clinical cytogenetics

Question 1

what are the subtelomeric regions?

Answer 1

gene rich areas just proximal to the telomere

Question 2

define metacentric/sub-metacentric/acrocentric

Answer 2

metacentric- centromere in the middle
sub-metacentric- centromere closer to one end
acrocentric- centromere at the end

Question 3

when is the best time in the “cell cycle” to view and analyze chromosomes?

Answer 3

better when chromosomes are longer and less condensed (like mitotic cells)

Question 4

what are some prenatal reasons to perform cytogenetics? (4)

Answer 4

risk for aneuploidy
abnormal US
FHx of chromosomal anomaly
advanced maternal age

Question 5

what are some reasons to perform postal natal cytogenetics? (6)

Answer 5

multiple congenital anomalies 
MR/growth delay
ambiguous genitalia
infertility/SAbs
leukemia/solid tumor 
FHx of chromosomal anomaly

Question 6

what are two tissue types that are appropriate for chromosome analysis?

Answer 6

lymphocytes and fibroblasts

need dividing cells

Question 7

from where are lymphocytes derived?

Answer 7

peripheral blood, cord blood, bone marrow

place in sodium heparin preservative

Question 8

from where are fibroblasts derived?

Answer 8

solid tissue bx, amniotic fluid, chorionic villi

place in sterile saline media

Question 9

what are the steps in chromosome analysis? (7)

Answer 9

• 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

Question 10

what is a normal female? normal male?

Answer 10

female 46, XX

male 46, XY

Question 11

what is aneuploidy?

Answer 11

having a chromosome number that is not equal to a multiple of haploid number (n = 23)

Question 12

how is trisomy 21 denoted? monosomy 22?

Answer 12

47, XX, +21 (most common aneuploidy)

45, XY, -22

Question 13

what are the 2 mechanisms of triploidy?

Answer 13

dispermy, disomic egg

Question 14

describe dispermy

Answer 14

normal egg fertilized by 2 sperm
most lost at 10-20 weeks after conception
abnormal placenta with growth-retarded fetus

Question 15

describe disomic egg

Answer 15

disomic egg had error in meiosis II or retention of polar body
severely growth retarded fetus, small placenta, large head
some survive to birth

Question 16

what is mosaicism?

Answer 16

presence of 2 or more cell lines in an individual or tissue sample

Question 17

what yield a larger abnormality with mosaicism?

Answer 17

error that occurs earlier in fetal life

Question 18

when do oogonia begin to develop in fetal life?

Answer 18

12th week

Question 19

when does oogonia development arrest?

Answer 19

arrests in prophase I around 20 weeks (dichtyotene stage)

Question 20

why is it proposed that more non-disjunction events occur in females with advanced maternal age?

Answer 20

because of oogenesis as an embryo vs. males who continually produce sperm throughout life

Question 21

what is nondisjunction?

Answer 21

improper separation of chromosomes during meiosis, can happen in meiosis I or II

Question 22

what happens when nondisjunction occurs in meiosis I?

Answer 22

homologs do not separate

Question 23

what happens when nondisjunction occurs in meiosis I?

Answer 23

sister chromatids fail to separate

Question 24

what is the m/c cause of trisomy 21?

Answer 24

maternal meiosis I erros

Question 25

what are all the possible causes of trisomy 21?

Answer 25

maternal meiosis errors (I/II)- 85-90% paternal meiosis erros (I/II)- 3-5% mitotic errors (mosaicism)- 3-5% robertsonian translocation/other - 5%

Question 26

does the presence on mosaic T21 help to predict severity?

Answer 26

No

Question 27

where is the translocation tha causes T21?

Answer 27

robertsonian translocation of chromosome 21 to chromosome 14, unbalanced 46, XY, der(14;21)(q10;q10),+21

Question 28

what is T18?

Answer 28

Edward's syndrome | fist clenching, rocker bottom feet, prominent occiput, low set ears

Question 29

was is T13?

Answer 29

Patau syndrome | midline defects, clefting, polydactyly

Question 30

what is turner syndrome?

Answer 30

45,X | can be mosaic with 46,XX or 46,XY cells

Question 31

what is Kleinfelter Syndrome?

Answer 31

``` 47, XXY males delayed puberty mall stature, long legs gynceomastia decreased IQ ```

Question 32

what is 47,XXX?

Answer 32

females | reduced IQ, learning disabilities, increased vulnerability to stress

Question 33

what is 47,XYY?

Answer 33

males tall stature ?behavioral problems, decreased IQ

Question 34

what is the SYR?

Answer 34

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

Question 35

what is the germinal ridge?

Answer 35

testes determining factor

Question 36

which chromosomes can be involved with robertsonian translocations?

Answer 36

acrocentric chromosomes | 13, 14, 15, 21, 22

Question 37

what defines "robertsonian translocation"?

Answer 37

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

Question 38

what can robertsonian translocations increase the risk of?

Answer 38

infertility or trisomic conception (esp females)

Question 39

contrast balanced vs. unbalanced translocations

Answer 39

balanced- no material lost/gained, just rearranged- normal phenotype unbalanced- abnormal phenotype

Question 40

what is the risk of mental or physical abnormalities in people with inherited translocations vs de novo?

Answer 40

inherited ~2% de novo 10-15% can lead to increased pregnancy loss/infertility

Question 41

define terminal vs interstitial deletion

Answer 41

loss of material terminal- one break, at the end interstitial- 2 breaks

Question 42

define paracentric vs. pericentric inversions

Answer 42

paracentric- avoid the centromere (same arm) | pericentric- include the centromere (both arms)

Question 43

what is FISH?

Answer 43

florescence in situ hybridization detects chromosomal abnormalities in interphase cells various probes

Question 44

what are the uses of different FISH probes?

Answer 44

microdeletion syndromes- unique probes centromere- repetitive probes libraries- painting probes

Question 45

what are the pros of FISH? (3)

Answer 45

- used in non-dividing cells (like SAb) - detect subtle rearrangements - quick turnaround

Question 46

what are the cons of FISH? (3)

Answer 46

- must know target - may not reliably detect tandem duplications - may not detect mosaicism reliably

Question 47

what are the 2 types of microarray analysis?

Answer 47

Comparative genetic hybridization (CGH) | Single nucleotide polymorphism (SNP)

Question 48

what is the basis of CGH?

Answer 48

patient and control DNA are labelled with different colors and analyzed for gains/losses

Question 49

what are the minimum sizes of gains/losses that can be detected by CGH?

Answer 49

gains- 500 kb | losses- 200 kb

Question 50

what is the basis of SNP?

Answer 50

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

Question 51

how is SNP analysis interpreted?

Answer 51

deletion- decreased intensity gain- increased intensity ROH- neutral intensity

Question 52

what are the pros of chromosome analysis? (4)

Answer 52

- view of entire genome - detect balanced rearrangements - detect polyploidy - provide insight to etiology

Question 53

what are the cons of chromosome analysis? (3)

Answer 53

- must have actively dividing cells - lower resolution of detection - longer turnaround time