Two novel germline KRAS mutations: expanding the molecular and clinical phenotype.

Noonan and Cardio-facio-cutaneous (CFC) syndromes are characterized by typical dysmorphic features, cardiac defects, short stature, variable ectodermal anomalies, and intellectual disability. Both belong to the Ras/mitogen-activated protein kinase pathway group of disorders and clinical features overlap other related conditions, notably LEOPARD and Costello syndromes. KRAS mutations account for about 2% of reported Noonan and <5% of reported CFC cases. The mutation spectrum includes recurrent missense changes clustering in particular domains of the KRAS protein and conferring gain-of-function. We report three patients from two unrelated families with novel missense KRAS mutations, p.K147E and p.Y71H. Both mutations affect a residue which is highly conserved in KRAS and other RAS isoforms. One of the families includes a mother and son pair who represent the first report of a vertically transmitted KRAS mutation. In addition, the mother and son pair had peripheral neuropathy, complicated by Charcot arthropathy in the mother. An unusual phenotypic effect of the specific KRAS mutation or a coincidence of two independent disorders may be considered. KRAS mutation-associated phenotypes appear to be subject to considerable clinical heterogeneity. All three cases highlight the challenges of clinical assessment in KRAS mutation-positive patients, and the utility of molecular testing as an adjunct to diagnosis.

Pathogenic aberrations revealed exclusively by single nucleotide polymorphism (SNP) genotyping data in 5000 samples tested by molecular karyotyping.

BACKGROUND: Several recent studies have demonstrated the use of single nucleotide polymorphism (SNP) arrays for the investigation of intellectual disability, developmental delay, autism or congenital abnormalities. In addition to LogR 'copy number' data, these arrays provide SNP genotyping data for gene level autozygosity mapping, estimating low levels of mosaicism, assessing long continuous stretches of homozygosity (LCSH), detection of uniparental disomy, and 'autozygous' regions. However, there remains little specific information on the clinical utility of this genotyping data. METHODS: Molecular karyotyping, using SNP array, was performed on 5000 clinical samples. RESULTS: Clinically significant 'LogR neutral' genotyping abnormalities were detected in 0.5% of cases. Among these were a single case of chimerism, 12 cases with low level chromosome mosaicism, and 11 cases with an LCSH associated with uniparental disomy. In addition, the genotyping data revealed several LCSH associated with clinically relevant 'recessive type' genetic defects. CONCLUSIONS: These results demonstrate the utility of SNP genotyping data for detection of clinically significant abnormalities, including chimerism/mosaicism and recessive Mendelian disorders associated with autozygosity. The incidence of clinically significant low level mosaicism inferred from these cases suggests that this has hitherto been underestimated and chromosome mosaicism frequently occurs in the absence of indicative clinical features. The growing appreciation among clinicians and demand for SNP genotyping data poses significant challenges for the interpretation of LCSH, especially where there is no detailed phenotypic description to direct laboratory analysis. Finally, reporting of unexpected or hidden consanguinity revealed by SNP array analysis raises potential ethical and legal issues.

Mutation update for the CSB/ERCC6 and CSA/ERCC8 genes involved in Cockayne syndrome.

Cockayne syndrome is an autosomal recessive multisystem disorder characterized principally by neurological and sensory impairment, cachectic dwarfism, and photosensitivity. This rare disease is linked to mutations in the CSB/ERCC6 and CSA/ERCC8 genes encoding proteins involved in the transcription-coupled DNA repair pathway. The clinical spectrum of Cockayne syndrome encompasses a wide range of severity from severe prenatal forms to mild and late-onset presentations. We have reviewed the 45 published mutations in CSA and CSB to date and we report 43 new mutations in these genes together with the corresponding clinical data. Among the 84 reported kindreds, 52 (62%) have mutations in the CSB gene. Many types of mutations are scattered along the whole coding sequence of both genes, but clusters of missense mutations can be recognized and highlight the role of particular motifs in the proteins. Genotype-phenotype correlation hypotheses are considered with regard to these new molecular and clinical data. Additional cases of molecular prenatal diagnosis are reported and the strategy for prenatal testing is discussed. Two web-based locus-specific databases have been created to list all identified variants and to allow the inclusion of future reports (www.umd.be/CSA/ and www.umd.be/CSB/).

Restricted IgG1 subclass of anti-Yo antibodies in paraneoplastic cerebellar degeneration.

Paraneoplastic cerebellar degeneration (PCD) occurs as a non-metastatic manifestation of cancer in a small proportion of patients with certain breast or gynaecological tumours, and is characterised by widespread Purkinje cell loss. Antibodies against a Purkinje cell cytoplasmic antigen, called Yo, that is expressed by the tumours, are present in the majority of these patients, but the pathogenic role of the antibodies is not clear. To characterise further the immune response in these cases, 13 anti-Yo positive sera were tested for IgG subclasses by immunohistochemistry and western blotting and, in four cases, PHA-stimulated cytokine secretion by peripheral blood lymphocytes was measured. Surprisingly, anti-Yo antibodies were entirely restricted to the IgG1 subclass, whereas antibodies against the small cell cancer-associated antigen, Hu, were found in all four IgG subclasses. There was a trend towards raised IgG1 levels in the total IgG of the anti-Yo positive patients and, in two, PHA-stimulated peripheral blood lymphocytes secreted raised levels of IFN-gamma. By contrast, in the other two cases tested, raised levels of IL-4 were secreted. Patients with PCD associated with anti-Yo antibodies appear to have strong immune responses that are polarised with respect to the IgG subclass and Th cytokine profiles.

Sixteen New Cases Contributing to the Characterization of Patients with Distal 22q11.2 Microduplications.

The chromosome region 22q11.2 has long been recognized to be susceptible to genomic rearrangement. More recently, this genomic instability has been shown to extend distally (involving LCR22E-H) to the commonly deleted/duplicated region. To date, 21 index cases with 'distal' 22q11.2 duplications have been reported. We report on the clinical and molecular characterization of 16 individuals with distal 22q11.2 duplications identified by DNA microarray analysis. Two of the individuals have been partly described previously. The clinical phenotype varied among the patients in this study, although the majority displayed various degrees of developmental delay and speech disturbances. Other clinical features included behavioral problems, hypotonia, and dysmorphic facial features. Notably, none of the patients was diagnosed with a congenital heart defect. We found a high degree of inherited duplications. Additional copy number changes of unclear clinical significance were identified in 5 of our patients, and it is possible that these may contribute to the phenotypic expression in these patients as has been suggested recently in a 2-hit 'digenic' model for 16p12.1 deletions. The varied phenotypic expression and incomplete penetrance observed for distal 22q11.2 duplications makes it exceedingly difficult to ascribe pathogenicity for these duplications. Given the observed enrichment of the duplication in patient samples versus healthy controls, it is likely that distal 22q11.2 duplications represent a susceptibility/risk locus for speech and mild developmental delay.