Identification of a Novel Mutation in SASH1 Gene in a Chinese Family With Dyschromatosis Universalis Hereditaria and Genotype-Phenotype Correlation Analysis

Noonan syndrome (MIM 163950) is an autosomal dominant disorder characterized by dysmorphic facial features, proportionate short stature and heart disease (most commonly pulmonic stenosis and hypertrophic cardiomyopathy). Webbed neck, chest deformity, cryptorchidism, mental retardation and bleeding diatheses also are frequently associated with this disease. This syndrome is relatively common, with an estimated incidence of 1 in 1,000-2,500 live births. It has been mapped to a 5-cM region (NS1) [corrected] on chromosome 12q24.1, and genetic heterogeneity has also been documented. Here we show that missense mutations in PTPN11 (MIM 176876)-a gene encoding the nonreceptor protein tyrosine phosphatase SHP-2, which contains two Src homology 2 (SH2) domains-cause Noonan syndrome and account for more than 50% of the cases that we examined. All PTPN11 missense mutations cluster in interacting portions of the amino N-SH2 domain and the phosphotyrosine phosphatase domains, which are involved in switching the protein between its inactive and active conformations. An energetics-based structural analysis of two N-SH2 mutants indicates that in these mutants there may be a significant shift of the equilibrium favoring the active conformation. This implies that they are gain-of-function changes and that the pathogenesis of Noonan syndrome arises from excessive SHP-2 activity.

Multiple-lentigines (ML)/LEOPARD (multiple lentigines, electrocardiographic-conduction abnormalities, ocular hypertelorism, pulmonary stenosis, abnormal genitalia, retardation of growth, and sensorineural deafness) syndrome is an autosomal dominant condition--characterized by lentigines and café au lait spots, facial anomalies, cardiac defects--that shares several clinical features with Noonan syndrome (NS). We screened nine patients with ML/LEOPARD syndrome (including a mother-daughter pair) and two children with NS who had multiple café au lait spots, for mutations in the NS gene, PTPN11, and found, in 10 of 11 patients, one of two new missense mutations, in exon 7 or exon 12. Both mutations affect the PTPN11 phosphotyrosine phosphatase domain, which is involved in <30% of the NS PTPN11 mutations. The study demonstrates that ML/LEOPARD syndrome and NS are allelic disorders. The detected mutations suggest that distinct molecular and pathogenetic mechanisms cause the peculiar cutaneous manifestations of the ML/LEOPARD-syndrome subtype of NS.

Loss of heterozygosity (LOH) and in silico expression analysis were applied to identify genes significantly downregulated in breast cancer within the genomic interval 6q23-25. Systematic comparison of candidate EST sequences with genomic sequences from this interval revealed the genomic structure of a potential target gene on 6q24.3, which we called SAM and SH3 domain containing 1 (SASH1). Loss of the gene-internal marker D6S311, found in 30% of primary breast cancer, was significantly correlated with poor survival and increase in tumor size. Two SASH1 transcripts of approximately 4.4 and 7.5 kb exist and are predominantly transcribed in the human breast, lung, thyroid, spleen, placenta and thymus. In breast cancer cell lines, SASH1 is only expressed at low levels. SASH1 is downregulated in the majority (74%) of breast tumors in comparison with corresponding normal breast epithelial tissues. In addition, SASH1 is also downregulated in tumors of the lung and thyroid. Analysis of the protein domain structure revealed that SASH1 is a member of a recently described family of SH3/SAM adapter molecules and thus suggests a role in signaling pathways. We assume that SASH1 is a new tumor suppressor gene possibly involved in tumorigenesis of breast and other solid cancers. We were unable to find mutations in the coding region of the gene in primary breast cancers showing LOH within the critical region. We therefore hypothesize that other mechanisms as for instance methylation of the promoter region of SASH1 are responsible for the loss of expression of SASH1 in primary and metastatic breast cancer.