Detection and characterization of seven novel protein S (PROS) gene lesions: evaluation of reverse transcript-polymerase chain reaction as a mutation screening strategy

The molecular genetic analysis of protein S deficiency has been hampered by the complexity of the protein S (PROS) gene and by the existence of a homologous pseudogene. In an attempt to overcome these problems, a reverse transcript-polymerase chain reaction (RT-PCR) mutation screening procedure was developed. However, the application of this mRNA-based strategy to the detection of gene lesions causing heterozygous type I protein S deficiency appears limited owing to the high proportion of patients exhibiting absence of mRNA derived from the mutation-bearing allele (“allelic exclusion”). Nevertheless, this strategy remains extremely effective for rapid mutation detection in type II/III protein S deficiency. Using the RT-PCR technique, a G-to-A transition was detected at position +1 of the exon IV donor splice site, which was associated with type I deficiency and resulted in both exon skipping and cryptic splice site utilization. No abnormal protein S was detected in plasma from this patient. A missense mutation (Asn 217 to Ser), which may interfere with calcium binding, was also detected in exon VIII in a patient with type III protein S deficiency. A further three PROS gene lesions were detected in three patients with type I deficiency by direct sequencing of exon-containing genomic PCR fragments: a single base-pair (bp) deletion in exon XIV, a 2-bp deletion in exon VIII, and a G0to-A transition at position-1 of the exon X donor splice site all resulted in the absence of mRNA expressed from the disease allele. Thus, the RT-PCR methodology proved effective for further analysis of the resulting protein S-deficient phenotypes. A missense mutation (Met570 to Thr) in exon XIV of a further type III-deficient proband was subsequently detected in this patient's cDNA. No PROS gene abnormalities were found in the remaining four subjects, three of whom exhibited allelic exclusion. However, the father of one such patient exhibiting allelic exclusion was subsequently shown to carry a nonsense mutation (Gly448 to Term) within exon XII.