X-linked ichthyosis

X-linked ichthyosis
X-linked recessive inheritance: Affected boys may inherit a deletion or mutation of the STS gene from their mothers
Classification and external resources
Specialty	medical genetics
ICD-10	Q80.1
ICD-9-CM	757.1
OMIM	308100
DiseasesDB	29136
eMedicine	derm/191
MeSH	D016114
Orphanet	461
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X-linked ichthyosis (XLI) (also known as "Steroid sulfatase deficiency," and "X-linked recessive ichthyosis") (from the Ancient Greek 'ichthys' meaning 'fish') is a skin condition caused by the hereditary deficiency of the steroid sulfatase (STS) enzyme that affects 1 in 2000 to 1 in 6000 males. XLI manifests with dry, scaly skin and is due to deletions or mutations in the STS gene. XLI can also occur in the context of larger deletions causing contiguous gene syndromes. Treatment is largely aimed at alleviating the skin symptoms.

The major symptoms of XLI include scaling of the skin, particularly on the neck, trunk, and lower extremities. The extensor surfaces are typically the most severely affected areas. The >4 mm diameter scales adhere to the underlying skin and can be dark brown or gray in color. Symptoms may subside during the summer.

The STS gene is located on the X chromosome at band Xp22.3. Thus, the syndrome is an X-linked condition, and it affects males and females differently. The 23rd pair of chromosomes is typically termed the "sex chromosomes". Females have two X chromosomes and males have one X and one Y chromosome. Therefore, in normal individuals, males carry a single copy of the STS gene and females carry two copies. This gene partially escapes X-inactivation and females normally express higher amounts of the STS enzyme than males.

XLI can occur through new deletions or mutations of the STS gene but is more commonly inherited from a carrier mother. A hemizygous deletion or mutation of the STS gene in a male results in complete absence of enzyme activity, while a female carrier of a mutation or deletion is heterozygous and still has a normal copy of the STS gene. Female carriers of an STS deletion or mutation still express the STS enzyme, although with decreased enzyme activity.

For this reason, XLI most commonly affects males, although individuals with numeric abnormalities of the sex chromosomes (45,X and 47,XXY) who also carry STS deletions or mutations would be exceptions to this rule.

In addition, a female could be affected if she were the offspring of an affected male and a carrier female and inherited a deletion or mutation of the STS gene on both X chromosomes.

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Wikipedia