Paula and Bobby
Parents of Lillie
What is Allan-Herndon syndrome?
Allan-Herndon syndrome is a rare disease. It is also known as AHDS.
A large X-linked kindred, first ascertained by Allan et al., (1944) was re-studied by Stevenson et al., (1990). Linkage was established to Xq21. Affected males had severe mental retardation, dysarthria, ataxia, athetoid movements, muscle hypoplasia and spastic paraplegia with hyperreflexia. The facies were elongated and characterised by bitemporal narrowing, a normal head circumference, large, simple or cup-shaped ears. Neck drop was a frequent sign ('limber neck'), apparently caused by weakening of the neck muscles. The authors suggested that the family briefly described by Davis et al., (1981) may have had the same condition. Bialer et al., (1992) studied this family in more detail and established the clinical similarities and mapping to Xq21. They pointed out that Wieacker-Wolff syndrome (qv) has similarities, but mental retardation is less severe and there is no spasticity in that condition. The family reported by Ulku et al., (1992) is difficult to classify because of limited clinical detail, but it may also fall into this group. Although the majority of cases have delayed myelination, this is not always the case (Azzoliniu et al., 2014)
Dumitrescu et al., (2004) and Friesema et al., (2004) described mutations in a monocarboxylate transporter gene MCT8 needed for triiodothyronine transport into neurons. Schwartz et al., (2005) provide an excellent, overview of the entity.
Passos-Bueno et al., (1993) reported a family with a form of X-linked mental retardation that also mapped to Xp11. This seemed to be associated with muscle atrophy and the patients were never able to hold their head against gravity, to sit unsupported, or to walk or speak. There were some similarities to Allan-Herndon syndrome. This family was re-evaluated by Zorick et al., (2004) and Maranduba et al., (2006) and mutations were found in MCT8. All obligate female carriers were mentally, normal. There was a deficient cellular uptake of T3. Not all patients have abnormal serum T3 levels (Boccone et al., 2013)
The family (with a SLC16A2 mutation), reported by Gika et al., (2010), had paroxysmal chorea-athetosis, dystonia, hypotonia and hypothyroidism, and one had delayed myelination and was initially thought to have Pelizaeus-Merzbacher disease. Others had been diagnosed with dystonic CP. One had a myopathic face.
* This information is courtesy of the L M D.
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What gene changes cause Allan-Herndon syndrome?
The syndrome is inherited in the following inheritance pattern/s:
X-Linked Recessive - Syndromes inherited in an X-linked recessive pattern generally only affect males. Males only have one X chromosome, and so one copy of a gene mutation on it causes the syndrome. Females, with two X chromosomes, only one of which will be mutated, are not likely to be affected.
In some cases, a genetic syndrome may be the result of a de-novo mutation and the first case in a family. In this case, this is a new gene mutation that occurs during the reproductive process.
The syndrome can be caused by mutations in the following gene/s location/s:
What are the main symptoms of Allan-Herndon syndrome?
The typical symptoms of the syndrome are:
How does someone get tested for Allan-Herndon syndrome?
The initial testing for Allan-Herndon syndrome can begin with facial genetic analysis screening, through the FDNA Telehealth telegenetics platform, which can identify the key markers of the syndrome and outline the type of genetic testing needed. A consultation with a genetic counselor and then a geneticist will follow.
Based on this clinical consultation with a geneticist, the different options for genetic testing will be shared and consent will be sought for further testing.
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