Edwin Reyniers

Fmr1 knockout mice: A model to study fragile X mental retardation

Male patients with fragile X syndrome lack FMR1 protein due to silencing of the FMR1 gene by amplification of a CGG repeat and subsequent methylation of the promoter region. The absence of FMR1 protein leads to mental retardation, aberrant behavior, and macroorchidism. Hardly anything is known about the physiological function of FMR1 and the pathological mechanisms leading to these symptoms. Therefore, we designed a knockout model for the fragile X syndrome in mice. The knockout mice lack normal Fmr1 protein and show macroorchidism, learning deficits, and hyperactivity. Consequently, this knockout mouse may serve as a valuable tool in the elucidation of the physiological role of FMR1 and the mechanisms involved in macroorchidism, abnormal behavior, and mental retardation.

A Point mutation in the FMR-1 gene associated with fragile X mental retardation.

The vast majority of patients with fragile X syndrome show a folate–sensitive fragile site at Xq27.3 (FRAXA) at the cytogenetic level, and both amplification of the (CGG)n repeat and hypermethylation of the CpG island in the 5′ fragile X gene (FMR–1) at the molecular level. We have studied the FMR–1 gene of a patient with the fragile X phenotype but without cytogenetic expression of FRAXA, a (CGG)n repeat of normal length and an unmethylated CpG island. We find a single point mutation in FMR–1 resulting in an Ne367Asn substitution. This de novo mutation is absent in the patients family and in 130 control X chromosomes, suggesting that the mutation causes the clinical abnormalities. Our results suggest that mutations in FMR–1 are directly responsible for fragile X syndrome, irrespective of possible secondary effects caused by FRAXA

A new chromosome 17q21.31 microdeletion syndrome associated with a common inversion polymorphism

Submicroscopic genomic copy number changes have been identified only recently as an important cause of mental retardation. We describe the detection of three interstitial, overlapping 17q21.31 microdeletions in a cohort of 1,200 mentally retarded individuals associated with a clearly recognizable clinical phenotype of mental retardation, hypotonia and a characteristic face. The deletions encompass the MAPT and CRHR1 genes and are associated with a common inversion polymorphism.

Mildly impaired water maze performance in male Fmr1 knockout mice

Fmr1 knockout mice constitute a putative model of fragile X syndrome, the most common form of heritable mental disability in humans. We have compared the performance of transgenic mice with an Fmr1 knockout with that of normal littermates in hidden- and visible-platform water maze learning, and showed that knockouts exhibit subnormal spatial learning abilities and marginal motor performance deficits. During 12 training trials of the hidden-platform task, escape latency and path length decreased significantly in knockouts and control littermates, and no effect of genotype was found. During four ensuing reversal trials, however, significant differences were found between knockouts and control littermates both in escape latency and path length. During the visible-platform condition, the reversal trials also revealed a difference between knockouts and normal littermates in escape latency, but not in path length. Possibly due to marginal motor incapacity, knockouts swam significantly slower than controls during these latter trials. During both probe trials of the hidden-platform task, knockouts as well as normal littermates spent more time in the target quadrant than in the other quadrants, and percent of time spent in the target quadrant was the same in both groups; swimming velocity was not significantly different between knockouts and normal littermates during these trials. Entries in the target area during the probe trials did show a significant effect of genotype on number of entries. The present results largely confirm and extend our previous findings. Impaired spatial abilities in Fmr1 knockouts might have been due to relatively low response flexibility or high memory interference in Fmr1 knockouts. It remains unclear, however, which brain region or neurochemical system might be involved in these disabilities. We conclude that Fmr1 knockout mice might be a valid model of fragile X mental retardation.

Long-term potentiation in the hippocampus of fragile X knockout mice.

To gain more insight in the physiological function of the fragile X gene (FMR1) and the mechanisms leading to fragile X syndrome, the Fmr1 gene has been inactivated in mice by gene targeting techniques. In the Morris water maze test, the Fmr1 knockout mice learn to find the hidden platform nearly as well as the control animals, but show impaired performance after the position of the platform has been modified. As malperformance in the Morris water maze test has been associated with impaired long-term potentiation (LTP), electrophysiological studies were performed in hippocampal slices of Fmr1 knockout mice to check for the presence of LTP. Judged by field extracellular excitatory postsynaptic potential recordings in the CA1 hippocampal area, Fmr1 knockout mice express LTP to a similar extent as their wild type littermates during the first 1-2 hr after high frequency stimulation. Also, short-term potentiation (STP) was similar in both types of mice. To investigate whether Fmr1 is involved in the latter stages of LTP as an immediate early gene, we compared Fmr1 mRNA quantities on northern blots after chemical induction of seizures. A transient increase in the transcription of immediate early genes is thought to be essential for the maintenance of LTP. As no increase in Fmr1 mRNA could be detected, neither in cortex nor in total brain, during the first 2 1/2 hr after pentylenetetrazol-induced seizures, it is unlikely that Fmr1 is an immediate early gene in mice. In conclusion, we found no evidence for a function of FMR1 in STP or LTP.

Transgenic mouse model for the fragile X syndrome

Transgenic fragile X knockout mice have been constructed to provide an animal model to study the physiologic function of the fragile X gene (FMR1) and to gain more insight into the clinical phenotype caused by the absence of the fragile X protein. Initial experiments suggested that the knockout mice show macroorchidism and cognitive and behavioral deficits, abnormalities comparable to those of human fragile X patients. In the present study, we have extended our experiments, and conclude that the Fmr1 knockout mouse is a reliable transgenic model to study the fragile X syndrome.

Fourteen new cases contribute to the characterization of the 7q11.23 microduplication syndrome.

Interstitial deletions of 7q11.23 cause Williams-Beuren syndrome, one of the best characterized microdeletion syndromes. The clinical phenotype associated with the reciprocal duplication however is not well defined, though speech delay is often mentioned. We present 14 new 7q11.23 patients with the reciprocal duplication of the Williams-Beuren syndrome critical region, nine familial and five de novo. These were identified by either array-based MLPA or by array-CGH/oligonucleotide analysis in a series of patients with idiopathic mental retardation with an estimated population frequency of 1:13,000-1:20,000. Variable speech delay is a constant finding in our patient group, confirming previous reports. Cognitive abilities range from normal to moderate mental retardation. The association with autism is present in five patients and in one father who also carries the duplication. There is an increased incidence of hypotonia and congenital anomalies: heart defects (PDA), diaphragmatic hernia, cryptorchidism and non-specific brain abnormalities on MRI. Specific dysmorphic features were noted in our patients, including a short philtrum, thin lips and straight eyebrows. Our patient collection demonstrates that the 7q11.23 microduplication not only causes language delay, but is also associated with congenital anomalies and a recognizable face.

Spatial learning, contextual fear conditioning and conditioned emotional response in Fmr1 knockout mice

Fmr1 knockout mice are an animal model for fragile X syndrome, the most common form of heritable mental retardation in humans. Fmr1 knockout mice exhibit macro-orchidism and cognitive and behavioural deficits reminiscent of the human phenotype. In the present study additional behavioural and cognitive testing was performed. Knockouts and control littermates were subjected to a spatial learning test using a plus-shaped water maze. Animals had to learn the position of a hidden escape platform during training trials. The position of this platform was changed during subsequent reversal trials. Previously reported deficits in reversal learning were replicated, but we also observed significant differences during the acquisition trials. A plus-shaped water maze experiment with daily changing platform positions failed to provide clear evidence for a working memory impairment, putatively underlying the spatial learning deficits. Two different test settings were used to examine the reported deficit of Fmr1 knockout mice in fear conditioning. Conditioned fear responses were observed in a contextual fear test, and the ability to acquire an emotional response was tested by means of response suppression in a conditioned emotional response procedure. Neither protocol revealed significant differences between controls and knockouts.

Further molecular and clinical delineation of co-locating 17p13.3 microdeletions and microduplications that show distinctive phenotypes

Background Chromosome 17p13.3 contains extensive repetitive sequences and is a recognised region of genomic instability. Haploinsufficiency of PAFAH1B1 (encoding LIS1) causes either isolated lissencephaly sequence or Miller–Dieker syndrome, depending on the size of the deletion. More recently, both microdeletions and microduplications mapping to the Miller–Dieker syndrome telomeric critical region have been identified and associated with distinct but overlapping phenotypes. Methods Genome-wide microarray screening was performed on 7678 patients referred with unexplained learning difficulties and/or autism, with or without other congenital abnormalities. Eight and five unrelated individuals, respectively, were identified with microdeletions and microduplications in 17p13.3. Results Comparisons with six previously reported microdeletion cases identified a 258 kb critical region, encompassing six genes including CRK (encoding Crk) and YWHAE (encoding 14-3-3ε). Clinical features included growth retardation, facial dysmorphism and developmental delay. Notably, one individual with only subtle facial features and an interstitial deletion involving CRK but not YWHAE suggested that a genomic region spanning 109 kb, encompassing two genes (TUSC5 and YWHAE), is responsible for the main facial dysmorphism phenotype. Only the microduplication phenotype included autism. The microduplication minimal region of overlap for the new and previously reported cases spans 72 kb encompassing a single gene, YWHAE. These genomic rearrangements were not associated with low-copy repeats and are probably due to diverse molecular mechanisms. Conclusions The authors further characterise the 17p13.3 microdeletion and microduplication phenotypic spectrum and describe a smaller critical genomic region allowing identification of candidate genes for the distinctive facial dysmorphism (microdeletions) and autism (microduplications) manifestations.

Multiplex ligation-dependent probe amplification to detect subtelomeric rearrangements in routine diagnostics

Subtelomeric rearrangements are believed to be responsible for 5–7% of idiopathic mental retardation cases. Due to the relative complexity and high cost of the screening methods used till now, only preselected patient populations including mostly the more severely affected cases have been screened. Recently, multiplex ligation‐dependent probe amplification (MLPA) has been adapted for use in subtelomeric screening, and we have incorporated this technique into routine diagnostics of our laboratory. Since the evaluation of MLPA as a screening method, we tested 275 unselected patients with idiopathic mental retardation and detected 12 possible subtelomeric aberrations: a der(11)t(11;20)(qter;qter), a 19pter duplication, a der(18)t(18;10)(qter; pter), a 15qter deletion, a 8pter deletion, a 6qter deletion, a der(X)t(X;1)(pter;qter), a der(X)t(X;3)(pter;pter), a 5qter duplication, a 3pter deletion, and two 3qter duplications. The patients can be subdivided into two groups: the first containing de novo rearrangements that are likely related to the clinical presentation of the patient and the second including aberrations also present in one of the parents that may or may not be causative of the mental retardation. In our patient cohort, five (1.8%) subtelomeric rearrangements were de novo, three (1.1%) rearrangements were familial and suggestively disease causing, and four (1.5%) were possible polymorphisms. This high frequency of subtelomeric abnormalities detected in an unselected population warrants further investigation about the feasibility of routine screening for subtelomeric aberrations in mentally retarded patients.