PubMed · 9211184
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P S Harper. 1997. Trinucleotide repeat disorders.. https://doi.org/10.1023/a%3A1005388218625
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The importance of the interaction between basic science and clinical practice has long been known but it has become even more evident in the past few decades with the impressive rate of development in the field of molecular genetics. This short article reviews molecular diagnosis of two different diseases for which scientific progress has immediately been translated into a dramatic improvement of the quality of medical care: the Fragile X Syndrome, paradigm of the new mutational mechanism of the unstable triplet repeats, and von Hippel-Lindau disease, a recent acquisition in the growing number of familial cancer syndromes.
The human genome, made of about 3 billion bases, encodes between 75 and 100,000 genes. However, most of the genome is made of non coding sequences, whose function is still unknown. When a base variation occurs in a DNA fragment, base change, deletion or insertion of one or several bases, a mutation or a polymorphism is generated depending whether this base change modifies or not the function of the encoded gene. In 1991, a new type of mutation has been discovered, namely the expansion beyond a critical length of a three-base repeat, called triplet. These anomalies due to genome instability are not rare and are now responsible for at least twelve diseases. It is expected that other diseases due to the same mechanism will be discovered in the near future. This article illustrates the concept of mutation by triplet expansion and presents 3 diseases frequently observed in Pediatrics: the fragile X syndrome, myotonic dystrophy and Friedreich's ataxia.
We have analysed the size of the non-expanded FRAXA CGG repeat in 385 male patients affected by mental retardation and in 182 unrelated normal chromosomes as control. The results show that intermediate alleles with more than 40 repeats were not significantly more frequent in patients than in controls. These data do not corroborate previous findings supporting the idea that intermediate alleles may have a deleterious effect on mental retardation.