3. Point mutations or small deletions in the cytoplasmic tail domain of the Fas

Question

3. Point mutations or small deletions in the cytoplasmic tail domain of the Fas receptor are associated with autoimmune lymphoproliferative syndrome (ALPS). In individuals heterozygous for such mutations ymphocytes do not die at their normal rate and accumulate to abnormally high numbers the Fas receptor entirely exhibit no clinical symptoms ms. By contrast, individuals heterozygous for mutations that eliminate expression of toplasmic ta planation for h ow heterozygous expression of a Fas receptor with only small changes to the ds to apoptosis defects and a clinical syndrome. Include a description of what happens to Fas receptors (6 points) receptor activation and initiati on of apoptotic sianaling. [Note: Assume the wildtype and mutant Fas e expressed at equal levels and have the same affinity for Fas ligand.] Feel free to draw a diagram. B) Using similar analysis, provide an explanation for why the absence of expression of Fas receptor from one chromosome does not lead to apoptosis defects. (2 points)

Explanation / Answer

Fas mutations associated with ALPS are dominant because they reduce the number of normal fas-fas ligand complexes by a factor of eight in heterozygotes .mutations that eliminates fas expression are recessive because they reduce the number of fas -fas ligand complexes by only a factor of two.

One eight of fas-fas ligand complexes on the lymphocytes from an individual with ALPS would be expected to be composed entirely of normal fas subunits.since half the fas protein in the lymphocytes is normal and there are three fas sub units per complex ,the probability of three normal fas sub units coming together in a complex is (1/2)3.

In an individual heterozygous for a mutation that eliminates the fas expression all the expressed fas protein would be normal.thus 100%of the fas fas ligand complex’s would be composed entirely of normal fas subunits . The total number of fas molecules however would be half the number present in an individual with the two normal genes for fas.

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