Chromosomal disease is based in three abnormalities-All result in abnormal expression of genes, which is the basis for the diseased state.
· Extra Genetic material
· Missing Genetic Material
· Rearranged Gentic Material
Genetic and Chromosomal analysis should not be a first line test. Other, non-chromosomal pathologies should first be explored, then if no other pathology can explain the presenting symptoms and tests, genetic analysis should be used in the following situations
· Problems in early growth and development
o Short stature
o Developmental delays
o Malformation
· Stillbirth and neonatal death
· Fertility Issues
· The following should be tested NOT as a last resort
o Family History
o Neoplasia
o Pregnancy in women older than 35
§ Higher incidence of non-disjunction
Chromosome structure
· consist of DNA, Histone Proteins and Non Histone Proteins
o Histones are responsible for chromosomal compaction
§ Produced in the cytoplasm and transported into the nucleus
o Non Histone proteins are NUMEROUS (in the hundreds)
§ Enzymes
§ Gene regulators
§ Structural proteins
o Length of DNA in a haploid cell is ~3.28 ft
Chromosome Nomenclature
· Kinetochore—Place of attachment of kinetochore microtubules. The kinetochore is made of protein only.
· Short Arm—Shorter of the two arms. Is designated by the letter p
· Long Arm—Longer of the two arms. It is designated by the letter q
· Centromere—Nucleoprotien complex that adjoins the two sister chromatids and separates the long and short arm. Centromere location is used as a classfication and description of the chromosome
o Metacentric—The centromere is located centrally, equidistant from both telomeres
o Submetacentric-_The centromere is located closer to one end than the other. This shows a clear distinction between long and short arm.
o Acrocentric—The centromere is almost at the very end of the chromosome. There are sometimes secondary constrictions and “satellites”
Chromosome Preparation—blood preparation
· Separate white cells off of a 5 ml sample an incubate with phytohemaglutinin for 3 days at 37 C.
· Add cochecine and then add to a hypotonic solution. The cells will swell. Drop the cell suspension from a specific height onto a glass slide.
· Air Dry, stain and photograph.
Karyotypeing
· Number of chromosomes is reported first and then sex chromosomes (46, XY is a human male)
· Arrangment is based on size and centromere location
o The largest are first with the smallest last
§ Exception 21 is smaller than 22
§ Sex chromosomes can be placed near the group they resemble most or at the bottom.
o Arrangment is into 7 groups
- Chromosomes 1, 2, and 3
- Largest of all
- Metacentric centromere
- Chromosomes 4 and 5
- Large
- Submetacentric
- Chromosomes 6-12
- Medium
- Submetacentric
- Chromosomes 13, 14, and 15
- Medium
- Acrocentric with satellites
- Chromosomes 16, 17, and 18
- Relativly Short
- Metacentric or Submetacentric
- Chromosomes 19 and 20
- Short
- Metacentric
- Chromosomes 21 and 22
- Short
- Acrocentric
Chromosome Banding
· Nomenclature
- Chromosome number
- Chromosome arm
- Region
- Band
- Sub-band (following a decimal)
EXAMPLE 1p23.3 is on the short arm of chromosome 1 in region 2 band 3.3
· Methodology
o Q-Band
§ Flurorescent
§ Shows AT rich areas
§ Heterochromatin is shown more
o G-Band
§ Stains darker in AT rich areas
§ Dark G bands correlate to bright Q bands
§ Heterochromatin is shown more
o R-Band
§ Reverse Stained
§ Stains GC rich bands
§ Dark R bands correlate to light G bands & dim Q bands
§ Euchromatin is shown more
o T-Band
§ Telomere Bands
§ Stains the telomeres
o C-Band
§ Centromere bands
§ Stains the Centromere
o NOR
§ Nucleolus organizing Region
§ Acrocentric chromosomes in humans
Chromosomal Abnormalities
· Ploidy
o Extra SETS of chromosomes
§ Exemption-Aneuploidy is another word for a –somy
o Caused by incomplete meiosis in the egg or by polyspermy
o Examples
§ Tetraploidy
§ Triploidy
ú Spontaneous abortion
ú Syndactyly
ú Large head compared to torso
ú Multiple congenital abnormalities
· Somy
o Extra or missing chromosomes
o Causes
§ Non disjunction events during gametogenesis
§ Mosaiscism-Non Disjunction during early zygote mitosis
o Examples
§ Trisomy 21 (47, XX, +21 or 47, XY +21)-Down Syndrome
ú 85% maternally derived
§ Trisomy 18 (47, XX, +18 or 47, XY, +18)
§ Turner Syndrome (45, X)
§ Kleinfelter Syndrome (47, XXY)
§ Trisomy X (47, XXX)
· Structural Aberrations
o Structural abberations are a result of chromosomal breakage and mis-repair.
o Breakage causes formation of “sticky ends” which can repair onto an inappropriate location
§ Different strands
§ Cyclic strands
§ Multiple centromeres
o Breakage can be caused by
§ Ionizing radiation
§ X-rays
§ Mutagens
§ Inherited conditions