At last, a good mutation? by Carl Wieland There are children who are so prone to infection that, if they survive at all, they have to spend their lives in an artificial ‘bubble’. This is the usual fate of those who have inherited two defective copies (one from each parent) of a gene which produces an enzyme called ADA (adenosine deaminase). Because they are unable to make ADA, toxic substances accumulate in their blood which slowly damage the body’s immune cells. However, in an unprecedented finding, a U.S. boy called Jordan Houghton has spontaneously recovered from his condition.1 All the evidence indicates that in one line of his immune cells, one of the faulty genes has apparently repaired itself. Geneticist Hagop Youssoufian at Brigham and Women’s Hospital, Boston, says about this 'fascinating' occurrence: ‘We finally have a clear example of a mutation doing something good’. ‘Back mutations’, replacing a letter in the DNA sequence which was faulty back to what it originally should have been, are not unknown. They certainly do not show us how significant information can arise de novo, as they merely (accidentally) ‘restore’ what should have been there. An occurrence like this (encouraging, but exquisitely rare) may actually not be mutational as such, as there are abundant error-checking, proof-reading and repair mechanisms in our genetic machinery. Youssoufian’s ‘at last’ statement highlights the fact that mutations, random accidental changes in copying hereditary information, are overwhelmingly a downhill process. Geneticists in hospitals are all too familiar with the harm they cause in people who inherit their effects. Reference - Cohen, P., 1996. Child's lethal gene fault heals itself. New Scientist, 151(2039):16.
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