Prenatal screening for genetic disorders


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Prenatal screening for genetic disorders

  • Prenatal screening for genetic disorders

    • FTS/IPS/MSS
    • Non-invasive prenatal testing (NIPT)
    • Use of microarray for prenatal diagnosis
  • Ethnicity based screening

  • Consanguinity and genetics



Prenatal Screening tests for: DS, T18, T13 and ONTD



Factors affecting FTS/IPS screening performance…and why you need to record them on the requisition

  • Gestational dating

    • FPR lowered by ~2% when GA estimated using scan
  • Maternal weight

    • Negative association between levels of maternal serum markers and maternal weight due to dilution effect produced by increased blood volume
    • Weight adjustment
      • increases DR by ~1% for a given FPR
      • reduces FPR by 0.2% for given DR
  • Ethnic origin

    • Adjusting tends to equalize the FPR among women of different ethnic groups


Factors affecting FTS/IPS screening performance…and why you need to record them on the requisition

  • Insulin-dependent diabetes mellitus

    • Some T2 markers are lower
  • Smoking

    • Affects risk calculations – indicate if any smoking during pregnancy (even at time of conception)
  • Assisted Reproduction

    • Maternal age used is age of donor or woman at time egg was harvested
    • IVF pregnancies have higher risk of false positive screen – correction factor used – so important to indicate


Prenatal Genetic Screening

  • Maternal age

    • Practice of using solely maternal age at EDD to identify at-risk pregnancies should be abandoned
      • DR for DS: 44%, FPR 16%
    • Maternal age should not be used as a basis for recommending invasive testing when non-invasive PN screening is available
    • Modify age-related risk by multiple biochemical markers with or without first trimester u/s assessment of nuchal translucency


Use of Ultrasound in Screening for Chromosomal Anomalies

  • Detailed u/s at 18-20 weeks’ gestation

  • Most major fetal anatomic abnormalities will be detected

    • Majority of ONTD
  • Soft markers

    • Should not be used alone
    • Use to modify any a priori risk established by age or prior screening
    • In absence of soft markers and anomalies – reduction of risk of 0.5 can be applied – only in tertiary level scan centre
    • See 2007 SOGC Ultrasound Soft Marker Guideline


Prenatal Screening with addition of Non-invasive Prenatal Testing (NIPT)

  • What is NIPT?

  • What is the evidence for NIPT?

  • What do the experts say?

  • What is current Ontario prenatal screening landscape?



Screening test to prenatally detect Down syndrome and other aneuploidies (extra or missing chromosomes)

  • Screening test to prenatally detect Down syndrome and other aneuploidies (extra or missing chromosomes)

    • trisomy 21, 18, 13
    • trisomy of sex chromosomes (XXX, XXY, XYY)
    • Turner syndrome (monosomy X)
    • triploidy (extra copy of all chromosomes)


NIPT measures circulating cell free fetal DNA (ccffDNA) present in maternal blood (from trophoblast)

  • NIPT measures circulating cell free fetal DNA (ccffDNA) present in maternal blood (from trophoblast)

  • Comprises ~10% of DNA in maternal blood

  • Increases with gestational age

  • ccffDNA analysis determines if normal, higher, or lower than expected quantity of particular DNA sequences found on select chromosomes (13, 18, 21, X, Y)

  • Performed on maternal blood sample

  • As early as 9 weeks’ gestation

  • Dating u/s – viability, accurate GA, exclude multiples



7 studies of “high risk” women

  • 7 studies of “high risk” women

  • High risk:

    • Screen positive
    • AMA (≥35 yrs)
    • Ultrasound findings
    • Family history indicating increased risk
      • Previous pregnancy with aneuploidy


By far most accurate performance for T21/18

  • By far most accurate performance for T21/18



4 studies on unselected pregnancies

  • 4 studies on unselected pregnancies

    • Most mixed risk, some after pos screen, AMA, fewer with neg or no screen
    • > 14,000 pregnancies total, largest (11,000 significant loss to follow up)
    • Similar performance
  • Not yet validated in low risk women, triplets or higher, pregnancies conceived with egg donation



Failed results

  • Failed results

    • 6.1% (0.8-9.9) untested for insufficient sample quality
    • 2% (436/22,222) no result after testing
    • Rarely happens with conventional screening
    • ccffDNA decreases with increased maternal BMI




What do the experts say?

  • American College of Obstetricians and Gynecologists 2012

    • Has been validated in industry sponsored studies on at risk populations
      • History of prior pregnancy with trisomy
      • Positive multiple marker test
      • Parental balanced Robertsonian translocation with increased risk of fetal trisomy 13/21
      • Maternal age >35
      • Fetal ultrasound findings with increased risk of aneuploidy


What do the experts say?

  • Society of Obstetricians & Gynecologists of Canada 2013

    • Non-invasive prenatal testing using massive parallel sequencing of cell-free fetal DNA to test for trisomies 21, 18, and 13 should be an option available to women at increased risk in lieu of amniocentesis. Pretest counselling of these women should include a discussion of the limitations of non-invasive prenatal testing.


What do the experts say?

  • Society of Obstetricians & Gynecologists of Canada 2013

    • No irrevocable obstetrical decision should be made in pregnancies with a positive non-invasive prenatal testing result without confirmatory invasive diagnostic testing.


Recommendation in Ontario

  • Offer all women prenatal screening using either FTS, IPS or MSS (SIPS or Quad)

  • IF screen +ve or at high risk for other reasons

  • High risk:

    • Advanced maternal age
    • Previous aneuploidy pregnancy
    • Personal hx of sex chromosome aneuploidy (Turner mosaic, XXX)
    • Abnormal u/s
    • Pregnancy conceived via reproductive technologies


Recommendation in Ontario

  • NIPT is not a replacement for diagnostic PN testing

  • Positive NIPT should be confirmed by diagnostic testing: amnio or CVS

  • Expected benefit of NIPT: fewer women having diagnostic tests with associated risk of pregnancy loss



Recommendation in Ontario

  • If result is negative → reassuring

  • NIPT cannot:

    • Completely rule out aneuploidy
    • Detect chromosome differences other than aneuploidy of chromosomes 13, 18, 21, X and Y
    • Detect single gene conditions
    • Detect congenital anomalies
  • Still offer MSAFP and 18-20 wk ultrasound



Recommendation in Ontario

  • If result is positive:

    • Genetic counselling
    • Confirmation by diagnostic testing
  • No irrevocable obstetrical decisions should be made in pregnancies with abnormal NIPT results without confirmatory invasive testing (CVS or amnio) - SOGC



Increasing demand from women (who can pay)

  • Increasing demand from women (who can pay)

  • Increasing uptake in most (urban) centres

  • 3 separate companies, 3 separate technologies

  • No mechanism to evaluate centrally

  • Has it become mainstream now, become a “standard of care”? (? Only for high risk women)

  • Heading for 2 tiered prenatal screening

  • Costs between $795 and $1200

  • 8-10 days for result



11 to 14 week scan has value to pregnancy care

  • 11 to 14 week scan has value to pregnancy care

    • NT may shift in importance (importance to other chromosomal, genetic and structural disorders)
    • Accurate dating/establishment of live fetus
    • Multiples/chorionicity affects management
    • Detects structural abnormalities


Likely primary providers will not have time/expertise to counsel in detail

  • Likely primary providers will not have time/expertise to counsel in detail

  • Refer to genetic counselling if your centre provides that service (significant impact on counselling demands)

  • Company websites:

    • Panorama™ by Lifelabs
    • Harmony™ by Ariosa
    • Verifi™ by Verinata Health (Medcan)




CMA is a technology used to determine if there are small extra (microduplication) or missing (microdeletion) pieces of genetic information. These gains and losses are called copy number variants (CNVs). A CNV can be: of no medical consequence; pathogenic resulting in physical and/or intellectual consequences; or protective against disease (e.g. HIV infection).

  • CMA is a technology used to determine if there are small extra (microduplication) or missing (microdeletion) pieces of genetic information. These gains and losses are called copy number variants (CNVs). A CNV can be: of no medical consequence; pathogenic resulting in physical and/or intellectual consequences; or protective against disease (e.g. HIV infection).



Microarray can detect submicroscopic copy number changes that are associated with clinically significant outcomes

  • Microarray can detect submicroscopic copy number changes that are associated with clinically significant outcomes

  • Systematic review to calculate utility of PN microarray in presence of normal conventional karyotype

    • 12,362 pregnancies
      • 2.4% overall had copy number changes with associated clinical significance
      • 6.5% if ascertained because of abnormal ultrasound
      • 1.0% if increased maternal age
      • 1.1% for other reasons (parental anxiety, abnormal MSS)


Microarray generally detects all aneuploides and unbalanced translocations identified on karyotyping

  • Microarray generally detects all aneuploides and unbalanced translocations identified on karyotyping

    • Does not identify balanced translocations/triploidy


Summary

  • Summary

    • Consider when doing invasive test like CVS or amnio
    • Added diagnostic value of microarray especially when karyotype gives normal chromosome result
    • Genetic consult will assist with which test is most appropriate depending on clinical presentation
      • karyotype, microarray or both
    • Challenges:
      • Variants of unknown significance
      • Incidental findings
    • Future:
      • Next generation sequencing, whole genome sequencing


Ethnicity based screening



Ethnicity based screening: Carrier testing for persons of Ashkenazi Jewish background



Ethnicity based screening: Carrier testing for persons of French Canadian background - Charlevoix/Saguenay-Lac-Saint-Jean area



Consanguinity

  • Deeply rooted cultural trend among 1/5 of world population

    • Mostly residing in Middle East, West Asia, North Africa and emigrants from these communities
    • Intra-familial unions account for 20-15+% of all marriages
    • First cousins 1/3 of all marriages
  • Def’n: union between 2 people who are related as second cousins (5th degree relatives) or closer

    • First cousins, first cousins once removed, second cousins




Consanguinity

  • Mainly cultural reasons for consanguineous marriages

    • Strategy to preserve transmission of cultural values and continuity
    • Maintenance of familial structure and property
    • Financial advantage relating to dowry, keeping property within family
    • Sometimes thought to be more stable unions
      • Same social relationships before and after marriage


Consanguinity

  • Genetic Risk: first cousin marriages compared to non-consanguineous marriages

    • Fertility rate slightly higher
    • Miscarriage rate not different
    • Stillbirths and infant mortality rates slightly higher
    • Congenital anomalies 2-3% higher than background population risk (2-3%)
    • Increased likelihood of autosomal recessive conditions
    • Increased risk for almost all multifactorial birth defects including congenital heart defects, clefting, club feet, etc
    • Complex diseases – really not known – could be significant if high susceptibility gene present in family


Consanguinity

  • Management

    • Premarital/preconception counselling
    • 3 to 4 generation FH
    • Appropriate testing based on FH and ethnic background
      • i.e. screen for common autosomal recessive conditions in pop and community
    • Refer to genetics if FH points to presence of genetic disorder


Consanguinity

  • Concerns

    • Fear of stigmatization
    • Belief that inherited disorders can only arise through cousin marriages on paternal side – specifically inquire re shared biologic relationships on mother’s side
    • Need to balance risks and benefits


Prenatal Screening Summary

  • Offer all pregnant women, regardless of age, PN screening for fetal aneuploidy (trisomy 13, 18, 21) through FTS, IPS, SIPS or Quad screening

  • + second trimester ultrasound for dating, assessment of fetal anatomy and detection of multiples

  • Maternal age should not be used as a basis for recommending invasive testing when non-invasive PN screening is available

  • Consider offering NIPT to high risk women or following a positive PN screen for aneuploidy



Prenatal Screening Summary

  • Take a FH to identify familial and/or ethnically related disorders and screen accordingly

  • Consider consanguinity and screen and test accordingly

  • Refer or consult genetics when in doubt



Useful Genetics Resources

  • GEC-KO website: Genetics Education Website

  • www.gecko-cegco.ca

  • NIPT fact sheets:

    • http://www.mountsinai.on.ca/care/pdmg/NIPT%20info%20sheet%20for%20parents%2029_11_2012.pdf
    • http://www.mountsinai.on.ca/care/pdmg/NIPT%20info%20sheet%20for%20For%20Healthcare%20Providers%2029_11_2012.pdf
  • Prenatal Screening Ontario Website

    • http://www.prenatalscreeningontario.ca/
  • morrison@cheo.on.ca, jcarroll@mtsinai.on.ca

  • Thanks to Shawna Morrison, GEC-KO program manager for assistance with this presentation




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