We report a large French and Belgian clinical and molecular study of 25 patients from 16 families presenting with suspected OFD1 syndrome. Mutations in OFD1 were identified in 16/25 patients.
The frequencies of the major clinical features were similar to those reported in the literature. Facial dysmorphism was a consistent feature, and included hypertelorism (67%), buccal frenulae (76%), hypoplasia of the nasal alae (58%), lingual hamartomas (52%), and cleft palate or/and lip (56%). Tooth abnormalities (44%), including hypodontia and malposition were not always seen in association with cleft palate and lip; 2/11 patients without cleft palate or/and lip presented noticeable tooth abnormalities. Considering distal abnormalities, brachydactyly involving predominantly the second, third, and fourth fingers was characteristic (64%).
Recently, the increasing use of renal ultrasound scan revealed that polycystic kidney disease (PKD) is commonly associated with OFD1 syndrome.3,4
Our study confirms these data, as PKD was present in 7/16 cases (44%). However, 8/9 patients were too young (from fetal period to 11.5 years of age) to draw definite conclusions about the absence of kidney involvement.
The central nervous system (CNS) may also be involved in as many as 40% of the cases.5
In our group, corpus callosum agenesis (10/14 cases) was more frequent than previously reported. However, results may be biased, as cerebral magnetic resonance imaging was not systematically performed, particularly when mental retardation was absent. Moderate to mild mental retardation was frequent (48%), but could not always be correlated with CNS malformations. Spicule‐like formations in metacarpals and/or phalanges were present in 3/4 cases. This previously reported sign is frequent and specific to OFD1 syndrome but hand x
rays were rarely performed (4/25 cases).
The majority of clinical features overlaps with those reported in the other OFD syndromes.27
Clinical classification remains imperfect. For example, OFD7 syndrome has been previously reported in only one family with renal involvement and mother to daughter transmission.28
Later, the birth of a new affected female baby with corpus callosum agenesis in the third generation suggested OFD1 syndrome. Subsequent linkage studies at the OFD1
locus showed compatible linkage despite a non‐significant lod score (1.7).22
The authors suggested that OFD7 should not be a distinct clinicopathological entity. OFD8 syndrome (OMIM #311200), characterised by polydactyly and tibial dysplasia, is described as a recessive X linked heritable disease, but this type could also be allelic with OFD1.29
Direct sequencing of OFD1
in 16 families revealed 11 different mutations. In 5/16 families, no mutation was found. The detection level of 67% in our series is higher than previously reported (50%).22
Surprisingly, the detection rate was higher in sporadic (80%) than in familial cases (50%), although compatible with dominant X linked inheritance (families 13 and 15), with the presence of spicule‐like formation metacarpals and/or phalanges in family 15. Similarly, linkage was compatible with the OFD1
locus in family 12, associated with PKD and corpus callosum agenesis. In the two sporadic cases without identified mutation (cases 14 and 16), the presence of PKD strongly suggests OFD1 syndrome. In such cases, complementary molecular studies would be necessary to detect intronic mutations, mutations within the promoter, or large rearrangements such as deletions, as recently described in a recent report.30
When comparing the phenotype of the families with or without a pathogenic OFD1
mutation, we found that the presence of polycystic kidneys and short stature and the absence of lingual hamartomas suggested the absence of an OFD1
mutation, although such data could not be useful at an individual level.
There have been 18 OFD1
mutations reported in the literature; we identified a further 29 in this study, including 15 deletions, four insertions, two nonsense, five missense, and three splice mutations.15,17,18,19
No mutation was found in exons 18–23. The majority of mutations occurred in exons 3, 8, 9, 13, and 16 (19/29 cases, 66%), suggesting that these exons may represent regions for mutational hotspots. This hypothesis has previously been postulated for exons 3, 13, and 16.18
Mutational analysis of OFD1
may be of interest for diagnosis and genetic counselling, especially in sporadic cases. Indeed, in family 2, family history is both compatible with dominant X linked and autosomal recessive mode of inheritance (OFD2 syndrome) with pseudodominance because of familial consanguinity. Identification of an out of frame deletion in exon 16 confirms OFD1 syndrome and therefore a risk of transmission to daughters of 50%. Another example is given by family 5. The mother presented with isolated bifid uvula, and identification of a nonsense mutation (L144X) only in the proband confirmed a de novo mutation and a consequent low risk of recurrence in other children, limited to the risk of gonadal mosaicism.31
When analysing the phenotype–genotype correlations, mental retardation and cleft lip/palate were reported more frequently in association with mutations in exons 3, 8, 9, 13, and 16 compared with mutations in other exons. PKD was more frequently associated with splice mutations compared with other mutations, and tooth abnormalities more frequently associated with mutations in coiled coil domains compared with other domains. Unfortunately, no correlation between CNS abnormalities and mutations within the LisH motif could be tested because of the small size of the sample.
Skewed X inactivation was found in 7/23 cases (30%) from six families, a higher rate than in the general population (10%).32
Skewed X inactivation was found in four cases from three families: in two severe cases, the paternal allele was inactivated, and in a moderate case, the maternal allele was inactivated. In family 2, the mother presented a moderate phenotype with skewed inactivation of unknown origin (parental origin could not be identified because of de novo mutation and unavailability of parental DNA samples), whereas her daughter has a severe phenotype with random inactivation. These results suggest that non‐random X inactivation may in part explain intrafamilial clinical variability in these three families. In the three sporadic cases with non‐random X inactivation, mental retardation and PKD were present in 2/3 cases, also suggesting that non‐random X inactivation could explain in part the severe phenotype in sporadic cases. Additional studies in other tissues and in other patients would be of interest.
In conclusion, we report here the identification of 11 new mutations in OFD1 in 16 French and Belgian families who presented with OFD1 syndrome. Slight phenotype–genotype correlations were identified, and the X inactivation study showed that skewed X inactivation could be partially involved in the pathogenesis of intrafamilial clinical variability. These results should be confirmed on larger samples.