clinical cytogenetics Flashcards
what are the subtelomeric regions?
gene rich areas just proximal to the telomere
define metacentric/sub-metacentric/acrocentric
metacentric- centromere in the middle
sub-metacentric- centromere closer to one end
acrocentric- centromere at the end
when is the best time in the “cell cycle” to view and analyze chromosomes?
better when chromosomes are longer and less condensed (like mitotic cells)
what are some prenatal reasons to perform cytogenetics? (4)
risk for aneuploidy
abnormal US
FHx of chromosomal anomaly
advanced maternal age
what are some reasons to perform postal natal cytogenetics? (6)
multiple congenital anomalies MR/growth delay ambiguous genitalia infertility/SAbs leukemia/solid tumor FHx of chromosomal anomaly
what are two tissue types that are appropriate for chromosome analysis?
lymphocytes and fibroblasts
need dividing cells
from where are lymphocytes derived?
peripheral blood, cord blood, bone marrow
place in sodium heparin preservative
from where are fibroblasts derived?
solid tissue bx, amniotic fluid, chorionic villi
place in sterile saline media
what are the steps in chromosome analysis? (7)
- obtain sample
- grow cells in media (time varies)
- harvest cells
- make/band slides
- scan slides to look for cells in metaphase
- analyze/capture/karyotype cells
- review/sign out results
what is a normal female? normal male?
female 46, XX
male 46, XY
what is aneuploidy?
having a chromosome number that is not equal to a multiple of haploid number (n = 23)
how is trisomy 21 denoted? monosomy 22?
47, XX, +21 (most common aneuploidy)
45, XY, -22
what are the 2 mechanisms of triploidy?
dispermy, disomic egg
describe dispermy
normal egg fertilized by 2 sperm
most lost at 10-20 weeks after conception
abnormal placenta with growth-retarded fetus
describe disomic egg
disomic egg had error in meiosis II or retention of polar body
severely growth retarded fetus, small placenta, large head
some survive to birth
what is mosaicism?
presence of 2 or more cell lines in an individual or tissue sample
what yield a larger abnormality with mosaicism?
error that occurs earlier in fetal life
when do oogonia begin to develop in fetal life?
12th week
when does oogonia development arrest?
arrests in prophase I around 20 weeks (dichtyotene stage)
why is it proposed that more non-disjunction events occur in females with advanced maternal age?
because of oogenesis as an embryo vs. males who continually produce sperm throughout life
what is nondisjunction?
improper separation of chromosomes during meiosis, can happen in meiosis I or II
what happens when nondisjunction occurs in meiosis I?
homologs do not separate
what happens when nondisjunction occurs in meiosis I?
sister chromatids fail to separate
what is the m/c cause of trisomy 21?
maternal meiosis I erros
what are all the possible causes of trisomy 21?
maternal meiosis errors (I/II)- 85-90%
paternal meiosis erros (I/II)- 3-5%
mitotic errors (mosaicism)- 3-5%
robertsonian translocation/other - 5%
does the presence on mosaic T21 help to predict severity?
No
where is the translocation tha causes T21?
robertsonian translocation of chromosome 21 to chromosome 14, unbalanced
46, XY, der(14;21)(q10;q10),+21
what is T18?
Edward’s syndrome
fist clenching, rocker bottom feet, prominent occiput, low set ears
was is T13?
Patau syndrome
midline defects, clefting, polydactyly
what is turner syndrome?
45,X
can be mosaic with 46,XX or 46,XY cells
what is Kleinfelter Syndrome?
47, XXY males delayed puberty mall stature, long legs gynceomastia decreased IQ
what is 47,XXX?
females
reduced IQ, learning disabilities, increased vulnerability to stress
what is 47,XYY?
males
tall stature
?behavioral problems, decreased IQ
what is the SYR?
sex-determining region ont he Y chromosome
conserved sequences is DNA binding domain that is believed to be a transcription factor
found in all mammals
what is the germinal ridge?
testes determining factor
which chromosomes can be involved with robertsonian translocations?
acrocentric chromosomes
13, 14, 15, 21, 22
what defines “robertsonian translocation”?
involves the fusion of 2 long arms and loss of the short arms, if the short arms contain only repetitive DNA, can have a balances translocation and normal phenotype
what can robertsonian translocations increase the risk of?
infertility or trisomic conception (esp females)
contrast balanced vs. unbalanced translocations
balanced- no material lost/gained, just rearranged- normal phenotype
unbalanced- abnormal phenotype
what is the risk of mental or physical abnormalities in people with inherited translocations vs de novo?
inherited ~2%
de novo 10-15%
can lead to increased pregnancy loss/infertility
define terminal vs interstitial deletion
loss of material
terminal- one break, at the end
interstitial- 2 breaks
define paracentric vs. pericentric inversions
paracentric- avoid the centromere (same arm)
pericentric- include the centromere (both arms)
what is FISH?
florescence in situ hybridization
detects chromosomal abnormalities in interphase cells
various probes
what are the uses of different FISH probes?
microdeletion syndromes- unique probes
centromere- repetitive probes
libraries- painting probes
what are the pros of FISH? (3)
- used in non-dividing cells (like SAb)
- detect subtle rearrangements
- quick turnaround
what are the cons of FISH? (3)
- must know target
- may not reliably detect tandem duplications
- may not detect mosaicism reliably
what are the 2 types of microarray analysis?
Comparative genetic hybridization (CGH)
Single nucleotide polymorphism (SNP)
what is the basis of CGH?
patient and control DNA are labelled with different colors and analyzed for gains/losses
what are the minimum sizes of gains/losses that can be detected by CGH?
gains- 500 kb
losses- 200 kb
what is the basis of SNP?
patient DNA is hybridized to a slide with DNA probes that are differentially labelled according to whether they are AA, AB, BB at many SNPs across the genome; computer calculates if there is gain/loss/loss of heterozygosity
how is SNP analysis interpreted?
deletion- decreased intensity
gain- increased intensity
ROH- neutral intensity
what are the pros of chromosome analysis? (4)
- view of entire genome
- detect balanced rearrangements
- detect polyploidy
- provide insight to etiology
what are the cons of chromosome analysis? (3)
- must have actively dividing cells
- lower resolution of detection
- longer turnaround time
