A new gene on the X involved in Fanconi anemia

X-linked VACTERL with hydrocephalus syndrome: further delineation of the phenotype caused by FANCB mutations

X-linked VACTERL-hydrocephalus syndrome (X-linked VACTERL-H) is a rare disorder caused by mutations in the gene FANCB which underlies Fanconi Anemia (FA) complementation group B. Cells from affected males have increased chromosome breakage on exposure to DNA cross-linking agents. Only five FANCB mutations found in six affected males, including an affected uncle and nephew, have been reported. We have identified FANCB mutations in a further four affected families. The VACTERL-H phenotype segregates as an X-linked recessive trait in three of these. Each mutation is predicted to truncate the FANCB open reading frame and results in highly skewed X-inactivation in unaffected carrier females. Phenotypic data were available on six affected males. Comparison of the clinical findings in our patients with published clinical data (total 12 patients) shows that ventriculomegaly, bilateral absent thumbs and radii, vertebral defects, renal agenesis, and growth retardation are the major phenotypic signs in affected males. Less frequent are brain, pituitary, ear and eye malformations, gastrointestinal atresias (esophageal, duodenal and anal), tracheoesophageal fistula, lung segmentation defects, and small genitalia. Three of six of our patients survived the perinatal period. One boy lived up to 2 years 10 months but developed aplastic anemia and died of renal failure. These data show that loss-of-function FANCB mutations result in a recognizable, multiple malformation phenotype in hemizygous males for which we propose clinical criteria to aid diagnosis.

An X chromosome gene, WTX, is commonly inactivated in Wilms tumor

Wilms tumor is a pediatric kidney cancer associated with inactivation of the WT1 tumor-suppressor gene in 5 to 10% of cases. Using a high-resolution screen for DNA copy-number alterations in Wilms tumor, we identified somatic deletions targeting a previously uncharacterized gene on the X chromosome. This gene, which we call WTX, is inactivated in approximately one-third of Wilms tumors (15 of 51 tumors). Tumors with mutations in WTX lack WT1 mutations, and both genes share a restricted temporal and spatial expression pattern in normal renal precursors. In contrast to biallelic inactivation of autosomal tumor-suppressor genes, WTX is inactivated by a monoallelic "single-hit" event targeting the single X chromosome in tumors from males and the active X chromosome in tumors from females.