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High-resolution mapping of the gene for cystinosis, using combined biochemical and linkage analysis
G Jean1, A Fuchshuber, M M Town
1INSERM U423, Hôpital Necker-Enfants Malades, Paris, France.
Insights
Infantile nephropathic cystinosis, a genetic disorder, involves high cystine levels. Linkage analysis in families confirmed the cystinosis gene locus on chromosome 17, refining its location.
Area of Science:
- Genetics
- Molecular Biology
- Pediatric Nephrology
Background:
- Infantile nephropathic cystinosis is an autosomal recessive disorder causing severe intracellular cystine accumulation due to lysosomal membrane transport defects.
- Affected children exhibit Fanconi syndrome and progressive kidney failure within the first decade.
- Measuring leukocyte cystine levels aids in diagnosing cystinosis and identifying carriers.
Purpose of the Study:
- To localize the gene locus responsible for cystinosis.
- To refine the genetic interval of the cystinosis gene.
- To assess the utility of carrier phenotypes in linkage analysis for autosomal recessive disorders.
Main Methods:
- Linkage analysis was performed on 18 cystinosis families, including simplex families.
- Phenotypes of heterozygous carriers, determined by leukocyte cystine content, were incorporated into the analysis.
- Genetic recombination events were analyzed to narrow the gene locus interval.
Main Results:
- Linkage analysis confirmed the cystinosis gene locus on the short arm of chromosome 17.
- The gene interval was refined to a genetic distance of 1 centimorgan (cM).
- No evidence of genetic heterogeneity was detected.
Conclusions:
- The cystinosis gene locus is confirmed on chromosome 17p.
- Incorporating carrier phenotypes enhances linkage analysis for simplex families with autosomal recessive traits.
- This approach provides a reliable method for investigating genetic disorders in families with limited affected individuals.
Abstract:
Infantile nephropathic cystinosis is an autosomal recessive disorder characterized biochemically by an abnormally high intracellular content of free cystine in different organs and tissues due to a transport defect of cystine through the lysosomal membrane. Affected children present with the Fanconi syndrome and usually develop progressive renal failure within the 1st decade of life. Measurement of free cystine in purified polymorphonuclear leukocytes provides an accurate method for diagnosis and detection of heterozygous carriers. In order to localize the gene locus for cystinosis we performed linkage analysis in 18 cystinosis families. However, since 17 of these were simplex families, we decided to include the phenotypes of the heterozygous carriers previously determined by their leukocyte cystine content in the linkage analysis. This approach allowed us to obtain highly significant results, confirming the localization of the cystinosis gene locus recently mapped to the short arm of chromosome 17 by the Cystinosis Collaborative Research Group. Crucial recombination events allowed us to refine the interval of the cystinosis gene to a genetic distance of 1 cM. No evidence of genetic heterogeneity was found. Our results demonstrate that the use of the previously determined phenotypes of heterozygous carriers in linkage analysis provides a reliable method for the investigation of simplex families in autosomal recessive traits.