¹Institute of Human Genetics, The Bartholin Building, Aarhus University, Denmark

²Department of Clinical Genetics, Aarhus University Hospital, Denmark

³Pediatric Department, Herning Central Hospital, Denmark

⁴Department of Clinical Biochemistry and Genetics, Odense University Hospital, Denmark

Correspondence to: Annette Haagerup MD, Institute of Human Genetics, University of Aarhus, DK-8000 Aarhus C, Denmark. Tel: +45 89 421672; Fax: +45 86 123137; E-mail: AH@humgen.au.dk

The molecular genetics of the autosomal recessive disorder pycnodysostosis was studied in five independent families from an outbred Caucasian population. We found two new mutations and one recently described mutation in the cathepsin K gene by sequencing DNA from eight patients with pycnodysostosis: a one base transition in exon 8, c926T > C, causing a single amino acid substitution leucine  proline, L309P; A 3' splice site mutation in intron 2, c121-1G > A, causing deletion of all exon 3, 41V-81Mdel; and the exon 3 missense mutation c236G > A leading to residue G79E. In three of the families patients were homozygous for 926T > C. In the remaining two families patients were heterozygous for 926T > C and 121-1G > A in one case, and for 926T > C and 236G > A in the other case. Assays using genomic DNA were developed for all three mutations. We tested 150 healthy control persons and observed the mutation frequencies: 0 to 300 for 121-1G > A and 236G > A and 1 to 150 for 926T > C. One patient from each family was haplotyped with eight microsatellite markers surrounding the cathepsin K gene on chromosome 1q21. A very rare, P = 1.8 ´ 10^-6 to P = 0.0004, and highly preserved area around the presumed disease locus was common to all the patients. This haplotype was found on seven chromosomes identical by state, IBS, out of the possible eight carrying the 926T > C mutation. Founder effect, locus homogeneity, and allele heterogeneity regarding pycnodysostosis within this population are discussed. Finally, the first pregnancy and delivery described in a patient with pycnodysostosis is reported. European Journal of Human Genetics (2000) 8, 431-436.

pycnodysostosis; cathepsin K (CTSK) gene; mutation; haplotype; founder effect