Loss of NDST1 N-sulfotransferase activity is associated with autosomal recessive intellectual disability.

Intellectual Disability (ID) is the major cause of handicap, affecting nearly 3% of the general population, and is highly genetically heterogenous with more than a thousand genes involved. Exome sequencing performed in two independent families identified the same missense variant, p.(Gly611Ser), in the NDST1 (N-deacetylase/N-sulfotransferase member 1) gene. This variant had been previously found in ID patients of two other families but has never been functionally characterized. The NDST1 gene encodes a bifunctional enzyme that catalyzes both N-deacetylation and N-sulfation of N-acetyl-glucosamine residues during heparan sulfate (HS) biosynthesis. This step is essential because it influences the downstream enzymatic modifications and thereby determines the overall structure and sulfation degree of the HS polysaccharide chain. To discriminate between a rare polymorphism and a pathogenic variant, we compared the enzymatic properties of wild-type and mutant NDST1 proteins. We found that the p.(Gly611Ser) variant results in a complete loss of N-sulfotransferase activity while the N-deacetylase activity is retained. NDST1 shows the highest and the most homogeneous expression in the human cerebral structures compared to the other members of the NDST gene family. These results indicate that a loss of NDST1 N-sulfation activity is associated with impaired cognitive functions.

NAPB and developmental and epileptic encephalopathy: Description of the electroclinical profile associated with a novel pathogenic variant.

Developmental and epileptic encephalopathies (DEE) are a group of neurodevelopmental disorders characterized by epileptic seizures associated with developmental delay or regression. DEE are genetically heterogeneous, and the proteins involved play roles in multiple pathways such as synaptic transmission, metabolism, neuronal development or maturation, transcriptional regulation, and intracellular trafficking. We performed whole exome sequencing on a consanguineous family with three children presenting an early onset (<6 months) with clusters of seizures characterized by oculomotor and vegetative manifestations, with an occipital origin. Before 1 year of age, interictal electroencephalographic recordings were well organized and neurodevelopment was unremarkable. Then, a severe regression occurred. We identified a novel homozygous protein-truncating variant in the NAPB (N-ethylmaleimide-sensitive fusion [NSF] attachment protein beta) gene that encodes the ßSNAP protein, a key regulator of NSF-adenosine triphosphatase. This enzyme is essential for synaptic transmission by disassembling and recycling proteins of the SNARE complex. Here, we describe the electroclinical profile of each patient during the disease course. Our findings strengthen the association between biallelic variants in NAPB and DEE and refine the associated phenotype. We suggest including this gene in the targeted epilepsy gene panels used for routine diagnosis of unexplained epilepsy.

TRAPPC2L-related disorder: first homozygous protein-truncating variant and further delineation of the phenotype.

The TRAPP (TRAfficking Protein Particle) complexes are evolutionarily conserved tethering factors involved in the intracellular transport of vesicles for secretion and autophagy processes. Pathogenic variants in 8 genes (of 14) encoding TRAPP proteins are involved in ultra-rare human diseases, called TRAPPopathies. Seven of them are autosomal recessive neurodevelopmental disorders with overlapping phenotypes. Since 2018, two homozygous missense variants in TRAPPC2L have been reported in five individuals from three unrelated families with early-onset and progressive encephalopathy, with episodic rhabdomyolysis. We now describe the first pathogenic protein-truncating variant in the TRAPPC2L gene found at a homozygous state in two affected siblings. This report provides key genetic evidence invaluable to establishing the gene-disease relationship for this gene and important insights into the TRAPPC2L phenotype. Regression, seizures and postnatal microcephaly initially described are not constant features. Acute episodes of infection do not contribute to the neurological course. HyperCKaemia is part of the clinical picture. Thus, TRAPPC2L syndrome is mainly characterised by a severe neurodevelopmental disorder and a variable degree of muscle involvement, suggesting that it belongs to the clinical entity of rare congenital muscular dystrophies.

Biallelic variants in PCDHGC4 cause a novel neurodevelopmental syndrome with progressive microcephaly, seizures, and joint anomalies.

PURPOSE: We aimed to define a novel autosomal recessive neurodevelopmental disorder, characterize its clinical features, and identify the underlying genetic cause for this condition. METHODS: We performed a detailed clinical characterization of 19 individuals from nine unrelated, consanguineous families with a neurodevelopmental disorder. We used genome/exome sequencing approaches, linkage and cosegregation analyses to identify disease-causing variants, and we performed three-dimensional molecular in silico analysis to predict causality of variants where applicable. RESULTS: In all affected individuals who presented with a neurodevelopmental syndrome with progressive microcephaly, seizures, and intellectual disability we identified biallelic disease-causing variants in Protocadherin-gamma-C4 (PCDHGC4). Five variants were predicted to induce premature protein truncation leading to a loss of PCDHGC4 function. The three detected missense variants were located in extracellular cadherin (EC) domains EC5 and EC6 of PCDHGC4, and in silico analysis of the affected residues showed that two of these substitutions were predicted to influence the Ca2+-binding affinity, which is essential for multimerization of the protein, whereas the third missense variant directly influenced the cis-dimerization interface of PCDHGC4. CONCLUSION: We show that biallelic variants in PCDHGC4 are causing a novel autosomal recessive neurodevelopmental disorder and link PCDHGC4 as a member of the clustered PCDH family to a Mendelian disorder in humans.

A large consanguineous family with a homozygous Metabotropic Glutamate Receptor 7 (mGlu7) variant and developmental epileptic encephalopathy: Effect on protein structure and ligand affinity.

BACKGROUND: Developmental and epileptic encephalopathies (DEE) are chronic neurological conditions where epileptic activity contributes to the progressive disruption of brain function, frequently leading to impaired motor, cognitive and sensory development. PATIENTS AND METHODS: The present study reports a clinical investigation and a molecular analysis by Next Generation Sequencing (NGS) of a large consanguineous family comprising several cases of developmental and epileptic encephalopathy. Bioinformatic prediction and molecular docking analysis were also carried out. RESULTS: The majority of patients in our studied family had severe developmental impairments, early-onset seizures, brain malformations such as cortical atrophy and microcephaly, developmental delays and intellectual disabilities. The molecular investigations revealed a novel homozygous variant c.1411G>A (p.Gly471Arg) in the GRM7 gene which was segregating with the disease in the family. Bioinformatic tools predicted its pathogenicity and docking analysis revealed its potential effects on mGlu7 protein binding to its ligand. CONCLUSION: Our results contribute to a better understanding of the impact of GRM7 variants for the newly described associated phenotype.

Molecular characterization of a 1p36 chromosomal duplication and in utero interference define ENO1 as a candidate gene for polymicrogyria.

While chromosome 1p36 deletion syndrome is one of the most common terminal subtelomeric microdeletion syndrome, 1p36 microduplications are rare events. Polymicrogyria (PMG) is a brain malformation phenotype frequently present in patients with 1p36 monosomy. The gene whose haploinsufficiency could cause this phenotype remains to be identified. We used high-resolution arrayCGH in patients with various forms of PMG in order to identify chromosomal variants associated to the malformation and characterized the genes included in these regions in vitro and in vivo. We identified the smallest case of 1p36 duplication reported to date in a patient presenting intellectual disability, microcephaly, epilepsy, and perisylvian polymicrogyria. The duplicated segment is intrachromosomal, duplicated in mirror and contains two genes: enolase 1 (ENO1) and RERE, both disrupted by the rearrangement. Gene expression analysis performed using the patient cells revealed a reduced expression, mimicking haploinsufficiency. We performed in situ hybridization to describe the developmental expression profile of the two genes in mouse development. In addition, we used in utero electroporation of shRNAs to show that Eno1 inactivation in the rat causes a brain development defect. These experiments allowed us to define the ENO1 gene as the most likely candidate to contribute to the brain malformation phenotype of the studied patient and consequently a candidate to contribute to the malformations of the cerebral cortex observed in patients with 1p36 monosomy.

Mosaicism of XX and XXY cells accounts for high copy number of Toll like Receptor 7 and 8 genes in peripheral blood of men with Rheumatoid Arthritis.

The X chromosome, hemizygous in males, contains numerous genes important to immunological and hormonal function. Alterations in X-linked gene dosage are suspected to contribute to female predominance in autoimmunity. A powerful example of X-linked dosage involvement comes from the BXSB murine lupus model, where the duplication of the X-linked Toll-Like Receptor 7 (Tlr7) gene aggravates autoimmunity in male mice. Such alterations are possible in men with autoimmune diseases. Here we showed that a quarter to a third of men with rheumatoid arthritis (RA) had significantly increased copy numbers (CN) of TLR7 gene and its paralog TLR8. Patients with high CN had an upregulated pro-inflammatory JNK/p38 signaling pathway. By fluorescence in situ hybridization, we further demonstrated that the increase in X-linked genes CN was due to the presence of an extra X chromosome in some cells. Men with RA had a significant cellular mosaicism of female (46,XX) and/or Klinefelter (47,XXY) cells among male (46,XY) cells, reaching up to 1.4% in peripheral blood. Our results present a new potential trigger for RA in men and opens a new field of investigation particularly relevant for gender-biased autoimmune diseases.

Clinical study of 19 patients with SCN8A-related epilepsy: Two modes of onset regarding EEG and seizures.

OBJECTIVE: To describe the mode of onset of SCN8A-related severe epilepsy in order to facilitate early recognition, and eventually early treatment with sodium channel blockers. METHODS: We reviewed the phenotype of patients carrying a mutation in the SCN8A gene, among a multicentric cohort of 638 patients prospectively followed by several pediatric neurologists. We focused on the way clinicians made the diagnosis of epileptic encephalopathy, the very first symptoms, electroencephalography (EEG) findings, and seizure types. We made genotypic/phenotypic correlation based on epilepsy-associated missense variant localization over the protein. RESULTS: We found 19 patients carrying a de novo mutation of SCN8A, representing 3% of our cohort, with 9 mutations being novel. Age at onset of epilepsy was 1 day to 16 months. We found two modes of onset: 12 patients had slowly emerging onset with rare and/or subtle seizures and normal interictal EEG (group 1). The first event was either acute generalized tonic-clonic seizure (GTCS; Group 1a, n = 6) or episodes of myoclonic jerks that were often mistaken for sleep-related movements or other movement disorders (Group 1b, n = 6). Seven patients had a sudden onset of frequent tonic seizures or epileptic spasms with abnormal interictal EEG leading to rapid diagnosis of epileptic encephalopathy. Sodium channel blockers were effective or nonaggravating in most cases. SIGNIFICANCE: SCN8A is the third most prevalent early onset epileptic encephalopathy gene and is associated with two modes of onset of epilepsy.

Abnormal function of the UBA5 protein in a case of early developmental and epileptic encephalopathy with suppression-burst.

Early myoclonic epilepsy (EME) or Aicardi syndrome is one of the most severe epileptic syndromes affecting neonates. We performed whole exome sequencing in a sporadic case affected by EME and his parents. In the proband, we identified a homozygous missense variant in the ubiquitin-like modifier activating enzyme 5 (UBA5) gene, encoding a protein involved in post-translational modifications. Functional analysis of the UBA5 variant protein reveals that it is almost completely unable to perform its trans-thiolation activity. Although recessive variants in UBA5 have recently been associated with epileptic encephalopathy, variants in this gene have never been reported to cause EME. Our results further demonstrate the importance of post-translational modifications such as the addition of an ubiquitin-fold modifier 1 (UFM1) to target proteins (ufmylation) for normal neuronal networks activity, and reveal that the dysfunction of the ubiquitous UBA5 protein is a cause of EME.

Heterogeneity of FHF1 related phenotype: Novel case with early onset severe attacks of apnea, partial mitochondrial respiratory chain complex II deficiency, neonatal onset seizures without neurodegeneration.

INTRODUCTION/OBJECTIVES: We report the case of a child prospectively followed in our institution for a severe, neonatal onset epilepsy presenting with severe attacks of apnea that were not initially recognized as seizure since they were not associated with any abnormal movement and since interictal EEG was normal. Recording of attacks using prolonged video-EEG recording allowed to confirm the diagnosis of epileptic seizures. RESULTS: Using whole exome sequencing we found a de novo heterozygous, missense mutation of FHF1 (p.Arg52His, NM_004113), a mutation that has been very recently described in 7 patients with an early onset epileptic encephalopathy. The initial workup showed a partial deficit of the complex II of the respiratory chain in muscle and liver. The prospective follow-up demonstrated that 2 drugs seemed to be more effective than the others: sodium blocker carbamazepine, and serotonin reuptake blocker fluoxetine. GABAergic drugs seemed to be ineffective. No drug aggravated the epilepsy. DISCUSSION: This case report contributes to the description of an emerging phenotype for this condition.

Early-onset epileptic encephalopathy as the initial clinical presentation of WDR45 deletion in a male patient.

Variants in the WD repeat 45 (WDR45) gene in human Xp11.23 have recently been identified in patients suffering from neurodegeneration with brain iron accumulation, a genetically and phenotypically heterogeneous condition. WDR45 variants cause a childhood-onset encephalopathy accompanied by neurodegeneration in adulthood and iron accumulation in the basal ganglia. They have been almost exclusively found in females, and male lethality was suggested. Here we describe a male patient suffering from a severe and early neurological phenotype, initially presenting early-onset epileptic spasms in clusters associated with an abnormal interictal electroencephalography showing slow background activity, large amplitude asynchronous spikes and abnormal neurological development. This patient is a carrier of a 19.9-kb microdeletion in Xp11.23 containing three genes, including WDR45. These findings reveal that males with WDR45 deletions are viable, and can present with early-onset epileptic encephalopathy without brain iron accumulation.

Evidence that homozygous PTPRD gene microdeletion causes trigonocephaly, hearing loss, and intellectual disability.

BACKGROUND: The premature fusion of metopic sutures results in the clinical phenotype of trigonocephaly. An association of this characteristic with the monosomy 9p syndrome is well established and the receptor-type protein tyrosine phosphatase gene (PTPRD), located in the 9p24.1p23 region and encoding a major component of the excitatory and inhibitory synaptic organization, is considered as a good candidate to be responsible for this form of craniosynostosis. Moreover PTPRD is known to recruit multiple postsynaptic partners such as IL1RAPL1 which gene alterations lead to non syndromic intellectual disability (ID). RESULTS: We describe a 30 month old boy with severe intellectual disability, trigonocephaly and dysmorphic facial features such as a midface hypoplasia, a flat nose, a depressed nasal bridge, hypertelorism, a long philtrum and a drooping mouth. Microarray chromosomal analysis revealed the presence of a homozygous deletion involving the PTPRD gene, located on chromosome 9p22.3. Reverse Transcription PCR (RT-PCR) amplifications all along the gene failed to amplify the patient's cDNA in fibroblasts, indicating the presence of two null PTPRD alleles. Synaptic PTPRD interacts with IL1RAPL1 which defects have been associated with intellectual disability (ID) and autism spectrum disorder. The absence of the PTPRD transcript leads to a decrease in the expression of IL1RAPL1. These results suggest the direct involvement of PTPRD in ID, which is consistent with the PTPRD -/- mice phenotype. Deletions of PTPRD have been previously suggested as a cause of trigonocephaly in patients with monosomy 9p and genome-wide association study suggested variations in PTPRD are associated with hearing loss. CONCLUSIONS: The deletion identified in the reported patient supports previous hypotheses on its function in ID and hearing loss. However, its involvement in the occurrence of metopic synostosis is still to be discussed as more investigation of patients with the 9p monosomy syndrome is required.

Contribution of copy number variants (CNVs) to congenital, unexplained intellectual and developmental disabilities in Lebanese patients.

BACKGROUND: Chromosomal microarray analysis (CMA) is currently the most widely adopted clinical test for patients with unexplained intellectual disability (ID), developmental delay (DD), and congenital anomalies. Its use has revealed the capacity to detect copy number variants (CNVs), as well as regions of homozygosity, that, based on their distribution on chromosomes, indicate uniparental disomy or parental consanguinity that is suggestive of an increased probability of recessive disease. RESULTS: We screened 149 Lebanese probands with ID/DD and 99 healthy controls using the Affymetrix Cyto 2.7 M and SNP6.0 arrays. We report all identified CNVs, which we divided into groups. Pathogenic CNVs were identified in 12.1% of the patients. We review the genotype/phenotype correlation in a patient with a 1q44 microdeletion and refine the minimal critical regions responsible for the 10q26 and 16q monosomy syndromes. Several likely causative CNVs were also detected, including new homozygous microdeletions (9p23p24.1, 10q25.2, and 8p23.1) in 3 patients born to consanguineous parents, involving potential candidate genes. However, the clinical interpretation of several other CNVs remains uncertain, including a microdeletion affecting ATRNL1. This CNV of unknown significance was inherited from the patient's unaffected-mother; therefore, additional ethnically matched controls must be screened to obtain enough evidence for classification of this CNV. CONCLUSION: This study has provided supporting evidence that whole-genome analysis is a powerful method for uncovering chromosomal imbalances, regardless of consanguinity in the parents of patients and despite the challenge presented by analyzing some CNVs.

Intragenic rearrangements in X-linked intellectual deficiency: results of a-CGH in a series of 54 patients and identification of TRPC5 and KLHL15 as potential XLID genes.

High-resolution array comparative genomic hybridization (a-CGH) enables the detection of intragenic rearrangements, such as single exon deletion or duplication. This approach can lead to the identification of new disease genes. We report on the analysis of 54 male patients presenting with intellectual deficiency (ID) and a family history suggesting X-linked (XL) inheritance or maternal skewed X-chromosome inactivation (XCI), using a home-made X-chromosome-specific microarray covering the whole human X-chromosome at high resolution. The majority of patients had whole genome array-CGH prior to the selection and we did not include large rearrangements such as MECP2 and FMR1 duplications. We identified four rearrangements considered as causative or potentially pathogenic, corresponding to a detection rate of 8%. Two CNVs affected known XLID genes and were therefore considered as causative (IL1RAPL1 and OPHN1 intragenic deletions). Two new CNVs were considered as potentially pathogenic as they affected interesting candidates for ID. The first CNV is a deletion of the first exon of the TRPC5 gene, encoding a cation channel implicated in dendrite growth and patterning, in a child presenting with ID and an autism spectrum disorder (ASD). The second CNV is a partial deletion of KLHL15, in a patient with severe ID, epilepsy, and anomalies of cortical development. In both cases, in spite of strong arguments for clinical relevance, we were not able at this stage to confirm pathogenicity of the mutations, and the causality of the variants identified in XLID remains to be confirmed.

Epileptic and nonepileptic features in patients with early onset epileptic encephalopathy and STXBP1 mutations.

PURPOSE: STXBP1 (MUNC18-1) mutations have been associated with various types of epilepsies, mostly beginning early in life. To refine the phenotype associated with STXBP1 aberrations in early onset epileptic syndromes, we studied this gene in a cohort of patients with early onset epileptic encephalopathy. METHODS: STXBP1 was screened in a multicenter cohort of 52 patients with early onset epilepsy (first seizure observed before the age of 3 months), no cortical malformation on brain magnetic resonance imaging (MRI), and negative metabolic screening. Three groups of patients could be distinguished in this cohort: (1) Ohtahara syndromes (n = 38); (2) early myoclonic encephalopathies (n = 7); and (3) early onset epileptic encephalopathies that did not match any familiar syndrome (n = 7). None of the patients displayed any cortical malformation on brain MRI and all were screened through multiple video-electroencephalography (EEG) recordings for a time period spanning from birth to their sixth postnatal month. Subsequently, patients had standard EEG or video-EEG recordings. KEY FINDINGS: We found five novel STXBP1 mutations in patients for whom video-EEG recordings could be sampled from the beginning of the disease. All patients with a mutation displayed Ohtahara syndrome, since most early seizures could be classified as epileptic spasms and since the silent EEG periods were on average shorter than bursts. However, each patient in addition displayed a particular clinical and EEG feature: In two patients, early seizures were clonic, with very early EEG studies exhibiting relatively low amplitude bursts of activity before progressing into a typical suppression-burst pattern, whereas the three other patients displayed epileptic spasms associated with typical suppression-burst patterns starting from the early recordings. Epilepsy dramatically improved after 6 months and finally disappeared before the end of the first year of life for four patients; the remaining one patient had few seizures until 18 months of age. In parallel, EEG paroxysmal abnormalities disappeared in three patients and decreased in two, giving place to continuous activity with fast rhythms. Each patient displayed frequent nonepileptic movement disorders that could easily be mistaken for epileptic seizures. These movements could be observed as early as the neonatal period and, unlike seizures, persisted during all the follow-up period. SIGNIFICANCE: We confirm that STXBP1 is a major gene to screen in cases of Ohtahara syndrome, since it is mutated in >10% of the Ohtahara patients within our cohort. This gene should particularly be tested in the case of a surprising evolution of the patient condition if epileptic seizures and EEG paroxysmal activity disappear and are replaced by fast rhythms after the end of the first postnatal year.

Ambiguous genitalia, microcephaly, seizures, bone malformations, and early death: a distinct MCA/MR syndrome.

We report on two siblings with hypotonia, ambiguous genitalia, microcephaly, ptosis, microretrognathia, thin lips, seizures, absent ossification of pubic rami, and brain abnormalities at the MRI. The two siblings died at 5 and 8 months, respectively. Molecular analysis indicated that SOX9, ARX, and DHCR7 genes were normal. Comparative genomic hybridization (CGH)-array analysis performed on the younger boy indicated two notable deletions, one on paternally inherited chromosome 4, and one on maternally inherited chromosome 5. The same deletions were found in a normal sister. Differential diagnoses and the possibility of a hitherto unreported syndrome are discussed.

Disruption of the ATP8A2 gene in a patient with a t(10;13) de novo balanced translocation and a severe neurological phenotype.

Mental retardation is a frequent condition that is clinically and genetically highly heterogeneous. One of the strategies used to identify new causative genes is to take advantage of balanced chromosomal rearrangements in affected patients. We characterized a de novo t(10;13) balanced translocation in a patient with severe mental retardation and major hypotonia. We found that the balanced translocation is molecularly balanced. The translocation breakpoint disrupts the coding sequence of a single gene, called ATP8A2. The ATP8A2 gene is not ubiquitously expressed, but it is highly expressed in the brain. In situ hybridization performed in mouse embryos at different stages of development with the mouse homologue confirms this observation. A total of 38 patients with a similar phenotype were screened for mutations in the ATP8A2 gene but no mutations were found. The balanced translocation identified in this patient disrupts a single candidate gene highly expressed in the brain. Although this chromosomal rearrangement could be the cause of the severe phenotype of the patient, we were not able to identify additional cases. Extensive screening in the mentally retarded population will be needed to determine if ATP8A2 haploinsufficiency or dysfunction causes a neurological phenotype in humans.

Deletion of YWHAE in a patient with periventricular heterotopias and pronounced corpus callosum hypoplasia.

BACKGROUND: Malformations of cortical development are not rare and cause a wide spectrum of neurological diseases based on the affected region in the cerebral cortex. A significant proportion of these malformations could have a genetic basis. However, genetic studies are limited because most cases are sporadic and mendelian forms are rare. METHODS: In order to identify new genetic causes in patients presenting defects of cortical organisation, array based comparative genomic hybridisation was performed in a cohort of 100 sporadic cases with various types of cortical malformations in search for inframicroscopic chromosomal rearrangements. RESULTS: In one patient presenting with periventricular nodular heterotopias and pronounced corpus callosum hypoplasia, a small (400 kb) 17p13.3 deletion involving the YWHAE gene was identified. It is shown that YWHAE is the only brain expressed gene in the deleted region and that the other genes in the interval are unlikely to contribute to the brain malformation phenotype of this patient. CONCLUSION: Most 17p13.3 deletions reported to date are large, such as the deletions causing Miller-Dieker syndrome, and involve several genes implicated in various steps of brain development. Haploinsufficiency of the mouse orthologue of YWHAE causes a defect of neuronal migration. However, the human counterpart of this phenotype was not known. The case described here represents the smallest reported deletion involving the YWHAE gene and could represent the human counterpart of the abnormal cortical organisation phenotype presented by the Ywhae heterozygous knockout mouse.

Characterization of a de novo balanced translocation in a patient with moderate mental retardation and dysmorphic features.

Moderate mental retardation (MR) could affect up to 3% of the general population. A proportion of these cases has a genetic origin. Genes responsible for mental retardation can be identified taking advantage of familial cases or patients carrying a chromosomal rearrangement. We have studied a female patient with mild mental retardation and dysmorphic features. Cytogenetic and molecular investigations revealed a de novo balanced translocation 46, XX, t(5;18)(q21.3;q21.32) in the patient. The karyotypes of the parents are normal. We mapped the breakpoints of the translocation on chromosomes 5 and 18 by fluorescence in situ hybridization (FISH). The characterization of the chromosomal breakpoints helped us identify a new candidate region containing a portion of a gene. This gene is called FER. It is a tyrosine kinase located on the chromosome 5q21.3. We found no known genes in the genomic region corresponding to the BAC spanning the 18q21.32 breakpoint. Molecular analysis showed that the FER gene was not interrupted by the translocation breakpoint on chromosome 5. Real-time quantitative PCR performed using RNA from the patient, compared to her parents and controls, showed no significant modification of FER expression ruling out a putative position effect, at least in the tissue tested. Our data suggest that FER is not implicated in the mental retardation phenotype observed in the reported patient. Therefore the MR phenotype might not be caused by the translocation.

In human pachytene spermatocytes, SUMO protein is restricted to the constitutive heterochromatin.

SUMO-1, a ubiquitin-like protein, is covalently bound to many proteins, leading to chromatin inactivation and transcriptional repression. The high concentration of SUMO-1 on the XY body in rodents suggests that this protein has an important role in facultative heterochromatin organization. In human, the precise role of SUMO in chromatin/heterochromatin organization remains to be defined. Here we describe the SUMO-1 distribution, during human male meiosis, in relation to the formation of the different types of heterochromatin. We show that, during late pachynema, SUMO-1 appears on the constitutive heterochromatin, but is excluded from the XY body facultative heterochromatin. At the SUMO-1 labelled areas, the presence of HP1alpha protein, as well as of trimethylated H3-K9 and H4-K20 histone modifications, supports a role for SUMO-1 in constitutive heterochromatin organization. We also establish that, on the constitutive heterochromatin, H4-K20me3 staining progressively decreases as SUMO-1 staining increases, suggesting that core histone(s), and histone H4 in particular, are direct targets for sumoylation. Our results also suggest that, in the context of global histone H4 hyperacetylation that precedes the histone-to-protamine transition at postmeiotic stages of spermatogenesis, histone H4 sumoylation may represent an important epigenetic marker replacing methylation on the constitutive heterochromatin.