Exploring Turner Syndrome and Its Chromosomal Features

What is Turner Syndrome?

Turner syndrome is a chromosomal disorder generally seen in females wherein there is either one fully or partially absent X chromosome. The term Turner syndrome is coined after Henry Turner, who first characterized the disease.

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We know that our DNA has 46 chromosomes and it is the blueprint of life. This 46 chromosome, as we know, is paired in 23 and each pair has one chromosome derived from each parent. It is the sex chromosome pair that decides an individual’s sex. It can be either XX (i.e. female) or XY (i.e. male).

Now, in order to make a human baby, a sperm cell and egg cell from male and female parents contain half of the chromosome, or 23 chromosomes needed to fuse and when they fuse, a single cell that has 46 chromosomes, referred to as zygote is formed, which later evolved into a fully grown human baby in mothers uterus. Now, how are these sex cells which contain only half the size of chromosome as compared to normal cells are produced?

The starting point for the development of sex cells, that is either sperm cell or egg cell is a precursor cell called germ cell with 46 chromosomes and when this precursor undergoes a process called meiosis, sex cells with 23 chromosomes are formed. Now, let’s look into the formation of these sex cells in detail..

During the initial stages of meiosis, the germ cells which carry 46 chromosomes ( 2 sex chromosomes and 44 autosomes or normal chromosomes) undergo replication resulting in the formation of two copies of each chromosome, which are connected together by centromere.

In the subsequent steps, the chromosomal pairs are split up and form two two cells, which contain 23 pairs of chromosomes. Then in the final stage of meiosis, two cells formed in the previous step split again and form four gamete cells, each containing 23 chromosomes including 1 sex chromosome.

These newly formed sperm cells and egg cells, each carrying 23 chromosomes fuse, to form a zygote and the end result is a cute little baby having 50 percent chromosome from dad and 50 percent from mom!

The above mechanism is what’s happening in the normal circumstances, that is each parent contributes one chromosome to each pair. But in some cases, one parent might contribute one extra chromosome and this is called trisomy, or may be one chromosome less, which is called monosomy..

Turner syndrome is characterized by Monosomy and that particularly affects the X chromosome.

What causes Turner Syndrome?

There are three common karyotype scenarios associated with Turner syndrome. In the most common scenario, the entire X chromosome is absent, the next most common scenario is mosaicism and the least common karyotype in Turner syndrome is where there’s only a part of the X chromosome missing. Let’s talk in detail about each of these scenarios and how they develop.

So, the most common type of Turner’s syndrome occurs when a complete X chromosome gets lost resulting in 45, X karyotype i.e. an individual has only 45 chromosomes and the X chromosome is completely missing. This occurs as a result of nondisjunction of sex chromosomes in the time of meiosis, mostly in sperm cells and less commonly in egg cells.

In other words, nondisjunction means, there is no splitting of chromosomes. So, after meiosis 1, one resulting cell ends up with two sex chromosomes and the other gets none. The final result after meiosis II, is that 2 cells with an extra chromosome, and two cells missing a chromosome. The nondisjunction can also occur in the meiosis II.

So, in that case, Meiosis I occurs perfectly, and both cells have a chromosome, but they don’t split apart in the meiosis II, and the end result is one cell with an extra chromosome, one cell missing a chromosome, and two cells with the correct number of chromosomes.

Turner’s syndrome develops when an egg cell with the right number of chromosomes combines with any of these sperm cells that have the missing chromosome.

Another common case of Turner’s syndrome is Mosaicism in which people have some cells in their body with the 45, X karyotype and others with a 46, XX karyotype.

This mosaicism occurs because of an error following conception. So, Conception of egg and sperm cells leads to a single celled zygote which divides repeatedly making all types of cells in the body and each of these divisions is called Mitosis.

The previously mentioned nondisjunction of sex chromosomes can also occur during mitosis in such cases you’d end up with one cell line that has three sex chromosomes, so 47 in total, and one cell line missing an X chromosome, so 45 chromosomes in total.

However, if the divisions in the early stage proceeded conventionally, then there will also be a normal cell line which has 46 intact chromosomes which further supply cells to the evolving fetus.

Now, the cell line carrying 47 chromosomes rarely survives, but the chance of survival of cell lines with 45 chromosome is high, and they continues to replicate and produce more cells with only one X chromosome, along with the 46, XX cell line, resulting in a blend of 45, X and 46, XX cells in the human body.

The third karyotype in Turner syndrome is the least common compared to the other two. Here, there’s only a part of the X chromosome missing.

That is in this case, only a section of the chromosome is deleted at some point during meiosis, but the rest of the chromosome is passed on to the gamete cells. Please note that such a thing can also occur during mitosis resulting in yet another mosaic karyotype.

All these mistakes occur randomly and therefore parents who have a child with Turner syndrome are not at a higher risk than others of having a second child with the same condition.

Having only one X chromosome affects the proper functioning of the female reproductive system and there will be an increase in the rate of egg loss. In normal cases, the loss of eggs occurs very slowly!

But in Turner syndrome affected individuals, by age 2, there are no eggs left resulting in menopause prior to menarche. Now, without eggs, there is failure in the ovary development producing non-functional, fibroid and streak gonads which no longer are able to produce sex hormones leading to hypogonadism and lower estrogen levels.

Also, to compensate for low estrogen levels, the body starts to release more and more FSH (follicle-stimulating hormone) and LH (luteinizing hormone) as they promote the secretion of estrogen by ovaries.

Also, the X chromosome contains many genes that are essential for growth and development of various tissues throughout the body. One of such genes is the short stature homeobox gene or SHOX. So, having a single X chromosome affects the function of many important genes like in this case, a single copy of the SHOX gene results in short stature.

Other than SHOX, there are a number of genes on the X chromosome. Therefore, Turner syndrome affected people are at risk of developing cardiovascular issues namely preductal coarctation of aorta and bicuspid aortic valve.

Also, there could be kidney problems, in particular, horseshoe kidney, otherwise known as Renal fusion syndrome that occurs during fetal development.

Affected people are also at risk of having lymphatic and skeletal malfunctions and are also predisposed to hypothyroidism and diabetes.

The overall abnormalities in connection with Turner syndrome mainly depends on the amount of cell one has with 45, X karyotype.

What are the symptoms of Turner syndrome?

It varies with the age of the person and how much portion of the second X chromosome is missing. Infants could have lymphedema in hands and feet which is nothing but tissue swelling due to lymph fluid build up.

Lymph linked swelling in the back of the neck producing cystic hygroma could also be present in infants which in due course decline as they grow. However, it leaves behind an additional skin on the neck, namely, neck webbing that displays a wider neck.

Cardiovascular risks like preductal coarctation of aorta and bicuspid aortic valve produces lower extremity cyanosis i.e. lower limb discoloration which is bluish or purplish resulting in congestive heart failure. Kids with horseshoe kidney syndrome are more expected to have urinary tract infections.

In adolescents, Skeletal problems like short stature, low-set ears, neck webbing, wide chest with widely spaced nipples, and arms that are angled out away at the elbows called cubitus valgus become evident.

Also, Turner syndrome causes streak ovaries and because of this, it is the most common cause of primary amenorrhea. Primary amenorrhea is the absence of menarche by 15 years of age and no breast development by age 13. Therefore, many females with Turner syndrome become infertile.

How Turner Syndrome is Diagnosed?

Karyotype analysis is the only option for the diagnosis of Turner syndrome which can be performed prior to birth by means of chorionic-villus sampling or amniocentesis.

There are some other tests that could be beneficial to figure out the developmental abnormalities linked with Turner syndrome.

For instance, congenital problems like cardiac abnormalities, cystic hygroma, horseshoe kidney are identified using ultrasound scanning and a fasting blood glucose test aid in detecting diabetes.

What is the treatment for Turner syndrome?

The therapy for Turner syndrome is supplementation of growth hormone during childhood to stimulate height and growth of bones.

Also, sex hormone replacement therapy is initiated at adolescence to stimulate development of breast and uterus. For infertile females, in vitro fertilization is also done for them to become pregnant.

Quick summary

Turner syndrome is a chromosomal disorder in females wherein there is either one fully or partially absent X chromosome.

The three common karyotypes seen in Turner syndrome are 45:X karyotype, 46:XX & 45:X mosaic karyotype and karyotype that miss some part of X chromosome.

The common characteristics of Turner syndrome are short stature, streak ovaries, neck webbing, lymphedema, congenital heart and renal anomalies.

Treatment includes growth hormone and sex hormone replacement therapy.