Turner syndrome causes

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Humans have 46 chromosomes. Chromosomes contain all of your genes and DNA, the building blocks of the body. Two of these chromosomes, the sex chromosomes, determine if you become a boy or a girl. Females normally have two of the same sex chromosomes, written as XX. Males have an X and a Y chromosome (written as XY).

In Turner syndrome, cells are missing all or part of an X chromosome. The condition only occurs in females. Most commonly, the female patient has only one X chromosome. Others may have two X chromosomes, but one of them is incomplete. Sometimes, a female has some cells with two X chromosomes, but other cells have only one.

Causes

Turner syndrome results from the following mechanisms.

Nondisjunction

During meiosis in either parent, a nondisjunction event can occur that leaves the gamete, either oocyte or spermatocyte, with neither X nor Y chromosome. When this gamete combines with a gamete from the other parent (with a normal X chromosome), the embryo lacks the normal two chromosomes. Normally, humans have 46 chromosomes, so this leaves the embryo with 45 chromosomes and a single X chromosome, denoted 45,X (or, sometimes 45,XO, where the "O" is used as a placeholder). This is found in 50% of individuals with Turner syndrome.

Chromosomal structure

An X chromosome can form a ring chromosome for example by losing a portion of the smaller arm, enabling the end of the long arm to wrap around. This is detrimental for the X chromosome in two ways. Either the lost portion itself makes the chromosome less functional, or it causes nondisjunction, as described above. Thus, the causes listed here are partly overlapping.

When such a ring chromosome combines with another ring chromosome in fertilization, the pair is denoted as 46, XrXp-, where rXp- means a ring chromosome missing the small (p) arm of the chromosome.

Another variant of abnormal chromosomal structure is chromosomes with two long arms of the X chromosomes attached, and are called isochromosomes.

Variants of chromosomal structure occur in 30% of individuals with Turner syndrome.

Nonfunctional Y

Very rarely, the embryo has a normal X chromosome and a portion of the Y chromosome. In these cases, the Y chromosome does not have a functional SRY (and so develops as a female), the diagnosis is XY gonadal dysgenesis. It is possible that some Turner syndrome diagnosis is due to gonadal dysgenesis, particularly when it is caused by a large deletion of the Y chromosome.

Mosaicism

Each of the causes mentioned above can occur as a mosaicism, that is, some of the cells carry the mutation and some don't. This happens if the error takes place in one cell after the very first divisions of the early embryo after fertilization. The exact mixture of the two different cell types depends on when the nondisjunction occurred. However, if the nondisjunction occurs after enough divisions, the fraction of abnormal cells is probably not large enough to show any significant effects. For instance, such a 45,X/46,XY individual will develop as a male, without Turner syndrome. Mosaicism is found in about 20% of individuals with Turner syndrome.

No single Y

There is no equivalent syndrome which results in a Y chromosome with no X, as such a condition is fatal in utero. Because an embryo with Turner syndrome doesn't have a Y chromosome (or, doesn't have a functional SRY on the Y chromosome), it will move along the path to female development.

Overview

Disease name] may be caused by [cause1], [cause2], or [cause3].

OR

Common causes of [disease] include [cause1], [cause2], and [cause3].

OR

The most common cause of [disease name] is [cause 1]. Less common causes of [disease name] include [cause 2], [cause 3], and [cause 4].

OR

The cause of [disease name] has not been identified. To review risk factors for the development of [disease name], click here.

Causes

Life-threatening Causes

• Life-threatening causes include conditions which may result in death or permanent disability within 24 hours if left untreated. There are no life-threatening causes of disease name, however complications resulting from untreated disease name is common.
• Life-threatening causes of [symptom/manifestation] include [cause1], [cause2], and [cause3].
• [Cause] is a life-threatening cause of [disease].

Common Causes

Common causes of [disease name] may include:

• [Cause1]
• [Cause2]
• [Cause3]

OR

• [Disease name] is caused by an infection with [pathogen name].
• [Pathogen name] is caused by [pathogen name].

Less Common Causes

Less common causes of [disease name] include:

• [Cause1]
• [Cause2]
• [Cause3]

Genetic Causes

• [Disease name] is caused by a mutation in the [gene name] gene.

Causes by Organ System

Cardiovascular	No underlying causes
Chemical/Poisoning	No underlying causes
Dental	No underlying causes
Dermatologic	No underlying causes
Drug Side Effect	No underlying causes
Ear Nose Throat	No underlying causes
Endocrine	No underlying causes
Environmental	No underlying causes
Gastroenterologic	No underlying causes
Genetic	No underlying causes
Hematologic	No underlying causes
Iatrogenic	No underlying causes
Infectious Disease	No underlying causes
Musculoskeletal/Orthopedic	No underlying causes
Neurologic	No underlying causes
Nutritional/Metabolic	No underlying causes
Obstetric/Gynecologic	No underlying causes
Oncologic	No underlying causes
Ophthalmologic	No underlying causes
Overdose/Toxicity	No underlying causes
Psychiatric	No underlying causes
Pulmonary	No underlying causes
Renal/Electrolyte	No underlying causes
Rheumatology/Immunology/Allergy	No underlying causes
Sexual	No underlying causes
Trauma	No underlying causes
Urologic	No underlying causes
Miscellaneous	No underlying causes

Causes in Alphabetical Order

List the causes of the disease in alphabetical order:

• Cause 1
• Cause 2
• Cause 3
• Cause 4
• Cause 5
• Cause 6
• Cause 7
• Cause 8
• Cause 9
• Cause 10

References

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