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Diseases that can be detected by prenatal diagnosis tests

Diseases that can be detected by prenatal diagnosis tests

There are several diseases that can be detected by prenatal diagnosis.
NIPT (new prenatal diagnosis), a non-confirmatory but highly accurate test, can diagnose three trisomies, and the confirmatory test can also examine sex chromosome abnormalities.

Why do chromosomal abnormalities occur?

Chromosomes are large molecules contained in the nucleus of a cell that are primarily composed of DNA, which carries genetic information.

Human cells contain 22 pairs of autosomes and one pair (XX or XY) of sex chromosomes, which together comprise 46 chromosomes. Each of these 22 pairs (autosomes) is numbered from 1 to 22.

An autosome is a single cell that divides into two, plus an X chromosome and a Y chromosome, making 46 chromosomes.

Originally, during cell division, autosomes have 46 chromosomes, and sex chromosomes have XX and XY. However, an abnormal number of chromosomes may occur due to a partial excess or a partial deficiency of chromosomes.

There are also structural abnormalities that occur when chromosomes are broken and then resynthesized between chromosomes.

numerical anomaly

It is an abnormality caused by a deficiency or excess of chromosomes, usually two chromosomes in a pair.

The normal "dysomy," which is paired in pairs, becomes one "monosomy," three "trisomies," and four "tetrasomies.

structural anomaly

This occurs when a break occurs in a chromosome and the chromosome is resynthesized between chromosomes.

It refers to a single abnormality in which the structure itself is changed within one chromosome, such as when part of a chromosome is broken off and attached to a different chromosome or turned upside down.

Types of chromosome aberrations (22 pairs of autosomes and trisomy)

Autosomal trisomies reported by type are summarized in the table below.

monotrisomy Death before implantation
two-trisomy miscarriage
trisomy miscarriage
tetrasomy miscarriage
pentothorisomy miscarriage
hexotrisomy miscarriage
7-trisomy Very rare births only in mosaic (complete type is lethal)
8-trisomy Very rare births
9-trisomy Very rare births
10-trisomy Very rare births only in mosaic (complete type is lethal)
11 trisomy miscarriage
12-trisomy Very rare births only in mosaic (complete type is lethal)
13-trisomy Birth Possible (Patou Syndrome)
14-trisomy Very rare births only in mosaic (complete type is lethal)
15-trisomy miscarriage
16-trisomy Very rare births only in mosaic (complete type is lethal)
17-trisomy miscarriage
18-trisomy Birth possible (Edwards' syndrome)
19 trisomy miscarriage
20 trisomy Very rare births only in mosaic (complete type is lethal)
21 trisomy Birth possible (Down's syndrome)
22 trisomy Very rare births

Most trisomies result in miscarriages, but there are rare cases of births.

Chromosomal Diseases Revealed by New-Information Prenatal Diagnosis (NIPT)

Among trisomies, 21 trisomy (Down syndrome), 13 trisomy (Patou syndrome), and 18 trisomy (Edwards syndrome) are eligible for the new prenatal diagnosis (NIPT).

21 trisomy (Down's syndrome)

It is a chromosomal abnormality in which three autosomal chromosomes 21 (trisomy) are present.

It occurs in approximately 1 in 600 births, making chromosome 21 trisomy the most frequent chromosomal abnormality.

Major symptoms include decreased muscle tone, characteristic facial features, intellectual disability, and developmental delay.

13 trisomy (Pato syndrome)

It is a chromosomal abnormality in which three autosomal chromosome 13s (trisomy) are present.

It is found at a frequency of about 1 in 1000 births.

Major symptoms include microcephaly, eye abnormalities, cleft lip and palate, inguinal hernia, and fifth finger single flexion line.

18 trisomy (Edwards syndrome)

It is a chromosomal abnormality in which three autosomal chromosomes 18 (trisomy) are present.

It is seen in approximately 1 in 6,000 births and appears to be more common in girls. (male:female = 1:3)

Major symptoms include growth retardation, congenital heart disease, respiratory, digestive, and musculoskeletal complications, and hearing loss.

*Mosaic type trimiso

The mosaic type is a mixture of normal and trisomic cells due to non-separation during the egg division process after fertilization, and since there are many normal cells, severe damage is often not seen.

The most frequent "Down syndrome

Down syndrome was discovered in 1959 as a chromosomal disorder caused by trisomy of chromosome 21.

It is known to be associated with moderate mental retardation, characteristic facial features, cardiac malformations, and hypotonia, as well as gastrointestinal disorders.

Most cases involve an excess of chromosome 21 due to a segregation anomaly that occurs during meiosis during the oogenesis process.

This is not hereditary, as both parents have normal chromosomes.

However, as the mother ages, the frequency of chromosome disjunctions increases.

It is believed that about 10% of meiosis also occurs in the father's meiosis.

Classification of Causes of Down Syndrome

*Most of what is seen in translocation Down syndrome is called Robertsonian translocation.

Unlike 21 trisomy, this translocation Down syndrome is not related to the age of the mother, but is more common when one of the parents, especially the mother, is the carrier of the translocation.

*Translocation type

This is a condition in which one of the parents' chromosome 21 is attached to another chromosome, resulting in a partial trisomy. In this case, one of the parents carries a translocated chromosome.

This Robertson-type translocation is in both parents' chromosomes,

1. both normal

2. possibility that one parent is a translocation carrier

The above are possible.

Translocation carriers are those with changes in chromosome structure but no genetic information and no symptoms.

1 occurs in neoplastic mutations as in the standard.

is hereditary.

The probability of (1) and (2) is 3:1 in favor of neoplastic mutations.

This Robertson translocation is a translocation in which the long arms of chromosomes 13, 14, 15, 21, and 22, which have centromeres at their ends, are linked together and two short arms are lost.

The centromere is the site where the long and short arms of a chromosome intersect.

Therefore, the number of chromosomes is 45 in the case of Robertsonian translocation as opposed to 46 in the normal case.

This is the most common type of translocation.

Among them, translocations between chromosomes 14 and 21 are common, and a child with that chromosome may have three possible types: normal karyotype, translocation carrier, or 21 trisomy.

A child with trisomy 21 would have translocation Down syndrome.

If the mother is a translocation carrier, the birth rate of Down syndrome infants appears to be 10%, and even lower if the father is a translocation carrier.

The next most common case is when a child with 21 trisomy is born due to Robertsonian translocations of chromosomes 13 and 21.

Carriers of translocations have one less chromosome number.

However, because there are no important genes in the lost centromere vicinity or short arm area, the *phenotype is not abnormal and is healthy.

*Phenotype

A phenotype is the expression of the genotype of an organism as a trait. It includes the morphology, structure, behavior, and physiological properties of the organism.

Sex chromosome disorders that can be identified by definitive diagnosis

Some chromosomal diseases include sex-linked diseases related to sexuality.

Sex chromosomes are usually XY for males and XX for females, but the most common sex chromosome abnormality is aneuploidy.

It was not discovered during amniotic fluid testing, which is performed when the mother's age is high and abnormalities are suspected, or diagnosed until puberty.

Trisomy (three sex chromosomes) and tetrasomy (four sex chromosomes) are less severe than autosomal trisomy and may go undetected for life.

However, because they are chromosomes that affect gender, infertility and genital malformations can occur.

Klinefelter's syndrome (XXY)

It occurs in approximately 1 case in 500 male births.

It is characterized by an excess of X chromosomes while the normal male karyotype is XY.

Major symptoms include gynecomastia, long limbs, low incidence of body hair, bone dysplasia and osteoporosis, heart disease, reduced mobility, and infertility.

The more X chromosomes there are, the greater the tendency to disability and also the more susceptible to heart disease.

In many cases, there are no symptoms at all and the disease may go unnoticed for the rest of one's life.

Triple-X syndrome (XXX)

It occurs in approximately 1 case per 1,000 female births.

It is characterized by an excess of X chromosomes while the normal female karyotype is XX.

Major symptoms include: height, learning disabilities, language delays, speech and language development delays, motor skill development delays, behavioral and emotional disturbances, and more.

Reproductive capacity is normal, as in normal women.

XYY Syndrome (XYY)

It occurs in approximately 1 case in 840 male births.

It is characterized by an excess of Y chromosomes while the normal male karyotype is XY.

It is also called XYY syndrome, etc., depending on the number of chromosomes.

Some reports indicate that taller stature, hyperactivity, and decreased intelligence appear, while others report higher intelligence.

There have been reports of genital and renal abnormalities, but no relationship to XYY syndrome has been proven.

The more Y chromosomes there are, the stronger the tendency toward the disorder. The reproductive capacity is normal, as in normal males.

These days, it is commonly viewed as a category of personality and may go unnoticed for the rest of one's life.

mosaic chromosome

Occurs in both men and women.

It comes in a variety of forms, and the incidence is said to be 1 in 10 to 10 billion people.

In addition to XX and XY, there are cases where XO, XXY, Y, etc. are mixed, sometimes with disorders, but there are also many cases with no problems at all.

Mosaic chromosomes are not detected by blood tests and require somatic cell testing.

sex chromosome monosomy

Regarding sex chromosome monosomies, unlike autosomes, monosomies of the X chromosome (XO, Turner syndrome), which have genes essential for survival, are viable, but monosomies of the Y chromosome (YO) are lethal and die soon after fertilization (YY, OO, etc. are also lethal for the same reason).

XO is also found in about 1 in 3,000 births, whereas excessive sex chromosome aberrations are found in about 1 in 500-600 births, and this only in about 1 in 3,000 births, which is thought to be due to the high probability of death in the fetus.

Turner's syndrome

Occurs only for women.

One of the X chromosomes is completely or partially absent (X, XO) while the normal female karyotype is XX.

Short stature, folds around the neck (pterygium), congenital heart disease, infertility, etc.

There is no intellectual disability of any kind, but 10% are known to have aortic stenosis.

Sex chromosome numerical abnormalities

Sex chromosome aberration syndrome is said to be caused by an abnormal number of chromosomes.

This is caused by chromosome disjunctions.

About half of the cases of Klinefelter's syndrome are often attributed to the father's chromosome (X and Y chromosomes) disjunction.

There is no maternal age dependency in Turner syndrome, but we do know that one X chromosome, which is present as a monosomy in about 80% of cases, is of maternal origin.

This means that in many cases, sex chromosomes of paternal origin are lost.

X-monosomy is found in 10% of spontaneous abortions.

Of all pregnancies, 15% result in miscarriage.

The probability of X monosomy leading to birth as Turner's syndrome is 1/200.

The reason for the difference between individuals who have the same X monosomy and those who have spontaneous abortions and those who go on to have births is not yet known.

Klinefelter's and Turner's syndromes have symptoms such as poor development of secondary sexual characteristics and failure of germ cell formation (infertility), but there is little or no loss of intelligence or survival impairment.

It is also fertile in XYYY men and XXXX women.

The Y chromosome has fewer genes itself and has no physiological function as a blueprint for proteins or DNA, so there is no XYY male pronounced disorder with one more Y.

What about the X chromosome, then?

The X chromosome has many genes in addition to sex determination.

Even if an X chromosome is altered, like a normal woman, she has one X activity, and even if she has three Xs, like an XXX woman, the other two Xs are inactivated (have no ability to be involved in the gene) and do not cause a prominent disorder.

It is also now known that the X chromosome is not fully inactivated and that some regions of the gene are involved in activation.

If this happens, it could have some impact.

Importance of genetic counselors

Genetic counselors are counselors who provide genetic information and psychological support to pregnant women and their families in need of genetic medical care, such as new prenatal diagnosis (NIPT).

If the test is positive, they will provide counseling on what to do in the future.

What genetic counselors can do

For example, if you have a child with Down syndrome, they can also provide you with information about the future support system of society.
They are very reassuring, clarifying the problems faced by pregnant women and their families, providing necessary information and psychological support for the final decision to give birth or not to give birth.

The genetic counselor can also answer questions about specialized areas of genetics.
Simply taking the test may cause further anxiety depending on the results.
However, there is a great possibility that the presence of a genetic counselor can make this anxiety a little lighter.