2. (25 points) In humans, insertion of retrotransposons has occasionally been as

Question

2. (25 points) In humans, insertion of retrotransposons has occasionally been associated with disease. An L1 element inserted into the gene for blood clotting fact VIII resulted in hemophilia in two separate individuals. An Alu insertion disrupted processing of an mRNA, resulting in neurofibromatosis in another person. In each case, the insertion was not present in the genotypes of either of the affected individual's parents. Given that both of these elements are very common in human repetitive DNA, how is that examples of these spontaneous mutations are not more common?

Explanation / Answer

Approximately 45% of the human genome is composed of transposable elements. Retrotransposons have played a major role in the evolution of the human genome. However, transposons have become inactive in the human genome now. Transposition of an Alu element is a rare phenomenon with a frequency of only one in 10^5 – 10^7 cells per generation.

Since transposon movement from one genomic location to the other can be destructive, it is not surprising to know that most of the transposon sequences in the human genome have become silent. This allows the genome to be relatively stable in spite of the prevalence of a large fraction of transposons.

Transposons require the enzyme 'Transposase' for their movement. In most of the cases, the expression of transposase genes is either suppressed by the accumulation of mutations or by epigenetic modifications. Hence, even though the TE sequences are intact, they can not be moved due to the absence of the functional enzymes. There are still a few active transposons in the human genome with functional transposase. When these transposons move, mutations occur.

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