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, 30 (4), 695-702

Germline BRAF Mutations in Noonan, LEOPARD, and Cardiofaciocutaneous Syndromes: Molecular Diversity and Associated Phenotypic Spectrum

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Germline BRAF Mutations in Noonan, LEOPARD, and Cardiofaciocutaneous Syndromes: Molecular Diversity and Associated Phenotypic Spectrum

Anna Sarkozy et al. Hum Mutat.

Abstract

Noonan, LEOPARD, and cardiofaciocutaneous syndromes (NS, LS, and CFCS) are developmental disorders with overlapping features including distinctive facial dysmorphia, reduced growth, cardiac defects, skeletal and ectodermal anomalies, and variable cognitive deficits. Dysregulated RAS-mitogen-activated protein kinase (MAPK) signal traffic has been established to represent the molecular pathogenic cause underlying these conditions. To investigate the phenotypic spectrum and molecular diversity of germline mutations affecting BRAF, which encodes a serine/threonine kinase functioning as a RAS effector frequently mutated in CFCS, subjects with a diagnosis of NS (N=270), LS (N=6), and CFCS (N=33), and no mutation in PTPN11, SOS1, KRAS, RAF1, MEK1, or MEK2, were screened for the entire coding sequence of the gene. Besides the expected high prevalence of mutations observed among CFCS patients (52%), a de novo heterozygous missense change was identified in one subject with LS (17%) and five individuals with NS (1.9%). Mutations mapped to multiple protein domains and largely did not overlap with cancer-associated defects. NS-causing mutations had not been documented in CFCS, suggesting that the phenotypes arising from germline BRAF defects might be allele specific. Selected mutant BRAF proteins promoted variable gain of function of the kinase, but appeared less activating compared to the recurrent cancer-associated p.Val600Glu mutant. Our findings provide evidence for a wide phenotypic diversity associated with mutations affecting BRAF, and occurrence of a clinical continuum associated with these molecular lesions.

Figures

FIGURE 1
FIGURE 1. BRAF domain structure and location of residues altered in Noonan, LEOPARD and cardiofaciocutaneous syndromes
BRAF protein domains are indicated (CR, conserved region; RBD, RAS binding domain, CRD, cysteine-rich domain, AL, activation loop). (A) Mutations identified in the present study. Amino acid substitutions identified in Noonan syndrome, LEOPARD syndrome or cardiofaciocutaneous syndrome are indicated in red, green and black, respectively. Novel mutations are indicated in italics, while mutations documented to occur as somatic changes in human cancers are underlined. (B) Mutations previously identified in cardiofaciocutaneous syndrome (including individuals with a phenotype resembling Costello syndrome during infancy) (above the cartoon) or human cancers (under the cartoon) (prevalence ≥1.5%, according the COSMIC database, http://www.sanger.ac.uk/genetics/CGP/cosmic/). A heterozygous condition for the p.Lys499Glu and p.Glu501Lys substitutions has also been observed in two individuals diagnosed with NS [Razzaque et al., 2007; Nystrom et al., 2008]. (C) Histogram showing distribution and relative prevalence of germline (above panel) and somatic (lower panel) BRAF mutations in human disease (updated to June 2008).
FIGURE 2
FIGURE 2. Facial dysmorphia and other features of BRAF mutation-positive subjects
(A) LEOPARD syndrome; (B) Noonan syndrome; (C) cardiofaciocutaneous syndrome.
FIGURE 3
FIGURE 3. Functional characterization of selected germline transmitted BRAF mutations
(A) Focus assays of NIH-3T3 cells transfected with each of the BRAF constructs coding for the p.Thr241Pro (LS), p.Glu275Lys (CFCS), p.Trp531Cys (NS), p.Leu597Val (NS), p.Thr599Arg (CFCS) and p.Lys601Gln (CFCS) mutants, the cancer-associated p.Val600Glu mutant, and the wild type (WT) protein. Data illustrate results of at least three experiments performed in duplicate. (B) MEK and ERK phosphorylation assays in cells transiently expressing one of the six selected BRAF mutants, the cancer-associated p.Val600Glu or wild type BRAF proteins. Blots are representative of three experiments performed (left), while phosphorylation ratios (right) are expressed as the mean of three replicates ± SD.

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