Session 5 Flashcards

Question

When is the fetal analomy scan performed?

Answer 1

18+0 and 20+6 weeks

Answer 2

In 1997, Lo et al showed that cffDNA from the Y chromosome of male fetuses was present in maternal blood

Answer 3

from 4-5 weeks but reliably from 7 weeks

Answer 4

cffDNA is rapidly cleared from circulation within an hour after delivery (mean half-life of 16-17 minutes)

Answer 5

Death of placental cells - breaking up the chromosomes into short fragments = average 166bp (size range is 200 – 300 bp)

Answer 6

- Low concentration - Need to use STECK tubes to stop nucleated cells that are collected releasing DNA into plasma at high levels - Much more maternal cfDNA - Half of the cffDNA is maternally inherited so hard to distinguish

Answer 7

1. Fetal sex determination in pregnancies at risk of a sex-linked disease. 2. Diagnosis of certain single gene disorder (bespoke NIPD) 3. Detection of fetal aneuploidy 4. Diagnosis of fetal blood type 5. Detection of the high risk allele using RHDO (relative haplotype dosage analysis)

Answer 8

cfDNA extracted from maternal plasma and RT-PCR be used to look for presence of SRY 1. Known X-linked condition (if female no invasive) 2. congenital adrenal hyperplasia (CAH) (if female give dexamethasone to avoid virilisation)

Answer 9

Presence of paternal specific SNPs Presence of universal fetal epigenetic markers (best are markers which can targeted by restriction enzyme to reudce maternal cfDNA levels)

Answer 10

Epigenetic (immunoprecipitation of fetal methylation pattern or restriction enzymes cutting of maternal) Size (<50% of aternal fragments are <300bp)

Answer 11

Plasma isolation of cfDNA Barcoding and multiplexing Sequencing Distribution of reads across chomosomes to look for aneuploidy compared to normal Can use fragment size to estimate fetal fraction and look for fetal specific markers too

Answer 12

SNPs that are heterozygous in Mum to determine whether fetus has inherited same X chromosome as reference sample Look at which SNPs are over represented to determine

Answer 13

Requires proband to be previous child of the couple Divides SNP genotypes in categories of usefulness: 1: Mum AA and Dad BB - used for fetal fraction 2: Mum AA and Dad AB - Fetal inheritance of paternal haplotype 3: Mum AB and Dad AA - Fetal inheritance of maternal haplotype Presence of category 3 paternal SNPs from mutated allele = inheritance of that allele by fetus More complex for the maternal= 1. If Mutant maternal allele SNPs matches paternal allele (alpha mat) then overrepresentation of category 4 maternal SNPs from mutated allele = inheritance of that allele by fetus 2. If normal maternal allele matches paternal allele (beta mat) then will be equal category 4 maternal SNPs from mutated allele = inheritance of that allele by fetus

Answer 14

Multiple pregnancy effects SNP alleles Low fetal fraction Consanguinity reduced number informative SNPs Only testing for haplotype not variant - risk of de novo

Answer 15

Paternal exclusion - requires parents to have different variants and then only excludes presence of paternal variant

Answer 16

FGFR2 and FGFR3 Based on specific ultrasound findings

Answer 17

During female embryo development, diploid primordial germ cells migrate into the embryonic ovary and undergo rapid proliferation (mitosis) – lasts from 15th week of gestation until 7th month – gives rise to ~7 million oogonia. Most oogonia die, but remaining (the primary oocytes) enter meiosis and pause at diplotene stage until puberty. Oocytes then begin to move through meiosis ready for ovulation. Meiosis 1 produces a secondary oocyte and a polar body (haploid nucleus but no cytoplasm - will die). Secondary oocytes then freeze at metaphase of meiosis 2 until fertilised.

Answer 18

Diploid primordial germ cells migrate into the embryonic testis and undergo to create spermatogonia Some stop proliferating and differentiate into primary spermatocytes ready for meoises. In meiosis I – division is symmetrical (generating identical diploid secondary spermatocytes); in meiosis II, four equally-sized haploid spermatids are produced (24 hours). Spermatids then differentiate into sperm (5 days)

Answer 19

8 weeks gestation

Answer 20

In the ampulla of the fallopian tube the sperm attaches to the outer layer of follicle cells of the oocyte and then reaches the zona pellucida (ZP). A cell-surface receptor on the sperm head binds to a ZP glycoprotein (PDILT gene). Acrosome (the head of the sperm) bursts – releasing enzymes that digest the ZP and allow sperm nucleus to entire oocyte. The cortical reaction occurs - cortical granules inside the oocyte fuse with the plasma membrane of the cell and enzymes inside expell to the zone pellucida – glycoproteins in the zona pellucida to crosslink and become impermable to sperm. Oocyte undergoes its second meiotic division Sperm’s tail and mitochondria degenerate Haploid oocyte and sperm nuclei form the pronuclei and undergo first mitotic division occurs i.e. the first mitosis of the union of sperm and oocyte is the actual fusion of their chromosomes Fertilised oocyte = totipotent zygote

Answer 21

The zona pellucida stops implantation until disregarded in the uterus Ectopic pregnancy

Answer 22

Cleavage: first plane is vertical; in the second round of division, one of the cells cleaves vertically and the other horizontally (rotational cleavage) - they can divide at different times to produce embryos with odd numbers of cells. Compaction: Loosely-associated blastomeres of the eight-cell embryo – flatten against each other and form a tightly-packed morula. Formation of a hollow ball of cell with a fluid filled cavity - the blastocele. Becomes the blastocyst - Outer layer of cells (trophoblast) – gives rise to the chorion (an extraembryonic membrane which provides the fetal component of the placenta). Inner cell mass (ICM) at one end of blastocele. The zona pellucida is degraded and the blastocyst implants into the uterus

Answer 23

Process where the orientation of the body is established; the embryo is converted from a bilaminar structure into a structure of three layers – ectoderm, endoderm and mesoderm

Answer 24

Endoderm = Lung, Liver, pancreas, thymus, endocrine glands Mesoderm = Blood, vessels, muscular tissue, connective tissue Ectoderm = Skin and its derivatives, nervous system

Answer 25

o Biopsy of the polar bodies just prior to conception (first polar body) and after fertilisation (second polar body). o Day 3 cleavage stage (5–8 cell embryonic stage) biopsy of 1-2 blastomeres. o Trophectoderm (blastocyst) biopsy is performed on day 5–6 (or rarely day 7) embryos that consist of ~120 cells; 5-10 cells are removed from the trophectoderm as it “hatches” through the zona pellucida. Blastocyst stage biopsy is advantageous for testing compared with biopsy at the cleavage stage because more cells are biopsied and tested

Answer 26

Somatic Microsatellite Mutations (SMMs) Polymorphic Submicroscopic Duplications/Submicroscopic microsatellite duplications (SMDs) Partial Chromosome Imbalance Copy Number Variant (CNV) Primer Binding Site Polymorphisms

Answer 27

nuchal oedema cardiac defects (mainly AVSDs) duodenal atresia (30% of cases have T21) echogenic bowel sandal gap clinodactyly of the fifth finger

Answer 28

Strawberry shaped head absent corpus callosum nuchal oedema heart defects horseshoe kidney diaphragmatic hernia exomphalos IUGR overlapping fingers and talipes or rocker bottom feet Decreased placental volume

Answer 29

Holoprosencephaly with associated facial abnormalities (cleft lip and palate) IUGR microcephaly Cardiac (mostly ASD or VSD) and renal abnormalities Often large and echogenic kidneys exomphalos

Answer 30

Lymphoedema renal anomalies

Answer 31

Severe IUGR Hydrops CNS abnormalities

Answer 32

Diploid (46 chromosomes) androgenetic pregnancy where both sets of chromosomes are paternally derived No formation of a fetus or embryo Presence of excess paternal material leads to trophoblastic hyperplasia with no formation of a fetus due to the absence of maternal chromosomal DNA Placenta has swollen villi Generally evacuated before 12 weeks gestation

Answer 33

Most common haploid sperm fertilises empty egg and doubles up to be 46,XX Rarer di-spermic fertilisation of empty egg

Answer 34

NLRP7 at 19q13.42 (OMIM: 609661) and KHDC3L at 6q13 (OMIM: 611687) Both thought to have roles in maintaining the maternal imprint within the ovum

Answer 35

Raised hCG

Answer 36

Invasive moles and Gestational choriocarcinomas

Answer 37

Does not detect balanced rearrangements Lower level mosaicism may be missed Triploidy will be missed or mis-classified Difficult interpretation of CNVs with limited phenotype info Incidental findings Postnatal findings (e.g. dev delay reduced penetrance)

Answer 38

Two eggs each fertilised by different sperm cells with their own placenta and sac (always dichorionic and diamniotic)

Answer 39

increased incidence of multiple follicle growth and multiple ovulation rate

Answer 40

Develop from same egg, which splits and forms two embryos

Answer 41

- 33% dichorionic/diamniotic (DC/DA)- division occurs during or before the morula stage (~ 4 days post fertilisation, 10 - 30 cells, before blastocyst stage) -66% monochorionic/diamniotic (MC/DA)- division occurs at the blastocyst stage (after morula stage, formation of the blastocoel cavity) involving division of the inner cell mass. - rare monochorionic/monoamniotic (MC/MA) embryos divide after amnion formed at day 9. These twins are occasionally conjoined

Answer 42

Affects 4-35% of MC/DA pregnancies Artery from one twin joined to the vein of the other allowing unidirectional blood flow from a donor twin to a recipient twin, resulting in asymmetrical fetal growth and fetal mortality in 80% or more of untreated cases.

Answer 43

In monochorionic placentas with artery-artery & vein-vein anastomoses one twin is acardiac & is perfused in a reverse direction by the 'pump' co-twin.

Answer 44

One twin suffers early fetal death and isn’t aborted - may be represented by areas of degenerate villi or amorphous debris in free membranes but is often difficult to detect CffDNA from vanished twin persists after its demise and can cause false positives for NIPD

Session 5 Flashcards

(68 cards)