chromosomes: Chromosome with two centromeres.

Frequency of Major Chromosomal Abnormalities Among Newborns

Abnormality	Prevalence at Birth
Trisomy 21	1/800
Trisomy 18	1/6,000
Trisomy 13	1/10,000
Unbalanced Rearrangement	1/230
Balanced Rearrangement	1/500
Robertsonian Translocation	1/1,000
Reciprocal Translocation	1/11,000

Generalizations About Chromosomal Abnormalities

• Developmental Delay: Most chromosome abnormalities are associated with developmental delay and intellectual disability.

• Facial Features: Many syndromes involve characteristic alterations of facial features.

• Growth Delay: Common in autosomal chromosome abnormalities.

• Congenital Malformations: Especially congenital heart defects, occur with increased frequency.

Gene Dosage and Genomic Disorders

Altered gene dosage due to duplication or deletion leads to clinical abnormalities reflecting the altered amount of gene product. These syndromes are often located in regions of segmental duplications.

Summary Table: Chromosome Abnormalities in Spontaneous Abortions

Type	Frequency (%)
Autosomal trisomies	50-55
Triploidy	15
Tetraploidy	5
Monosomy X	20
Unbalanced structural rearrangements	2
Other (monosomies, sex chromosomal trisomies, etc.)	3-8

Example: Karyotype Nomenclature

• 46,XY: Normal male

• 47,XX,+21: Female with trisomy 21

• 47,XY,+21/46,XY: Male mosaic for trisomy 21 and normal cells

• 46,XY,del(4)(p14): Male with deletion of short arm of chromosome 4

• 46,XX,dup(5p): Female with duplication of short arm of chromosome 5

• 46,XY,t(11;22)(q23;q22): Male with balanced reciprocal translocation between chromosomes 11 and 22

Additional info: These notes expand on the original slides by providing definitions, explanations, and context for key terms and concepts, as well as summarizing the main tables and figures for clarity and completeness.