PGD-AS: Accuracy and Interpretations
Homo Sapiens has 46 chromosomes: 22 pairs of autosomes, and
two sex chromosomes, X+X or X+Y. PGD tests human pre-embryos
for the presence of the correct number of chromosomes
13,18,21,X and Y; any combination other than
13,13,18,18,21,21,X,X (Normal female) or
13,13,18,18,21,21,X,Y (Normal male) is considered ABNORMAL.
A monosomy (absence of one chromosome) of any autosome is
lethal in humans, only a few babies with autosomal monosomy
have survived beyond birth, with severe abnormatilites.
TIley all had monosomy of chromosome 21.
A trisomy (one extra chromosome) can occur with any of the
chromosomes. However, the most common is a trisomy of
chromosome 21, leading to Down's syndrome. According to the
latest National Vital Statistics report, in the year 2002
out of 3,993,973 births, 1,850 had Down syndrome and 1,253
had "other chromosomal anomalies". Among these "other
chromosomal anomalies" the most common is an abnormal number
of sex chromosomes. Per each 100,000 recognized human
pregnancies, around 1,400 abort due to an abnormal number of
sex chromosomes, about 100 boys born with Kleinfelter
syndrome (instead of XY have XXY, XXXY, XXYY, or even XXXXY
sets of sex chromosomes) and about 50 girls born with Turner
syndrome (instead of XX have X, XXX, or XXXX).
Apart trom chromosome 21, the only other autosomal trisomies
with any significant frequency in newborn are trisomies 18
(Edward's syndrome) and 13 (Patau's syndrome); frequencies
of either syndrome range from one in 2,000 to one in 15,000.
Apart from these five chromosomes (13, 18, 21, X, Y), the
only other chromosomes noticeably affecting the outcome of
established pregnancies are chromosomes 16 (trisomy 16 found
in 1,229 among 15,000 spontaneous abortions) and 22 (extra
copy was present in 424 out of 15,000 spontaneous
abortions). These embryos never reach term, but they affect
the outcome of hurnan pregnancies.
Aneuploidy for any other chromosome is lethal at the very
first stages of embryo development, before a pregnancy can
even be established.
Interpretation of the Results
Triploid, tetraploid, or polyploid embryos are those having
full extra sets of all 23 chromosomes. These embryos
originate from an oocyte fertilized by two spennatozoa, by
diploid spermatozoon, or from an oocyte which failed to
extrude the second polar body. Polyploidization also occurs
during embryo cleavage; at the blastocyst stage it is a
normal step in trophectoderm formation. Haploid embryos
originate trom parthenogenetically activated oocytes; they
have only one set of23 maternal chromosomes.
Any combination of auto somes other than normal (disomy),
monosomy or trisomy will be called complex abnormality.
These reveal some very serious errors in oocyte maturation
and/or embryo cleavage. Chaotic embryo cleavage is the most
probable mechanism by which complex chromosomal
abnormalities may originate. Chaotic cleavage means that
during mitosis, when the zygote and, subsequently,
blastomeres divide into two daughter cells, the chromosomes
segregate between two sister blastomeres randomly or
chaotically. Most of these embryos are also morphologically
abnormal and very few of them progress beyond the cleavage
stage. Such embryos should not be considered for transfer.
If more than one cell is analyzed from an individual embryo
(marked in PGD Report as a), b), ...), and conflicting
results are obtained, this may be an indication of a FISH
error (see below) or embryo mosaicism. Embryo mosaicism may
be considered as a "mild" case of chaotic cleavage.
Mosaicism is a result of mitotic error and as such arises
during embryo cleavage, when one of the blastomeres divides
into two genetically unequal 'daughter blastomeres'. This
may lead to an embryo having both normal and abnormal cells,
i.e., mosaic embryo. If an embryo is suspected of having
genetically normal cells and cells with autosomal monosomy,
such embryos should be AVOIDED during embryo transfer.
Monosomic cells may be selected out during further embryo
development; however, if their population rises above some
critical level, the embryo dies before or shortly after
implantation, like any other embryo with autosomal monosomy.
Since chromosome 21 is an exception (see above), mosaic
embryos with monosomy 21 should not be considered for
If an embryo is suspected to have genetically normal cells
and cells with autosomal trisomy, such embryos may develop
into an abnormal mosaic baby. Some newborns with Down's,
Edward's, and Patau's syndrome are actually mosaics. Mosaic
embryos with trisomies should never be considered for
Some embryos may be revealed as abnormal even prior to FISH,
during embryo biopsy or after blastomere fixation. If a
single blastomere has more then one nucleus, it is called a
multinucleate blastomere. Even if each nucleus (separated in
PGD Report by [...]) is genetically normal, or if they add
to a normal set of chromosomes, the corresponding embryo may
be genetically abnormal. Multinucleation indicates some
gross abnormalities in the timing between cell division,
cytokinesis and nuclear division, karyokinesis. If cleavage
results in one blastomere retaining both nuclei, then it's
'sister blastomere', will have none. Absence of a nucleus (anucleate
blastomere) may be similar in its origins to multinucleation,
but it may also be considered as an extreme example of
embryo fragmentation. Although anucleate blastomeres cannot
give any indications as to the embryo genetic background, it
should be noted that their presence lowers embryo viability.
If multiple morphologically normal blastomeres were
analyzed, and none of them had a nucleus, such an embryo may
be considered 'not viable' due to gross errors in embryo
Current estimates of FISH errors fluctuate around 10% for
the detection of autosomal numerical abnormalities, and
around 0% for sex determination. Due to the standard
practice: "if not proven normal means abnormal", the rate of
actual misdiagnosis is significantly lower. However, PGD
technique cannot reveal all cases of embryo mosaicism, and
for this reason, prenatal diagnosis via CVS or amniocentesis
is strongly recommended.
Advantages of PGD-AS Procedure