Chapter 8, Problem 47P

You learned in Problem 21 in Chapter 7 that the neurodegenerative disease ALS can be caused by expansion of a hexanucleotide repeat region (5′-GGGGCC-3′) outside of the open reading frame (but within the first intron) of the gene called C9ORF72. While a normal C9ORF72 allele has 2–23 copies of the hexanucleotide repeat unit, dominant disease-causing alleles have hundreds or even thousands of copies.

Researchers observed that the first intron of the C9ORF72 disease allele is transcribed not only from the normal template strand of DNA, but also from the nontemplate strand. Even more unusual, both types of repeatregion transcripts are translated in all six reading frames in an AUG-independent manner—a process called repeat-associated non-ATG translation, or RAN translation. These discoveries led to the hypothesis that the proteins made from the repeats might contribute to ALS.

a.	What polypeptides are made from the repeat-region transcripts?
b.	According to the RAN translation hypothesis, why are disease-causing C9ORF72 alleles dominant to normal alleles?
c.	How would you classify the mutant alleles? Do they cause a loss of function or a gain of function? Are they amorphic, hypomorphic, hypermorphic, neomorphic, or antimorphic? (Note: More than one answer might be possible.)