Over 50 male patients with duplications of the Xq28 region have now been reported; several previous authors have reviewed the sizes of the duplications and commented on possible correlation between the phenotype and the genes included in the duplicated regions.4, 7
Although spasticity, severe learning disability, axial hypotonia and frequent chest infections have been common findings within this group of patients, gastrointestinal symptoms have been much rarer. Gastro-oesophageal reflux has been mentioned as a feature in some patients,3
but other bowel symptoms have been infrequent. In the report by Meins et al
one patient had suspected gluten enteropathy because of poor weight gain but no other bowel symptoms.
Many neurological and skeletal phenotypes have been associated with FLNA mutations.14
Missense mutations, generally thought to lead to gain of function, cause the skeletal phenotypes of otopalatodigital syndrome, Melnick–Needles syndrome and frontometaphyseal dysplasia. FLNA loss-of-function mutations are associated with the PVNH phenotype and are usually embryonically lethal in male patients.15
Virtually, all of these mutations are frameshift or truncating and likely to be subject to nonsense-mediated RNA decay. There have been two reports of surviving male patients with PVNH associated with duplications of the Xq28 region,15, 16
suggesting that the amount of FLNA is critical for neuronal migration, and either increased or decreased expression can interfere with the normal migration process. This hypothesis is also supported by animal studies.17
Chronic intestinal pseudo-obstruction has not been a prominent feature in patients with FLNA mutations who were ascertained because they had PVNH, and this may be because they often do not survive. Interestingly, one heterozygous female patient was noted to have problems with gastrointestinal motility.15
Surviving male patients with milder FLNA mutations, where some FLNA function is retained, have been described18
but again, gastrointestinal problems were not noted as a prominent feature.
studied a family that was ascertained because of intestinal pseudo-obstruction, and identified a 2-bp deletion in FLNA. Affected members of this family do not appear to have been screened for the presence of PVNH. Although the deletion caused a frameshift, the presence of an alternative ATG site 22 codons downstream of the initial ATG enabled the production of a shorter FLNA protein. The authors thus hypothesised that the first 27 FLNA residues might be crucial for enteric neuron structure and function. Alternatively, the bowel symptoms could have resulted from an overall reduced FLN expression. Hehr19
described a further patient with PVNH and severe constipation who had an FLNA splice-site mutation leading to the production of both a normal and a shorter mRNA. These findings might be consistent with either of the above hypotheses. FLNA protein expression in our families has not yet been studied in detail, and thus suggestions as to how the intestinal phenotype is caused are purely speculative; one hypothesis is that increased FLNA dosage significantly compromises enteric neuron development in addition to affecting neuronal migration. The findings in Hehr's patient suggest that the bowel problems may, like neuronal migration, be affected by both under- and overexpression of FLNA. Phenotyping of the reported patients with FLNA mutations or duplications has not always been comprehensive. Patients ascertained through chronic intestinal pseudo-obstruction have not always been investigated for PVNH and vice versa. Studies of filamin expression, more detailed phenotyping and careful scrutiny of the MRI images of patients with both FLNA mutations and duplications should help to resolve this issue.
OMIM entry 300048 concerns the phenotype of X-linked chronic idiopathic intestinal pseudo-obstruction (CIIP) associated with additional features including patent ductus arteriosus, short bowel, malrotation and coagulopathy. This entity includes the family reported by Fitzpatrick et al12
(our family 8), who had all of the above features, along with a similar patient reported by Pollock et al.20
Two further cases were reported by Harris et al
and the Italian family with CIIP originally reported by Aurecchio et al
and then investigated by Gargiulo,11
is included. Although this Italian kindred was subsequently identified to have a loss-of-function mutation in FLNA, no mutation was identified in Fitzpatrick's family or in our family 9. It is not clear whether FLNA was studied in the patient reported by Pollock et al.20
The presence of thrombocytopaenia with large platelets and the absence of a filamin mutation set these two families apart from Gargiulo's kindred. Their duplication detected with the P049 MLPA kit demonstrated only duplication of exons 4 and 11 of the FLNA gene, and they were the only patients of our cohort in which thrombocytopaenia was a feature. Neither of the patients from families 8 and 9 had mental retardation, hypotonia or spasticity, and they were not facially so dysmorphic as the patients with large duplications. Thrombocytopaenia and cardiac defects are clearly not confined to patients with intragenic FLNA duplications; however, as Parrini et al23
reported that insufficiency of the aortic valve, PDA or idiopathic thrombocytopaenia were present in 20 of the 35 patients with FLNA mutations, they ascertained in their study of PVNH. Vascular events such as cerebrovascular accidents were also documented in female patients heterozygous for an FLNA mutation and a hemizygous male patient had a neonatal haemorrhage and maturation arrest of the bone marrow.15
There is clear demonstration, therefore, of the association between PVNH, CIIP, cardiac defects and thrombocytopaenia in patients with FLNA point mutations and intragenic duplications. A systematic search for these haematological and vascular features in the group of patients with larger Xq duplications may help to delineate the thrombocytopaenia and bone marrow phenotype further.
Unger et al24
reported a patient with a missense mutation in FLNA and a phenotype suggestive of FG syndrome. This child had presented with relative macrocephaly, an unusual head shape and severe and chronic constipation. He had a prominent forehead with a frontal cowlick, a small mouth and simple unfolded ears. Until recently, FG syndrome has been considered as a group of genetically heterogeneous X-linked conditions, and reported affected male patients have had diverse phenotypes, mainly including mental retardation, macrocephaly and severe constipation. Since the finding of MED12 mutations in the original FG family25
and Unger's report of an FLNA mutation, more distinctive phenotypes for FG syndrome have emerged. It is possible that some of the ‘FG' patients without MED12 or FLNA mutations may have FLNA duplications. Interestingly, FG syndrome had been considered as a possible diagnosis in family 1.
Few authors have commented in great detail on the facial gestalt of the Xq28 duplication patients. Gargiulo for example, concentrated on the intestinal phenotype rather than including a description or photographs of facial features. We believe that there is a recognisable face (). Families 1 and 2 were recognised as having the same syndrome on the basis of facial features as well as bowel symptoms several years before molecular diagnosis. Patient 4 was originally referred with a possible diagnosis of Angelman syndrome, but on the basis of his facial appearance, MECP2 duplication testing was undertaken. Most of our patients had a narrow ‘pinched' appearance of the nose with a translucent quality of the skin and prominent veins. The eyes appeared deep-set and the chin prominent. The lower lip was frequently everted and small; widely spaced teeth were noted in some patients. A frontal cowlick was often present. We would agree with Van Esch et al2
that the ears appear prominent and the face hypotonic, though a flat nasal bridge tended to be present only in younger patients. Although microcephaly has frequently been reported with Xq28 duplications, this is not universally the case and several of our patients had head circumferences within the normal range even though their other growth parameters were impaired. This relative macrocephaly was associated with a prominent appearance of the forehead. An asymmetric skull shape has been commented upon in several patients both within our cohort and in the literature. Our patients looked remarkably similar to patient 3 of Smyk et al4
and patient II2
reported by Friez5
. The Xq28 facial gestalt seems to be more recognisable from mid-childhood onwards and less recognisable in patients who have associated microcephaly. MRI brain scans had been carried out on several of our patients as detailed above. None of the patients were reported to have the classical appearance of PVNH, although abnormal periventricular white matter was reported in some (see ), and these scans deserve further scrutiny. Abnormalities of the corpus callosum and delayed myelination were frequently reported and these features were also mentioned in the male patient with a duplication reported by Fox.15
Figure 3 Composite of faces showing, from left to right, two siblings from family 1, one from family 2 and the proband from family 4. All demonstrate similar hypotonic facies, narrow nose, prominent forehead and deep-set eyes. The mouth and chin are relatively (more ...)
In summary, we propose that duplication of the Xq28 region is a significant cause of intestinal pseudo-obstruction, especially in male patients presenting with severe learning difficulties and hypotonia, and that this and the presence of characteristic facial features are helpful in recognition of the phenotype. Though Xq28 duplications will, for the most part, contain several genes, our observations suggest that it is the involvement of the FLNA gene that appears to be responsible for the presence of intestinal pseudo-obstruction. This is corroborated by evidence from previously reported patients with FLNA mutations and by animal studies. The occurrence of additional features including patent ductus arteriosus, short bowel, malrotation, thrombocytopaenia, maturation arrest of the bone marrow and agenesis of the corpus callosum, in addition to PVNH and CIIP, suggests that duplication of FLNA is acting as a hypomorphic mutation with pleiotrophic effects in affected male patients. Male patients with larger duplications had the additional neurological features of mental retardation, hypotonia or spasticity, but it seems feasible that many of the phenotypic features of the Xq28 duplication may be due to FLNA involvement alone.