Haploinsufficiency of novel FOXG1B variants in a patient with severe mental retardation, brain malformations and microcephaly

We have investigated the chromosome abnormalities in a female patient exhibiting a severe cognitive disability associated with complete agenesis of the corpus callosum and microcephaly. The patient carries a balanced de novo translocation t(2;14)(p22;q12), together with a neighbouring 720 kb inversion in chromosome 14q12. By combined fluorescence in situ hybridisation and Southern hybridisation, the distal inversion breakpoint on chromosome 14 was mapped to a region harbouring genes and ESTs derived predominantly from brain tissue. RT-PCR studies indicated that these transcripts comprise the 3' ends of novel splice variants of the winged helix transcription factor FOXG1B (also referred to in previous studies as FOXG1A and FOXG1C, as well as Brain Factor 1), the mouse orthologue of which is essential for normal development of the telencephalon. Analysis of these novel FOXG1B transcripts indicated that they are all disrupted by the breakpoint in the patient. Moreover, we have identified novel orthologous Foxg1 transcripts in the mouse and other vertebrates, which validates the functional importance of these variants and provides a direct genetic link between the patient phenotype and that of the heterozygous Foxg1 knockout mice. These results, together with previously published studies on patients with similar disorders and proximal 14q deletions, strongly suggest that several disorders associated with malformations of the human brain may be directly caused by mutations or alterations in the FOXG1B gene.

Novel mutations in BCOR in three patients with oculo-facio-cardio-dental syndrome, but none in Lenz microphthalmia syndrome

Oculo-facio-cardio-dental (OFCD) syndrome is a rare X-linked dominant condition with male lethality characterized by microphthalmia, congenital cataracts, facial dysmorphic features, congenital heart defects, and dental anomalies. Mutations in BCOR (BCL6 co-repressor) located in Xp11.4 have been described to cause OFCD syndrome. Lenz microphthalmia syndrome is inherited in an X-linked recessive pattern comprising microphthalmia/anophthalmia, mental retardation, malformed ears, digital, skeletal, and urogenital anomalies (synonym: microphthalmia with associated anomalies (MAA)). One locus for MAA has been mapped to Xq27-q28. Nonetheless, linkage and subsequent mutation analysis revealed a single missense mutation (p.P85L) in BCOR in a large family with presumed Lenz microphthalmia syndrome (MAA2). We describe novel mutations in BCOR in three patients with OFCD syndrome, two small deletions (c.2488_2489delAG and c.3286delG) and a submicroscopic deletion of about 60 kb encompassing at least BCOR exons 2-15. No BCOR mutation was detected in eight patients with Lenz microphthalmia syndrome. Our data confirm that BCOR is the causative gene for OFCD syndrome; however, the failure to identify any mutation in patients with Lenz microphthalmia syndrome together with the oligosymptomatic phenotype in the reported MAA2 patients suggest that BCOR is not the major gene for this syndrome.

SONIC HEDGEHOG mutations causing human holoprosencephaly impair neural patterning activity

Holoprosencephaly (HPE) is a common forebrain malformation associated with mental retardation and craniofacial anomalies. Multiple lines of evidence indicate that loss of ventral neurons is associated with HPE. The condition is etiologically heterogeneous, and abnormalities in any of several genes can cause human HPE. Among these genes, mutations in SONIC HEDGEHOG ( SHH) are the most commonly identified single gene defect causing human HPE. SHH mediates a number of processes in central nervous system development and is required for the normal induction of ventral cell types in the brain and spinal cord. Although a number of missense mutations in SHH have been identified in patients with HPE, the functional significance of these mutations has not yet been determined. We demonstrate that two SHH mutations that cause human HPE result in decreased in vivo activity of SHH in the developing nervous system. These mutant forms of SHH fail to regulate genes properly that are normally responsive to SHH signaling and do not induce ventrally expressed genes. In addition, the immunoreactivity of the mutant proteins is altered, suggesting that the conformation of the SHH protein has been disrupted. These studies are the first demonstration that mutations in SHH associated with human HPE perturb the in vivo patterning function of SHH in the developing nervous system.