Founder effect and linkage disequilibrium have been successfully exploited to map single gene disorders, and the study of isolated populations is emerging as a major approach to the investigation of genetically complex diseases. In the search for genetic isolates ranging from Pacific islands to Middle East deserts, the 10 million Gypsies resident in Europe have largely escaped the attention of geneticists. Because of their geographical ubiquity, lack of written history and the presumed social and cultural nature of their isolation, Gypsies are construed as not meeting the criteria for a well defined founder population. Gypsy society has a complex structure with subdivisions and stratifications that are incomprehensible to the surrounding populations. Marginalization by the health care systems in most countries results in a lack of information on causes of morbidity and mortality and little is known about hereditary disorders or the population genetic characteristics of Gypsies. This study is the first example of mapping a disease gene in endogamous Gypsy groups. Using lod score analysis and linkage disequilibrium, we have located a novel demyelinating neuropathy to a narrow interval on chromosome 8q24. We show that the disease, occurring in Gypsy groups of different identity and history of migrations, is caused by a single mutation whose origin predates the divergence of these groups.

The presence of two different mutations carried by the same CF allele has been demonstrated in four out of 44 Bulgarian CF patients during a systematic search of the entire coding sequence of the CFTR gene. Two of the double mutant alleles include one nonsense and one missense mutation and although the nonsense mutation can be considered to be the main defect, the amino acid substitutions are good candidates for disease-causing mutations as well. One double mutant carries two missense mutations whose contribution to the CF phenotype is difficult to evaluate. The findings suggest that double mutant alleles may be more common than expected and could account for some of the problems in phenotype-genotype correlations. Such alleles may have important implications for molecular diagnosis and genetic counselling.

Over 1500 adenomatous polyposis coli (APC) gene mutations have already been identified as causative of familial adenomatous polyposis (FAP). However, routine genetic testing fails to detect mutations in about 10% of classic FAP cases. Recently, it has been shown that a proportion of mutation-negative FAP cases bear molecular changes in deep intronic and regulatory sequences. In this study, we used direct sequencing, followed by multiplex ligation-dependent probe amplification (MLPA) of genomic DNA from family members, affected by classic FAP. We first reported the family as mutation negative. With the launch of a new version of MLPA kit, we retested the family and a novel full deletion of promoter 1B was detected. The exact breakpoints of the deletion were determined by array comparative genomic hybridization (CGH) and long range polymerase chain reaction (PCR), followed by direct sequencing. The total APC expression levels were investigated by quantitative polymerase chain reaction (qPCR) assay and allele-specific expression (ASE) analysis. The APC gene expression was highly reduced, which indicates causative relationship. We suggest that there is a significant possibility that APC promoter 1B mutations could be found in mutation-negative FAP patients. In the light of our findings it seems reasonable to consider targeted genetic re-analysis of APC promoter 1B region in a larger cohort of unsolved cases.

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