IUGR – THE GENETIC POINT OF VIEW

Michal Feingold ZadokGenetic InstituteLabor & Delivery Ward Assaf Harofe Medical Center

Fetal Growth Restriction – The Aim

To differentiate fetuses with pathologicalgrowth restriction due to a variety ofmaternal, fetal or placental causes frompregnancies with a small but healthy fetuswhose growth is appropriate

70% of SGA fetuses are constitutionally small with no additive risk for adverse outcomes

Fetal Growth Restriction – Definition(s)

EFW < 10th centile for GA EFW< 5th centile for GA EFW< 3rd centile for GA AC< 5th centile for GA AC< 3rd centile for GA

BW < 2500 gr

•EFW < 10th centile for GA (ACOG, 2013)

•EFW or AC < 10th centile for GA (SOGC,2013)

•Fetal AC or EFW < 10th centile for GA (RCOG, 2014)

From: Birth weight Standards in the Live-Born Population in Israel. Dollberg S. et al. IMAJ 2005

n=787,710

Genetic counselling and prenatal diagnosis – WHEN?

• Fetal growth restriction + a structural defect or midtrimester onset (ACOG Practice Bulletin #134, 2013)

• Severe SGA (<3rd centile) with structural anomalies or SGA detected before 23w (RCOG Guideline #31, 2014)

• IUGR + fetal abnormality / soft markers / no supportive evidence for placental insufficiency (SOGC, Practice Guidelines, 2013)

IUGR + HIGH RISK FOR ANEUPLOIDY

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Karyotype: for “gross” chromosomal anomalies: resolution 7-10Mb

CMA: for microchromosomalanomalies: resolution few kb

Sequencing: for point mutations in single gene disease

IUGR and Chromosomal Anomalies• n= 458 growth restricted fetuses • Abnormal karyotype 19%

- 9.3% (Zhu et al. Prenatal Diagnosis, 2016)

• Most common: • Triploidy – more common <26w• Trisomy – mainly T18, but also T21 and T13, more common >26w

• incidence of aneuploidies with:• Maternal age (not triploidy)• Earlier GA at diagnosis (18-25w)/ later (37%12%)• Associated fetal malformations / isolated (40%2%)• Normal or increased AFI reduced (40% 8%)

• 38% in IUGR+polyhydramnios (Sickler GK et al, 1997)

• Normal Doppler in UA and /or Umb A (44%12%)

Fetal growth retardation: Associated malformations and chromosomal abnormalities. Snijders et al. Am J Obstet Gynecol,1993

SHOULD WE KARYOTYPE ALL?

Microdeletion / Microduplication Syndromes

• Complex phenotypes due to dosage imbalance of multiple, unrelated genes which happen to be contiguous on chromosome

• Many are associated with IUGR

• Diagnosed by CMA: measures gains & losses of DNA throughout the genome

• Does not diagnose balanced rearrangements and low level of mosaicism

IUGR and Pathogenic CMA results

• n=2858 prenatal samples with known abnormality• 2.6% when isolated “anomaly”

• 13.6% when associated structural anomalie/s(Shaffer et al. Prenatal Diagnosis, 2012)

• 6% (45/755) of fetuses with suspected growth or structural anomalies (Wapner et al. NEJM, 2012)

• 5.7% (4/70) pathogenic CNV’s in normal karyotype IUGR fetuses (Zhu H et al. 2016)

CONCLUSIONS:

• 5.7% added value in FGR with/without associated US anomalies- In midtrimester

- In asymmetric cases - certain CNV’s may affect placentation?

• 5.7% VOUS

SHOULD WE “CHIP” ALL?

Single Gene Disorders

• Mutation = a permanent change in the nucleotide sequence of DNA

• Many single gene disorders can manifest with prenatal IUGR

• In the absence of other “clues” – diagnosis is difficult...or not?

• WES = whole exome sequencing: comprises ~1.5% of genome, harbors ~ 85% of pathogenic mutations

• WGS = whole genome sequencing

Next Generation Sequencing• TARGETED / WHOLE EXOME (WES) / WHOLE GENOME (WGS)• ADVANTAGES:

• Massively parallel sequencing of millions of DNA fragments simultaneously Relevant when no leading diagnosis or single candidate gene (“genetic heterogeneity”)

• Low cost – high throughput sequencing• Fast: entire genome sequencing <1d

• DISADVANTAGES:• Poor coverage & sequence inaccuracy (GC-rich regions, repetitive regions…)• Data analysis:

– time consuming– bioinformatics knowledge required

IUGR and WES results

• No data on that…

• Most data on WES in post natal setting 25%-55% (68%) diagnostic yield

• Little data on prenatal WES for anomalous fetus10%-45% (57%) diagnostic yield (very small numbers)

• Will probably become valuable diagnostic testing option for multiple anomalous fetuses including IUGR

SHOULD WE “WHOLE EXOME” ALL?

Genomic Imprinting• Most genes are expressed equally from both parental alleles• GENOMIC IMPRINTING: epigenetic modification (no

change in DNA sequence) by which certain genes are expressed in a parent-of-origin-specific manner and results in monoallelic gene expression

• Silent, inactivated allele usually referred to as imprinted (i.e., maternally imprinted = mat. silent)

• ~200 imprinted genes known• Many involved in growth regulation

(also in behavioral phenotypes)• Abnormal imprinting might cause

severe phenotypes

1 Deletion – 2 SyndromesAngelman Syn –

Deletion 15q11-13• Severe MR

• Seizures

• Absent speech

• Paroxysms of laughter

• Tongue protrusion

• Stiff, ataxic gait

• Characteristic facies

• Absence of maternal contribution

Prader Willi Syn –

Deletion 15q11-13• Moderate MR

• Neonatal hypotonia

• Hypogonadism

• Hyperphagia → Obesity

• Short stature

• Small hands and feet

• Characteristic facies

• Absence of paternal contribution

Map of imprinted regions in human genome

Maternally expressedPaternally expressed

UPD (Uniparental Disomy)• Most common phenomenon involved in imprinting effects in humans• The inheritance of a pair of chromosomes of one parent with no

contribution of the other parent• Main mechanism – “trisomic / monosomic rescue” due to

• maternal meiosis I nondisjunction• trisomic rescue of a Robertsonian translocation

• Most chromosomes no relevant clinical abnormality with UPD • Clinically relevant: involves abnormal imprinting its association with trisomy mosaics homozygosity for AR mutations

• IUGR not uncommon due to the UPD itself (imprinting disruption) ±“residual trisomy”

From: Shaffer et al. ACMG guidelines, 2001

UPD not describedMat UPDPat UPDUPD described; abn. phenotype not clearly definedUPD described; no abn. phenotype

From: Prenatal testing for uniparental disomy: indications and clinical relevance. Kotzot. Ultrasound Obstet Gynecol, 2008

Confined Placental Mosaicism (CPM)

• Presence of chromosomal abnormalities in the extraembryonic tissue which are absent from the fetal tissue

• 1-2% of CVS sampling

• Trisomy 16 CPM: most common in association with IUGR

• Risk factor for UPD (“trisomic rescue”) – important for “imprinted” chromosomes

• Risk for IUGR even without UPD

(dysfunctional mosaic placenta)

UPD testing in prenatal diagnosis• When to consider? Fetuses with complete trisomy or mosaics on CVS for relevant

chromosomes (6,7,11,14,15)Fetuses with mosaicism on amniotic fluid chromosomes (for

relevant chromosomes)Prenatal diagnosed Robertsonian translocation or possible

isochromosome involving chromosome 14 or 15 – both familial translocations and de novoFetuses with anomalies identifies by US that are consistent

with features found in UPD syndromes

ACMG Statement on Diagnostic Testing for UPD (2001)

Russel Silver Syndrome• Clinical characteristics:1. Pre and postnatal growth retardation2. Proportionate short stature (151 cm in males,

139 cm in females)3. Normal HC - relative sparing of the head

(“asymmetric”) 4. Triangular facies and other dysmorphic features5. Facial & limb asymmetry6. Significant risk for DD and learning disabilities

• Genetically heterogenous:• Most cases unknown molecular etiology• 10% maternal UPD7• ~ 35-50% imprinting abnormality (loss of IC1

methylation of paternal 11p15.5)• Other rare mechanisms

IUGR Summary – Main Points

• “Gross” chromosomal anomalies ~ 9-19%Bad prognostic factors: midtrimester, polyH2O, malformations, no

evidence for placental insufficiency

• Microdeletion / microduplication syndromes ~ 3-6% Also in “placental insufficiency” like fetuses

• Other single gene disorders – missing data on WES/WGS in IUGR cases

• Imprinting defects / Uniparental disomy (UPD)

• Confined placental mosaicism (CPM)

SO, genetic counselling and prenatal diagnosis to “high risk for aneuploidy” AND “placental insufficiency” like IUGR

IUGR – “genetic” management• Detailed fetal anomaly US scan (“genetic sonogram”)• Fetal echo• Other tests as indicated (fetal MRI?)

• Invasive genetic testing:- Karyotype for all? - CMA for all?- WES / WGS?

• NIPT?

• Other genetic testing as indicated by additional findings , family history, screening tests

Only with additional “morbid” prognostic signs??

Thank u for your attention