Clinical Phenotypes Associated to Engrailed 2 Gene Alterations in a Series of Neuropediatric Patients

The engrailed homeobox protein (EN) plays an important role in the regionalization of the neural tube. EN distribution regulates the cerebellum and midbrain morphogenesis, as well as retinotectal synaptogenesis. In humans, the EN1 and EN2 genes code for the EN family of transcription factors. Genetic alterations in the expression of EN2 have been related to different neurologic conditions and more particularly to autism spectrum disorders (ASD). We aimed to study and compare the phenotypes of three series of patients: (1) patients with encephalic structural anomalies (ESA) and abnormalities in the genomic (DNA) and/or transcriptomic (RNAm) of EN2 (EN2-g), (2) ESA patients having other gene mutations (OG-g), and (3) ESA patients free of these mutations (NM-g).

Subjects and Methods: We have performed a descriptive study on 109 patients who suffer from mental retardation (MR), cerebral palsy (CP), epilepsy (EP), and behavioral disorders (BD), showing also ESA in their encephalic MRI. We studied genomic DNA and transcriptional analysis (cDNA) on EN2 gene (EN2), and in other genes (OG): LIS1, PTAFR, PAFAH1B2, PAFAH1B3, FGF8, PAX2, D17S379, D17S1866, and SMG6 (D17S5), as a routine genetic diagnosis in ESA patients.

Results: From 109 patients, fifteen meet the exclusion criteria. From the remaining 94 patients, 12 (12.8%) showed mutations in EN2 (EN2-g), 20 showed mutations in other studied genes (OG-g), and 62 did not showed any mutation (NM-g). All EN2-g patients, suffered from MR, nine EP, seven BD and four CP. The proportions of these phenotypes in EN2-g did not differ from those in the OG-g, but it was significantly higher when comparing EN2-g with NM-g (MR: p = 0.013; EP: p = 0.001; BD: p = 0.0001; CP: p = 0.07, ns). Groups EN2-g and OG-g showed a 100 and a 70% of comorbidity, respectively, being significantly (p = 0.04) greater than NM-group (62.9%).

Conclusion: Our series reflects a significant effect of EN2 gene alterations in neurodevelopmental abnormalities associated to ESA. Conversely, although these EN2 related anomalies might represent a predisposition to develop brain diseases, our results did not support direct relationship between EN2 mutations and specific clinical phenotypes.

Developmental basis of the rostro-caudal organization of the brainstem respiratory rhythm generator

The Hox genetic network plays a key role in the anteroposterior patterning of the rhombencephalon at pre- and early-segmental stages of development of the neural tube. In the mouse, it controls development of the entire brainstem respiratory neuronal network, including the pons, the parafacial respiratory group (pFRG) and the pre-Bötzinger complex (preBötC). Inactivation of Krox20/Egr2 eliminates the pFRG activity, thereby causing life-threatening neonatal apnoeas alternating with respiration at low frequency. Another respiratory abnormality, the complete absence of breathing, is induced when neuronal synchronization fails to develop in the preBötC. The present paper summarizes data on a third type of respiratory deficits induced by altering Hox function at pontine levels. Inactivation of Hoxa2, the most rostrally expressed Hox gene in the hindbrain, disturbs embryonic development of the pons and alters neonatal inspiratory shaping without affecting respiratory frequency and apnoeas. The same result is obtained by the Phox2a(+/-) mutation modifying the number of petrosal chemoafferent neurons, by eliminating acetylcholinesterase and by altering Hox-dependent development of the pons with retinoic acid administration at embryonic day 7.5. In addition, embryos treated with retinoic acid provide a mouse model for hyperpnoeic episodic breathing, widely reported in pre-term neonates, young girls with Rett's syndrome, patients with Joubert syndrome and adults with Cheyne-Stokes respiration. We conclude that specific respiratory deficits in vivo are assignable to anteroposterior segments of the brainstem, suggesting that the adult respiratory neuronal network is functionally organized according to the rhombomeric, Hox-dependent segmentation of the brainstem in embryos.

Huntingtin-associated protein 1 interacts with Ahi1 to regulate cerebellar and brainstem development in mice

Joubert syndrome is an autosomal recessive disorder characterized by congenital malformation of the cerebellum and brainstem, with abnormal decussation in the brain. Mutations in the Abelson helper integration site 1 gene, which encodes the protein AHI1, have been shown to cause Joubert syndrome. In this study, we found that mouse Ahi1 formed a stable complex with huntingtin-associated protein 1 (Hap1), which is critical for neonatal development and involved in intracellular trafficking. Hap1-knockout mice showed significantly reduced Ahi1 levels, defective cerebellar development, and abnormal axonal decussation. Suppression of Ahi1 also decreased the level of Hap1; and truncated Ahi1, which corresponds to the mutations in Joubert syndrome, inhibited neurite outgrowth in neuronal culture. Reducing Hap1 expression suppressed the level and internalization of TrkB, a neurotrophic factor receptor that mediates neurogenesis and neuronal differentiation, which led to decreased TrkB signaling. These findings provide insight into the pathogenesis of Joubert syndrome and demonstrate the critical role of the Ahi1-Hap1 complex in early brain development.

Genetic basis of Joubert syndrome and related disorders of cerebellar development

Over three decades have passed since Marie Joubert described the original proband for Joubert syndrome, a rare neurological disorder featuring absence of the cerebellar vermis (i.e. midline). Efforts at deciphering the molecular basis for this disease have been complicated by the clinical and genetic heterogeneity as well as extensive phenotypic overlap with other syndromes. However, progress has been made in recent years with the mapping of three genetic loci and the identification of mutations in two genes, AHI1 and NPHP1. These genes encode proteins with some shared functional domains, but their role in brain development is unclear. Clues may come from studies of related syndromes, including Bardet-Biedl syndrome and nephronophthisis, for which all of the encoded proteins localize to primary cilia. The data suggest a tantalizing connection between intraflagellar transport in cilia and brain development.

Differential transcription ofBarhl1homeobox gene in restricted functional domains of the central nervous system suggests a role in brain patterning

The mouse Barhl1 homeogene, member of the BarH subfamily, play a crucial role in the cerebellum development and its human ortholog BARHL1 has been proposed as a positional and functional candidate gene for the Joubert syndrome, a form of cerebellar ataxia. The Barhl1 expression has been demonstrated to be induced by the transcription factor Math1 involved in BMP responses. We isolated the mouse Barhl1 by screening of a cDNA library with the Xenopus Xvent-2, member of the BarH subfamily, which acts in the BMP4 pathway during embryonic patterning and neural plate differentiation. We studied the detailed Barhl1 expression pattern and showed its transcription in spatio-temporally and functionally restricted domains of the developing central nervous system (CNS). Using our new optical microscopy technology, we compare the transcript steady state level and cell density in the Barhl1-expressing regions. We found that Barhl1 was transcribed in superior and inferior colliculi in the dorsal mesencephalon at a relatively low transcriptional level. In the diencephalon, Barhl1 was found higher expressed first within the basal plate and later in the mammillary region. In the cerebellum, Barhl1 showed the highest transcriptional level restricted to the anterior and posterior rhombic lips giving rise to the external and internal cerebellar granular cells and to the deep nuclei. In the spinal cord, Barhl1 showed similar expression level than in the cerebellum and is delimited to a subset of dorsal interneurons. Therefore, our results indicated that Barhl1 homeodomain gene is exclusively transcribed in restricted CNS domain at differential transcription levels which suggest a highly regulated transcriptional mechanism. In addition, these regional and cellular specificities indicated that Barhl1 may be involved in the differentiation of the specific subsets of neuronal progenitors.

BARHL1 homeogene, the human ortholog of the mouse Barhl1 involved in cerebellum development, shows regional and cellular specificities in restricted domains of developing human central nervous system

The mouse homeobox gene Barhl1 plays a central role in cerebellum development and its expression is activated by the transcription factor Math1 which is involved in bone morphogenetic protein response pathways. We studied the human ortholog BARHL1 and we found that human, mouse, monkey, rat, and zebrafish orthologs were highly conserved and are members of the BarH homeogene family, containing Drosophila BarH1 and BarH2. The N-terminus of BARHL1 protein presents two FIL domains and an acidic domain rich in serine/threonine and proline, while the C-terminus contains a canonical proline-rich domain. Secondary structure analysis showed that outside the three helixes of the homeodomain, BARHL1 protein has essentially random coil structure. We isolated BARHL1 and defined its expression pattern in human embryonic and fetal central nervous system (CNS) and compared it to the mouse Barhl1 transcription. BARHL1 mRNA was found exclusively in the CNS restricted to p1-p4 prosomeres of the diencephalon, to the dorsal cells of the mesencephalon, to the dorsal dl1 sensory neurons of the spinal cord, and to the rhombic lips yielding the cerebellar anlage. Detailed analysis of BARHL1 expression in fetal cerebellar cell layers using our new optic microscopy technology showed BARHL1 expression in external and internal granular cells and also in mouse adult granular cells, in agreement to Barhl1 null mouse phenotype affecting the differentiation and migration of granular cells. These findings indicate that the regional and cellular specificities of BARHL1 transcriptional control well correspond to the mouse Barhl1 transcription and suggest a potential role of this gene in the differentiation of BARHL1-expressing neuronal progenitors involved in the pattern formation of human cerebral and cerebellar structures.

Attention deficit/hyperactivity disorder (ADHD): complex phenotype, simple genotype?

Complex genetic traits refer to those phenotypes not fitting patterns of Mendelian segregation and/or assortment but exhibiting a preferential familial clustering that cannot be explained by cultural or environmental causes. Attention-deficit/hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder of childhood and probably the most controversial. ADHD has been considered a complex genetic trait based upon the absence of a clear-cut boundary between affected and unaffected status. Furthermore, its high comorbidity with other disorders strongly suggests complex epistatic or pleiotropic effects acting in common with the environmental influences. This implies that the same gene or genes is or are associated with different and concurrently occurring phenotypes. In this study, we will review clinical and epidemiological aspects related to the ADHD phenotype, which are considered either as categorical or continuous traits. We also will discuss genetic models underlying the complexity of this behavioral phenotype and the probable role of epistatic interactions between major genes contributing to the ADHD phenotype.

Bar homeodomain proteins are anti-proneural in the Drosophila eye: transcriptional repression of atonal by Bar prevents ectopic retinal neurogenesis

Atonal (Ato)/Math (Mammalian atonal homolog) family proneural proteins are key regulators of neurogenesis in both vertebrates and invertebrates. In the Drosophila eye, Ato is essential for the generation of photoreceptor neurons. Ato expression is initiated at the anterior ridge of the morphogenetic furrow but is repressed in the retinal precursor cells behind the furrow to prevent ectopic neurogenesis. We show that Ato repression is mediated by the conserved homeobox proteins BarH1 and BarH2. Loss of Bar causes cell-autonomous ectopic Ato expression, resulting in excess photoreceptor clusters. The initial ommatidial spacing at the furrow occurs normally in the absence of Bar, suggesting that the ectopic neurogenesis within Bar mutant clones is not due to the lack of Notch (N)-dependent lateral inhibition. Targeted misexpression of Bar is sufficient to repress ato expression. Furthermore, we provide evidence that Bar represses ato expression at the level of transcription without affecting the expression of an ato activator, Cubitus interruptus (Ci). Thus, we propose that Bar is essential for transcriptional repression of ato and the prevention of ectopic neurogenesis behind the furrow.

Description, nomenclature, and mapping of a novel cerebello-renal syndrome with the molar tooth malformation

Cerebello-oculo-renal syndromes (CORSs) and Joubert syndrome (JS) are clinically and genetically heterogeneous autosomal recessive syndromes that share a complex neuroradiological malformation resembling a molar tooth on brain axial images, a condition referred to as "molar tooth on imaging" (MTI) or the "molar tooth sign." The current literature on these syndromes is complex, with overlapping and incomplete phenotypes that complicate the selection of clinically homogeneous cases for genetic purposes. So far, only one locus (JBTS1 on 9q34) has been mapped, in two families with JS. Here, we describe a large consanguineous family with JS and nephronophthisis, representing a novel cerebello-renal phenotype. We have mapped this condition to the pericentromeric region of chromosome 11 and have named the locus "CORS2." The acronym "CORS" is proposed for all loci associated with JS, CORSs, and related phenotypes sharing the MTI, because this neuroradiological sign seems to be the unifying feature of these clinically heterogeneous syndromes.

RECOGNITION OF THE CLINICAL SIGNS AND SYMPTOMS OF JOUBERT SYNDROME

Joubert Syndrome is a rare, autosomal recessive disorder that affects the cerebellum and brain stem. It presents with a distinct respiratory pattern and profound tachypnea in the newborn period. This article provides an overview of the condition and discusses the embryologic origins of this syndrome. A focused history and systematic physical assessment provide a step-by-step guide to enhance the early recognition of clinical signs and symptoms of this disorder. A series of clinical photographs and a brief case report offer insight into the classic presentation of this uncommon disorder. The diagnosis of Joubert syndrome is confirmed with magnetic resonance imaging, which reveals a classic neuroradiologic finding, characterized as the molar tooth sign. A discussion of the range of developmental outcomes and complex multispecialty care and intensive support that these infants and their families require is also provided.