Prion disease is neurodegenerative disorders, which is caused by misfolded prion

Question

Prion disease is neurodegenerative disorders, which is caused by misfolded prion protein. Recessive mutation in an autosomal gene PRNP have been linked to prion disease. A number of mutations have been identified in this gene that may result in prion disease. Based on data from a survey of 50,000 human population, calculate the penetrance for the following four mutations: Mutation 1 (D178N): the frequency of mutant allele (p1) is 1%, 2 homozygous p1p1 people were diagnosed as prion patient. Mutation 2 (V210I): the frequency of mutant allele (p2) is 3%, 25 homozygous p2p2 people were diagnosed as prion patient. Mutation 3 (M232R): the frequency of mutant allele (p3) is 2%, 20 homozygous p3p3 people were diagnosed as prion patient. Mutation 4 (T188R): the frequency of mutant allele (p4) is 5%, 10 homozygous p4p4 people were diagnosed as prion patient.

Explanation / Answer

Penetrance can be defined as the proportion of the individuals with the mutation who exhibit clinical symptoms for the condition.

1) Frequency of mutation in p1 is 1%, thus out of 50,000 individuals, 500 people should be homozygous to the mutation, but only 2 people were diagnosed with the condition. Therefore, 2/500 people show clinical symptoms. Therefore, the penetrance of the mutation is {(2/500)*100}= 0.4%. This mutation has a penetrance of 0.4%.

2) Frequency of mutation in p2 is 3%, thus out of 50,000 individuals, 1500 people should be homozygous to the mutation, but only 25 people were diagnosed with the condition. Therefore, 25/1500 people show clinical symptoms. Therefore, the penetrance of the mutation is {(25/1500)*100}= 1.66%. This mutation has a penetrance of 1.66%.


3) Frequency of mutation in p3 is 2%, thus out of 50,000 individuals, 1000 people should be homozygous to the mutation, but only 20 people were diagnosed with the condition. Therefore, 20/1000 people show clinical symptoms. Therefore, the penetrance of the mutation is {(20/1000)*100}= 2%. This mutation has a penetrance of 2%.


4) Frequency of mutation in p4 is 5%, thus out of 50,000 individuals, 2500 people should be homozygous to the mutation, but only 10 people were diagnosed with the condition. Therefore, 10/2500 people show clinical symptoms. Therefore, the penetrance of the mutation is {(10/2500)*100}= 0.4%. This mutation has a penetrance of 0.4%.

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