Molecular Karyotyping: Genomic Techniques for Chromosome Analysis

Molecular Karyotyping

• Molecular karyotyping is the application of genomic techniques to assess the integrity and dosage of the karyotype genome-wide.
• Determining the most appropriate approach for diagnostic or research purposes is a rapidly evolving area.

Clinical Indications for Chromosome Analysis

• Problems of early growth and development
• Stillbirth and neonatal death
• Fertility problems
• Family history
• Neoplasia
• Pregnancy in a woman of advanced age

Tissue Sampling

• Skin biopsy: a minor surgical procedure that provides samples of tissue that can produce fibroblasts in culture.
• White blood cells can be transformed in culture to form lymphoblastoid cell lines that are potentially immortal.
• Bone marrow: obtained only by a relatively invasive procedure of marrow biopsy.
• Fetal cells: derived from amniotic fluid (amniocytes) or obtained by chorionic villus biopsy.

Chromosome Abnormalities

• Numerical or structural abnormalities: involve one or more autosomes, sex chromosomes, or both simultaneously.
• Aneuploidy: abnormal chromosome number due to an extra or missing chromosome, always associated with physical or mental maldevelopment or both.
• Reciprocal translocations: an exchange of segments between nonhomologous chromosomes, relatively common but usually have no phenotypic effect, may increase the risk of abnormal offspring.

Abnormalities of Chromosome Number

• Heteroploidy: a chromosome complement with any number other than 46.
• Euploidy: an exact multiple of the haploid chromosome number (N).
• Aneuploidy: any other chromosome number.
• Triploidy and Tetraploidy: 3N or 4N, e.g., 92,XXXX or 92,XXYY.

Triploidy and Tetraploidy

• Triploid infants can be liveborn but do not survive long.
• Triploidy is observed in 1% to 3% of recognized conceptions, often resulting from fertilization by two sperm (dispermy).

Aneuploidy

• Trisomy: three instead of the normal pair of a particular chromosome.
• Monosomy: only one representative of a particular chromosome.
• Aneuploidy affects 5% of all clinically recognized pregnancies.
• Trisomy 21 is the most common type of trisomy in liveborn infants.
• Karyotype: 47,XX or XY,+21.

Nondisjunction

• Failure of a pair of chromosomes to disjoin properly during meiosis or mitosis.
• Results in aneuploidy, such as trisomy 21.

Abnormalities of Chromosome Structure

• Less common than aneuploidy, occurring in 1 in 375 newborns.
• Chromosome rearrangement occurs spontaneously at a low frequency.
• May be induced by breaking agents such as ionizing radiation, viral infections, and chemicals.

Types of Abnormalities of Chromosome Structure

• Balanced: no genetic material is lost or gained.
• Unbalanced: genetic material is lost or gained, resulting in abnormality.
• Deletions: loss of a chromosome segment.
• Duplications: duplication of a chromosome segment.
• Marker chromosomes: very small, unidentified chromosomes.
• Ring chromosomes: formed when a chromosome undergoes two breaks and the broken ends reunite in a ring structure.
• Isochromosomes: a chromosome with one arm missing (partial monosomy) and the other duplicated in a mirror-image fashion (partial trisomy).
• Dicentric chromosomes: a rare type of abnormal chromosome formed by the fusion of two chromosome segments, each with a centromere.

Balanced Rearrangements

• Do not usually have a phenotypic effect if they are balanced.
• Inversions: a type of balanced rearrangement where a segment of a chromosome is reversed.
• Translocations: a type of balanced rearrangement where a segment of a chromosome is exchanged with a segment of another chromosome.
• Insertions: a type of balanced rearrangement where a segment of a chromosome is inserted into a nonhomologous chromosome.

Mosaicism

• The presence of two or more different chromosome complements in an individual.
• Can result from nondisjunction in an early postzygotic mitotic division.
• Mosaicism can be detected by conventional karyotyping, interphase FISH analysis, or array CGH.

Incidence of Chromosome Anomalies

• Three autosomal trisomies: trisomy 21, trisomy 18, and trisomy 13.
• Four types of sex chromosomal aneuploidy: Turner syndrome (usually 45,X), Klinefelter syndrome (47,XXY), 47,XYY, and 47,XXX.
• Triploidy and tetraploidy account for a small percentage of cases, particularly in spontaneous abortions.