MspI RFLP at CRYB1 locus (17q11.2----17q12)

Prenatal diagnosis of neurofibromatosis type 1: from flanking RFLPs to intragenic microsatellite markers

Even though the neurofibromatosis type 1 (NF1) gene was cloned more than 3 years ago, the process of identifying mutations has not been fruitful, and genetic counselling is mainly based on the use of linked markers. Since 1990, we have analysed 130 NF1 families and have performed six prenatal diagnoses. In each case, genetic counselling has relied on linked markers and informativity was achieved in all of them. The use of intragenic microsatellite polymorphisms (IVS27AAAT2.1, IVS27AC28.4, IVS27AC33.1, and IVS38GT53.0) has increased the informativeness in our series of NF1 families to an average of 90 per cent, providing accurate diagnosis and confirmation of the disease status.

Molecular analysis of a patient with neurofibromatosis 1 and achondroplasia

The gene for von Recklinghausen neurofibromatosis (NF1) is on proximal 17q; the location of the gene for achondroplasia (ACH) is unknown. We have begun a molecular analysis of a patient with mental retardation, NF1 and ACH, a clinical presentation suggestive of a contiguous gene syndrome. In addition, this individual has a 47,XYY chromosome constitution. To define a possible chromosome 17 deletion, we investigated the copy number of DNA sequences linked to NF1 with conventional and pulsed-field gel electrophoresis (PFGE). We found no evidence for a deletion on chromosome 17. These results make it unlikely that this patient harbors a single deletion in the NF1 region causing both NF1 and ACH and suggest different mechanisms for the de novo occurrence of 2 autosomal dominant disorders in this individual.

The achondroplasia gene is not linked to the locus for neurofibromatosis 1 on chromosome 17

We have investigated genetic linkage of von Recklinghausen neurofibromatosis (NF1) and achondroplasia (ACH) using chromosome-17 markers that are known to be linked to NF1. Physical proximity of the two loci was suggested by the report of a patient with mental retardation and the de novo occurrence of both NF1 and ACH. Since the chance of de novo occurrence of these two disorders in one individual is 1 in 600 million, this suggested a chromosomal deletion as a single unifying molecular event and also that the ACH and NF1 loci might be physically close. To test this, we performed linkage analysis on a three-generation family with ACH. We used seven DNA probes that are tightly linked to the NF1 locus, including DNA sequences that are known to flank the NF1 locus on the centromeric and telomeric side. We detected two recombinants between the ACH trait and markers flanking the NF1 locus. In one recombinant, the flanking markers themselves were nonrecombinant. Multi-point linkage analysis excluded the ACH locus from a region surrounding the NF1 locus that spans more than 15 cM (lod score less than -2). Therefore, analysis of this ACH pedigree suggests that the ACH locus is not linked to the NF1 locus on chromosome 17.

A genetic map of human chromosome 17p

A genetic linkage map was constructed with 18 loci from the short arm and pericentric region of chromosome 17 typed on the CEPH reference families. The genetic map includes three markers extracted from the CEPH public database. Nine loci could be ordered using a threshold of odds of at least 1000:1 against alternative orders during the map construction process. With a reduced tolerance of 100:1, a total of 13 loci could be placed on the map spanning a distance of approximately 60 cM in females and 46 cM in males. There were statistically significant differences between the male and the female genetic maps. The order inferred from the genetic data was consistent with the physical localizations of these probes obtained from somatic cell hybrids and tumor deletion studies. This map should be useful for genetic fine mapping of 17p loci.