We have carried out mutation analysis on the FANCB gene in a pedigree with X linked VACTERL‐H. A mutation which causes abnormal splicing of the FANCB transcript was identified. This mutation causes a frameshift in the FANCB open reading frame which results in a premature stop codon and is likely to cause nonsense mediated decay of the abnormally spliced FANCB mRNA. This is the first mutation to be associated with X linked VACTERL‐H syndrome.
Our data, together with the reported clinical findings in four other unrelated affected males from different pedigrees, show that the growth retardation and bilateral radial ray defects associated with FANCB
mutations appear to be highly penetrant (six of six affected).13
Head and kidney abnormalities, together with hypogonadism are also very common associations (five of six affected). However, owing to the small number of reported cases, ascertainment bias for these phenotypic associations cannot be excluded at the moment. Unfortunately, it is not possible to discern from published data on the other reported males with FANCB
mutations whether hydrocephalus was part of the phenotype in these individuals.13
Three other VACTERL‐H pedigrees have been reported in which the phenotype is likely to be X linked.8,9,10
Symmetrical radial ray abnormalities and hydrocephalus were present in 10 of 10 affected males from these families, and renal agenesis and genital abnormalities are reported in all three pedigrees. Other congenital anomalies including vertebral defects, congenital heart disease, tracheo‐oesophageal fistula, and gut atresias have been described as part of the X linked VACTERL‐H syndrome in these reports. However, none of the affected males has survived, and necropsies were not always carried out. It is therefore difficult to establish whether these additional congenital anomalies occurred in affected males from all three families.
Interestingly, although hydrocephalus appears to be a highly penetrant trait, the anatomical causes appear to vary both within and between families. The affected fetus of the proband in our family had a patent aqueduct and his uncle was reported to have an Arnold‐Chiari malformation associated with a lower spina bifida occulta. Interestingly, in the two affected male cousins reported by Genuardi et al
, one also had hydrocephalus associated with an Arnold‐Chiari malformation.8
In contrast, the affected males who underwent necropsy in the two families reported by Wang et al
and Lomas et al
were reported to have aqueduct stenosis.9,10
Thus, although the phenotypes observed in our family and the three other reported families are very similar, genetic heterogeneity cannot be ruled out at the moment. Amniocytes from an affected male reported by Lomas et al
did not show increased chromosome breakage when challenged with mitomycin C, and chromosome breakage studies were not carried out in the family reported by Genuardi et al
However, chromosomes from affected males reported by Wang et al
were found to have increased spontaneous breakages and sensitivity to mitomycin C, implicating a mutation in FANCB
as the underlying cause of the VACTERL‐H phenotype.9
It is clinically important to discriminate between Fanconi anaemia and VACTERL association. Hydrocephalus and growth retardation are not taken to be features of the VACTERL association. However, VACTERL can be readily distinguished from Fanconi anaemia by the increased sensitivity of Fanconi anaemia cells to DNA cross linking agents such as diepoxybutane.5
Distinguishing cases of X linked Fanconi anaemia‐B from autosomal recessive forms expressing the VACTERL‐H phenotype may be less straightforward, particularly as the majority of affected males with Fanconi anaemia‐B are likely to occur as single cases.11,12,13
However, for genetic counselling it is important for this distinction to be made so that the parents and relatives of affected males can be given accurate information about the recurrence risks. The FANCB
gene has been shown to undergo X inactivation, and the mutant allele to be preferentially inactivated in carrier females.13
X inactivation studies on suspected carrier females can therefore be helpful in suspected cases of Fanconi anaemia‐B: both carrier women in this report, and the three carrier females reported by Meetei et al
, have 100% skewing of X inactivation in peripheral blood leucocytes.13
Selection against blood cells expressing mutations in X linked genes is a well documented feature of several severe X linked recessive conditions, and complete skewing of X inactivation in the general population is uncommon.14,15,16
Thus, if non‐random X inactivation is observed in the mothers of males with VACTERL‐H, this should raise the suspicion of Fanconi anaemia‐B.13
Detailed fetal ultrasonography to monitor for radial ray defects, hydrocephalus, and renal abnormalities may be offered to women at risk of having an affected male fetus as a means of non‐invasive prenatal diagnosis. However, until the full phenotypic spectrum of Fanconi anaemia‐B is defined, chromosome breakage studies and mutation analysis on male fetuses at risk of Fanconi anaemia‐B will be necessary for accurate prenatal diagnosis.
There appears to be strong selection against cells expressing the mutant FANCB
allele in female carriers: 100% skewed X inactivation was observed in fibroblasts, blood cells, and urothelial cells from the carriers reported by Meetei et al
, and these individuals and the carrier females reported in this study all have normal clinical phenotypes.13
X inactivation occurs randomly in early embryogenesis, and once inactivated, the inactive status of an X chromosome is clonally persistent.17
The absence of congenital abnormalities in FANCB
carrier females suggests that selection against cells expressing the mutant FANCB
allele occurs early in their development.13
If so, early selection against cells expressing the FANCB
mutant allele may substantially reduce any additional cancer risks conferred by FANCB
mutations. Although this cannot be fully excluded at the moment, none of the three female carriers reported by Meetei et al
had been diagnosed with malignancy (ages 12, 29, and 43 years), and neither of the female carriers reported here have had cancer (ages 23 and 49 years, respectively).
In summary, we show that mutations in FANCB are a cause of X linked VACTERL‐H syndrome. Analysis of further cases and their family pedigrees will help to further define the clinical phenotype associated with FANCB mutations. The data reported here are of relevance to the genetic counselling of relatives of males with the VACTERL‐H phenotype and proven Fanconi anaemia‐B families.