Cathepsin K gene mutations and 1q21 haplotypes in patients with pycnodysostosis in an outbred population

The molecular genetics of the autosomal recessive disorder pycnodysostosis was studied in five independent families from an outbred Caucasian populati on. We found two new mutations and one recently described mutation in the c athepsin K gene by sequencing DNA from eight patients with pycnodysostosis: a one base transition in exon 8, c926T > C, causing a single amino acid su bstitution 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 famili es patients were homozygous for 926T > C. In the remaining two families pat ients were heterozygous for 926T > C and 121-1G > A in one case, and for 92 6T > C and 236G > A in the other case. Assays using genomic DNA were develo ped for all three mutations. We tested 150 healthy control persons and obse rved the mutation frequencies: 0 to 300 for 121-1G > A and 236G > A and 1 t o 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 x 10(-6) to P = 0.0004, and highly preserved area aro und the presumed disease locus was common to all the patients. This haploty pe was found on seven chromosomes identical by state, IBS, out of the possi ble 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 pa tient with pycnodysostosis is reported.