DNA analysis in patients with hereditary fructose intolerance

Hereditary fructose intolerance

Hereditary fructose intolerance (HFI) is an inborn error of carbohydrate metabolism that is inherited as an autosomal recessive condition. The disease is caused by a catalytic deficiency of aldolase B and is characterized by severe abdominal symptoms and hypoglycaemia which follow the ingestion of fructose, sucrose or sorbitol. The exact prevalence of HFI in different populations is unknown but studies from Switzerland suggest a disease frequency of about 1 in 20,000 live births, thus predicting a carrier frequency of greater than 1% and a gene prevalence that approaches polymorphic frequency. It is notable that many patients who endure severe symptoms during early infancy develop a marked aversion to harmful foodstuffs and thereby survive to adulthood. Although exposure to fructose may prove to be fatal in this disorder, institution of a strict exclusion diet is curative. HFI, when treated rigorously after diagnosis, is thus compatible with a long and healthy life. HFI vividly illustrates the interplay of dietary factors and heredity in the development of human disease. The recent identification of genetic lesions that cause this disorder further demonstrates the remarkable clinical benefits that may accrue from the study of the molecular basis of inherited diseases and its population genetics: it is now possible to detect asymptomatic disease carriers and diagnose the disorder in affected families by non-invasive analysis of small samples of genomic DNA. Moreover, the systematic investigation of natural mutations in the human gene for aldolase B has allowed regions that are critical for catalytic function of this enzyme to be identified as part of an extended study of its molecular biology.

Mapping of a restriction fragment length polymorphism within the human aldolase B gene

Peripheral blood DNA was hybridized to the full-length cDNA and the cloned structural gene of human aldolase B. With PvuII endonuclease a restriction fragment length polymorphism was detected that was present in the heterozygous state in about 21% of the individuals tested. A map of the human aldolase gene was constructed for the two groups of individuals found to produce different fragments after PvuII digestion. This allowed the localization of the polymorphic site within the gene, which was found to be due to the loss of a PvuII site in the last intron upstream from the 3' end. This polymorphism may be used as a genetic marker to study individuals affected by hereditary fructose intolerance.

Molecular mechanisms of McArdle's disease (muscle glycogen phosphorylase deficiency). RNA and DNA analysis

Lack of muscle glycogen phosphorylase activity leads to McArdle's disease, a rare metabolic myopathy. To investigate its molecular basis at the nucleic acid level, we isolated muscle phosphorylase cDNA clones from a human cDNA library in Escherichia coli plasmid pBR 322. Subcloning of one insertion of M13 bacteriophage permitted its definite identification by sequencing. Northern blot experiments revealed one specific messenger RNA of 3.4 kilobases found uniquely in tissues expressing muscle phosphorylase. We show that McArdle's disease exhibits a molecular heterogeneity at the messenger RNA level. In eight unrelated cases of McArdle's disease in which no inactive proteins had been detected, we assayed muscle biopsies for phosphorylase mRNA by Northern blotting. In five cases, no muscle phosphorylase mRNA could be detected, while in three other cases, normal length mRNA was present in lower amounts. Moreover, Southern blot analysis of DNA isolated from white blood cells in four McArdle patients revealed no major deletion or rearrangements of the phosphorylase gene as compared with controls.

Diagnosis of genetic disease using recombinant DNA

Recombinant DNA technology promises to make an important contribution to the analysis and diagnosis of inherited human disease. Direct detection and analysis of various genetic defects at the DNA level are now possible using cloned gene or oligonucleotide probes. In addition, the use of restriction fragment length polymorphisms associated with linked DNA segments should permit not only the diagnosis of hitherto undetectable disease states but also the chromosomal localization of the loci responsible. The eventual isolation of disease loci should lead to a better understanding of the molecular basis of inherited disease.