Characteristics of Hypohidrotic Ectodermal Dysplasia
Hypohidrotic ectodermal dysplasia (HED) is a rare genetic condition characterized by a reduced ability to sweat, missing teeth, and fine sparse hair. Individuals affected by HED share a similar facial appearance with thin, dark skin beneath the eye with extra folds or wrinkles, a depressed “saddle” nose, small narrow jaw, and small pointed teeth. Eruption of the teeth may be delayed, or only a few teeth may erupt. Additional features include dry eyes, eczema, asthma, ear wax impaction, dry nasal concretions, respiratory illness, sinusitis, or sparseness of saliva. Nails, facial hair in males, and the appearance of pubic hair in adolescence are normal. With the exception of heat intolerance, general health and overall development, including intelligence, is within normal limits.
HED can be diagnosed after infancy on the basis of physical features in most affected individuals. Newborns may have dark circles around their eyes because that skin is thin and the underlying blood vessels create a bluish shadow. The skin also may appear to be “peeling” at birth. Infants may not be able to tolerate heat and may be irritable in warm environments or may have unexplained fevers. More often, the diagnosis is not made until the teeth do not erupt at the expected age or the teeth appear to be pointed when they do erupt.
Diagnosis can be made through molecular genetic testing, which is available for all three types of HED. Changes or mutations in the EDA, EDAR, EDARADD, and WNT10A genes are most commonly associated with HED. These genes tell the body to make proteins that are needed early in life (before birth and shortly after) for the normal development of sweat glands, teeth, hair, skin, and other mucous glands.
Mode of Inheritance
HED may be inherited in one of three patterns: X-linked recessive, autosomal recessive and autosomal dominant. Ninety-five percent of randomly selected individuals with HED have the X-linked recessive form. The remaining 5% have either the autosomal recessive or autosomal dominant form of HED. The mode of inheritance may be determined in some instances by family history and in others by molecular genetic testing.It is important to note that this research report focuses on the X-linked recessive form or XLHED.
Carrier testing is possible for XLHED, if the syndrome-causing mutation in the family is known. Prenatal testing is also possible for pregnancies at increased risk for XLHED.
How Is HED Passed from Parent to Child?
The risk to family members depends on the pattern of inheritance. Therefore, it is important to know which of the three types affects your family. For XLHED, the mutated gene, called EDA, is found on the X chromosome.
Males Affected by XLHED
A man affected by XLHED passes his X chromosome to his daughters and his Y chromosome to his sons. Because the changed gene is only on the X chromosome, his daughters will be carriers and the sons will not be affected. Daughters may then pass on the gene to her children. Fathers cannot pass on the mutated gene to their sons because they pass it on their Y chromosome.
Female Carriers of XLHED
A woman with XLHED, or a "carrier", has the EDA gene on one of her X chromosomes. Women who are carriers for XLHED may have some symptoms, such as thin hair, reduced sweating, one or more missing teeth, and sometimes have difficulty breastfeeding. The way each woman experiences symptoms of XLHED is unique.
Since women have two X chromosomes, when one X is changed, the healthy X can help compensate. This working copy can partially (or fully in some cases) control the development of some, or all, of the skin, hair, nails, teeth, and sweat glands. This helps women have less severe XLHED symptoms than men.
There is a 50% chance that a woman with XLHED will pass the gene to her son. Sons who inherit this gene will be affected by XLHED. This is because their only copy of the X chromosome does not work correctly. If the woman who is a carrier has a daughter, there is a 50% chance the daughter will inherit the changed gene. Then she can also pass XLHED to a future child, like her mother did.
Although not specifically known, it is estimated that at least one in 5,000-10,000 newborns has HED. This is probably an underestimate of the prevalence, as many cases may be missed during infancy before the cardinal features become obvious.
Thank you to Tessa Field, B.S., Kenneth Huttner, M.D., and Seema Jamal, M.Sc., LCGC for their contributions to this article.