Haploinsufficiency of HDAC4 causes brachydactyly mental retardation syndrome, with brachydactyly type E, developmental delays, and behavioral problems.

Brachydactyly mental retardation syndrome (BDMR) is associated with a deletion involving chromosome 2q37. BDMR presents with a range of features, including intellectual disabilities, developmental delays, behavioral abnormalities, sleep disturbance, craniofacial and skeletal abnormalities (including brachydactyly type E), and autism spectrum disorder. To date, only large deletions of 2q37 have been reported, making delineation of a critical region and subsequent identification of candidate genes difficult. We present clinical and molecular analysis of six individuals with overlapping deletions involving 2q37.3 that refine the critical region, reducing the candidate genes from >20 to a single gene, histone deacetylase 4 (HDAC4). Driven by the distinct hand and foot anomalies and similar cognitive features, we identified other cases with clinical findings consistent with BDMR but without a 2q37 deletion, and sequencing of HDAC4 identified de novo mutations, including one intragenic deletion probably disrupting normal splicing and one intragenic insertion that results in a frameshift and premature stop codon. HDAC4 is a histone deacetylase that regulates genes important in bone, muscle, neurological, and cardiac development. Reportedly, Hdac4(-/-) mice have severe bone malformations resulting from premature ossification of developing bones. Data presented here show that deletion or mutation of HDAC4 results in reduced expression of RAI1, which causes Smith-Magenis syndrome when haploinsufficient, providing a link to the overlapping findings in these disorders. Considering the known molecular function of HDAC4 and the mouse knockout phenotype, taken together with deletion or mutation of HDAC4 in multiple subjects with BDMR, we conclude that haploinsufficiency of HDAC4 results in brachydactyly mental retardation syndrome.

Mutations in SCARF2 are responsible for Van Den Ende-Gupta syndrome.

Van Den Ende-Gupta syndrome (VDEGS) is an extremely rare autosomal-recessive disorder characterized by distinctive craniofacial features, which include blepharophimosis, malar and/or maxillary hypoplasia, a narrow and beaked nose, and an everted lower lip. Other features are arachnodactyly, camptodactyly, peculiar skeletal abnormalities, and normal development and intelligence. We present molecular data on four VDEGS patients from three consanguineous Qatari families belonging to the same highly inbred Bedouin tribe. The patients were genotyped with SNP microarrays, and a 2.4 Mb homozygous region was found on chromosome 22q11 in an area overlapping the DiGeorge critical region. This region contained 44 genes, including SCARF2, a gene that is expressed during development in a number of mouse tissues relevant to the symptoms described above. Sanger sequencing identified a missense change, c.773G>A (p.C258Y), in exon 4 in the two closely related patients and a 2 bp deletion in exon 8, c.1328_1329delTG (p.V443DfsX83), in two unrelated individuals. In parallel with the candidate gene approach, complete exome sequencing was used to confirm that SCARF2 was the gene responsible for VDEGS. SCARF2 contains putative epidermal growth factor-like domains in its extracellular domain, along with a number of positively charged residues in its intracellular domain, indicating that it may be involved in intracellular signaling. However, the function of SCARF2 has not been characterized, and this study reports that phenotypic effects can be associated with defects in the scavenger receptor F family of genes.

A de novo 2.1-Mb deletion of 13q12.11 in a child with developmental delay and minor dysmorphic features.

We report on a patient with an interstitial deletion at 13q12.11. He had mild developmental delay, craniofacial dysmorphism, a pectus excavatum, narrow shoulders, malformed toes, and café-au-lait spots. Array CGH analysis disclosed a de novo deletion spanning 2.1 Mb,within cytogenetic band 13q12.11.The deletion produces hemizygozity for 16 known genes, among which GJA3, GJB2, GJB6, IFT88, LATS2, and FGF9 have potential clinical significance. The observed phenotype may be due to mutation in one of the 16 genes, or to a combination of deletion and/or mutation in a number of them.

Compound heterozygous deletions of PMP22 causing severe Charcot-Marie-Tooth disease of the Dejerine-Sottas disease phenotype.

Dejerine-Sottas disease (DSD) is a particular phenotype of the Charcot-Marie-Tooth (CMT) disease spectrum that is genetically heterogeneous. It represents a severe form of hypertrophic axonal and demyelinating neuropathy. Although it is predominantly inherited as an autosomal recessive condition, autosomal dominant inheritance has also been described. To date, the autosomal recessive forms of DSD are classified into several CMT type 4 (CMT4) subclasses based on allelic heterogeneity. We present a 7-year-old boy with a severe form of CMT disease consistent with the autosomal recessive phenotype of DSD. He was found to be a compound heterozygote for mutations in the PMP22 gene resulting in homozygous deletion of exons 2 and 3. The maternally inherited allele was the typical 1.5 Mb deletion involving PMP22 seen with hereditary neuropathy with liability to pressure palsy (HNPP). The paternally inherited allele was a deletion of exons 2 and 3. Both parents presented with a typical clinical picture of HNPP. To our knowledge, this is the first patient reported with large deletions involving both PMP22 alleles. Our patient has also developed severe gastroesophageal reflux disease (GERD), a clinical feature not previously reported with CMT or DSD. The correlation of the phenotype and the molecular defects observed in this patient may set a new subcategory in the classification of DSD.

Prenatal diagnosis of monosomy 1p36: a focus on brain abnormalities and a review of the literature.

Monosomy 1p36 is an increasingly recognized chromosomal anomaly. We describe two patients with monosomy 1p36 who had brain abnormalities detected on prenatal ultrasound. The first patient was ascertained prenatally with ultrasound abnormalities, including ventriculomegaly, a single umbilical artery, a unilateral club foot, a ventricular septal defect, and intra-uterine growth retardation. Amniocentesis showed a normal karyotype. A postnatal MRI showed moderate to severe non-obstructive hydrocephalus, bilateral colpocephaly, and abnormal myelination of the anterior limb of the internal capsule. A postnatal karyotype demonstrated a deletion of 1p36.3 that was not detected prenatally due to low resolution. Molecular studies by array comparative genome hybridization (CGH) identified a terminal deletion of approximately 10 Mb. Our second patient was a fetus who had brain abnormalities suggestive of holoprosencephaly identified on prenatal ultrasound. Amniocentesis showed 46,XX,der(1)t(1;20)(p36.1;p12.2), that was found to be maternally inherited. Fetal autopsy demonstrated hydrocephalus, focal polymicrogyria, and cerebellar hypoplasia. However, holoprosencephaly was not confirmed. In addition to describing two patients with monosomy 1p36 who had abnormal brain anatomy on prenatal ultrasounds, we review the literature of other prenatally detected patients with monosomy 1p36 and review brain abnormalities seen both prenatally and postnatally.

Connexin mutation testing of children with nonsyndromic, autosomal recessive sensorineural hearing loss.

OBJECTIVE: The etiology of hearing loss is heterogeneous and falls into the two broad categories of genetic and environmental. In the genetic subgroup, 70% are non syndromic. Fifty percent of nonsyndromic sensorineural deafness is due to a mutation in the connexin 26 gene. This article presents the detection rate of connexin mutations in a multiethnic Canadian population. METHODS: A study of patients with nonsyndromic hearing loss seen over a period of 2 years who had connexin 26 mutation testing. RESULTS: Nine of the 18 patients had connexin 26 mutations. CONCLUSION: The majority of our patients with connexin 26 mutations had moderate to profound hearing loss. Testing for connexin mutations should be standard care because it accounts for a large proportion of individuals with nonsyndromic hearing loss. Reasons for testing include ruling out a syndromic cause, predicting moderate to profound hearing loss, and the need for language intervention, cochlear implants, and genetic counselling.

A mutation in the HSN2 gene causes sensory neuropathy type II in a Lebanese family.

Hereditary sensory and autonomic neuropathy (HSAN) type II is an autosomal recessive disorder clinically characterized by distal and proximal sensory loss that is caused by the reduction or absence of peripheral sensory nerves. Recently, a novel gene called HSN2 has been found to be the cause of HSAN type II in five families from Newfoundland and Quebec. Screening of this gene in an HSAN type II Lebanese family showed a 1bp deletion mutation found in a homozygous state in all affected individuals. This novel mutation supports the hypothesis that HSN2 is the causative gene for HSAN type II.

Differential impact of the FMR1 gene on visual processing in fragile X syndrome.

Fragile X syndrome (FXS) is the most common form of heritable mental retardation, affecting approximately 1 in 4000 males. The syndrome arises from expansion of a trinucleotide repeat in the 5'-untranslated region of the fragile X mental retardation 1 (FMR1) gene, leading to methylation of the promoter sequence and lack of the fragile X mental retardation protein (FMRP). Affected individuals display a unique neurobehavioural phenotype that includes striking visual-motor deficits. Here we provide neurobiological and behavioural evidence that supports the hypothesis that these visual-motor deficits are attributable to a magnocellular (M) visual pathway impairment. Immunohistochemical staining of a lateral geniculate nucleus (LGN) of a normal human male revealed high FMRP basal expression selectively within the M layers, suggesting an increased susceptibility of these neurons to the lack of FMRP as occurs in FXS. Similar staining of monkey LGNs for quantification purposes revealed that the difference is not an artefact of cell size differences between M and parvocellular (P) neurons. Further, Nissl staining of the LGNs of a male FXS patient revealed alaminar nuclei comprised of a homogenous population of small sized neurons, providing anatomical and morphological support for the idea that an M pathway pathology exists in FXS. Consistent with these neurobiological data, we have found that male patients with FXS have reduced sensitivity for psychophysical stimuli that probe the M pathway but not for those that probe the P pathway, a complementary visual stream that performs a separate set of early visual operations. Finally, male patients with FXS performed poorly on a global motion task but not on a form perception task, suggesting that the M pathway thalamic deficit may have a selective impact on cortical visual functioning in the parietal lobe, which is known to be a major recipient of M pathway afferents via the primary visual cortex. Together, these findings provide the first evidence that the loss of a single gene product, FMRP, in humans leads to abnormal neuroanatomical morphology of the LGN and a concomitant selective visual deficit of the M pathway.