11-18-05

Solution to Problem S-11


Problem S-11: "f alleles of CFTR gene"

(a) The CFTR protein contains 1480 amino acids. Thus, it takes 1480x3=4440 nucleotides of mRNA to code for this protein. But, since the mRNA is 6129 nucleotides long, there are 6129-4440=1689 untranslated nucleotides in the mRNA. Untranslated regions are at the 5' and 3' ends and the summation of their lengths in this case is 1689 nucleotides. Exon 1 is entirely part of the untranslated 5' region, and exon 24 is entirely part of the 3' untranslated region. Mutations in these regions certainly do occur, but almost all of them do not have a noticeable phenotypic effect, because they do not prevent a correct CFTR protein from getting made. So, an allele with a mutation somewhere in exon 1 or exon 24 would still most likely be considered to be a wild-type F allele.

(b)
* A missense mutation in exon 12 or 13 results in a protein that has just one amino acid different in the protein (somewhere between about aa #450 to #550 or so). The protein is still the same size (1480 amino acids) as the wildtype protein.

* A nonsense mutation in exon 12 or 13 results in a truncated (shortened) protein that is about 450 to 550 or so amino acids long.

* A frame-shift mutation in exon 12 or 13 results in a protein that has a completely different amino acid sequence beyond the site of the mutation, and almost always the protein is truncated (due to there being a UAG, UAA, or UGA sequence quite soon while reading the mRNA "out of frame".) So, in this case the first 450-550 or so amino acids are correct, then there would be a stretch (of some length) of random amino acid sequence, with the total length of the protein probably being much shorter than the wildtype protein.

* An in-frame deletion results in a protein that is missing an amino acid at a certain position in the protein (the rest of the amino acid sequence is unaltered). In the case of the most common f allele, delta-F-508, the missing amino acid is the 508'th in the protein, so the protein is 1479 amino acids long, identical to the wildtype protein except that it is missing phenylalanine at position 508.

* Splicing mutations are located close to exon-intron boundaries. If the mutation prevents intron 12 from being removed, or causes the removal to be inaccurate, the most likely result will be the same as for a frame-shift mutation. That is, as translation continues into the "intron 12 region" in the mRNA, nucleotide triplet sequences of the intron will be read as codons, and quite soon a UAA, UAG, or UGA sequence will occur that will terminate translation.

In all of the cases above, if the mutated allele is identified as "f " rather than "F ", it means that the protein made by this allele does NOT fold up into a properly functioning ion channel protein.