MOTHER (4] The diagram shows types for three DNA polymorphisms on chromosome FAT

Question

MOTHER (4] The diagram shows types for three DNA polymorphisms on chromosome FATHER 15 in the parents of a child. Marker A maps within the PWS / Angelman syndrome critical region; the other two markers map distal to this region. Write possible marker genotypes for their child if he has: (a) PWS due to a deletion 3+4 bl Angelman syndrome due to a deletion (c] PWS due to uniparental disomy (d) Angelman syndrome due to uniparental disomy le] PWS due to an imprinting error If) Angelman syndrome due to a mutation in the UBE3A gene.

Explanation / Answer

PWS is a disorder caused by an epigenetic phenomenon known as imprinting. Deletion of the same region on the maternal chromosome causes Angelman syndrome. PWS and AS represent the first reported instances of imprinting disorders in humans.

a) Prader- Willi syndrome due to a deletion.

This means, the copie coming from the father will be the one containing the deletion. The marker genotype might be -/3 or -/4 (absense of marker) while B and C can be present in both copies of the child. It could be possible as well if the deletion is pretty big or complete.

b) Angelsman syndrome due to a deletion.

This is the opposite case. So child´s genotype for marker A could be 1/-   or 2/ - . Same happens with markers B and C.

c) PWS due to uniparental disomy.

This is the situation where two copies of a chromosome come from the same parent. In this case, the mother.

So the child´s genotype could be 3/3 or 4/4 in all three markers (A,B and C).

d) In this case both copies come from the father in order to angelsman desease to develop (remember it exists imprinting in this region). So the child´s genotype could be 1/1 or 2/2 in all three markers.

e) Each individual must erase the paternal imprinting from their parents and write their own based on their sex. In an imprinting error, in the germinal line, these marks are not erased and thus maternal imprinting can appear in a chromosome transmited by the father. Meaning genes from the PWS region are not expressed when they should.

But remember imprinting is a epigenetic phenomenon, this means the genotype remains the same. In this case for the three markers the kid could have 1/3, 2/3, 1/4, 2/4 genotype, and still have the PWS desease.

f) This is a punctual mutation. This means we could not see it with a generic DNA polymorphism like we are doing here. We would still see 1/3, 2/3, 1/4, 2/4 genotypes and the kid would still carrie Angelman syndrome due to a deffective UBE3A gene.

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