Development of the cerebellar defect in ataxic SELH/Bc mice

Insights into the Etiology of Mammalian Neural Tube Closure Defects from Developmental, Genetic and Evolutionary Studies

The human neural tube defects (NTD), anencephaly, spina bifida and craniorachischisis, originate from a failure of the embryonic neural tube to close. Human NTD are relatively common and both complex and heterogeneous in genetic origin, but the genetic variants and developmental mechanisms are largely unknown. Here we review the numerous studies, mainly in mice, of normal neural tube closure, the mechanisms of failure caused by specific gene mutations, and the evolution of the vertebrate cranial neural tube and its genetic processes, seeking insights into the etiology of human NTD. We find evidence of many regions along the anterior–posterior axis each differing in some aspect of neural tube closure—morphology, cell behavior, specific genes required—and conclude that the etiology of NTD is likely to be partly specific to the anterior–posterior location of the defect and also genetically heterogeneous. We revisit the hypotheses explaining the excess of females among cranial NTD cases in mice and humans and new developments in understanding the role of the folate pathway in NTD. Finally, we demonstrate that evidence from mouse mutants strongly supports the search for digenic or oligogenic etiology in human NTD of all types.

Purkinje cell compartmentalization in the cerebellum of the spontaneous mutant mouse dreher

The cerebellar morphological phenotype of the spontaneous neurological mutant mouse dreher (Lmx1a(dr-J)) results from cell fate changes in dorsal midline patterning involving the roof plate and rhombic lip. Positional cloning revealed that the gene Lmx1a, which encodes a LIM homeodomain protein, is mutated in dreher, and is expressed in the developing roof plate and rhombic lip. Loss of Lmx1a causes reduction of the roof plate, an important embryonic signaling center, and abnormal cell fate specification within the embryonic cerebellar rhombic lip. In adult animals, these defects result in variable, medial fusion of the cerebellar vermis and posterior cerebellar vermis hypoplasia. It is unknown whether deleting Lmx1a results in displacement or loss of specific lobules in the vermis. To distinguish between an ectopic and absent vermis, the expression patterns of two Purkinje cell-specific compartmentation antigens, zebrin II/aldolase C and the small heat shock protein HSP25 were analyzed in dreher cerebella. The data reveal that despite the reduction in volume and abnormal foliation of the cerebellum, the transverse zones and parasagittal stripe arrays characteristic of the normal vermis are present in dreher, but may be highly distorted. In dreher mutants with a severe phenotype, zebrin II stripes are fragmented and distributed non-symmetrically about the cerebellar midline. We conclude that although Purkinje cell agenesis or selective Purkinje cell death may contribute to the dreher phenotype, our data suggest that aberrant anlage patterning and granule cell development lead to Purkinje cell ectopia, which ultimately causes abnormal cerebellar architecture in dreher.

A consideration of the evidence that genetic defects in planar cell polarity contribute to the etiology of human neural tube defects

A variety of human birth defects originate in failure of closure of the embryonic neural tube. The genetic cause of the most common nonsyndromic defects, spina bifida (SB) or anencephaly, is considered to be combinations of variants at multiple genes. The genes contributing to the etiology of neural tube closure defects (NTDs) are unknown. Mutations in planar cell polarity (PCP) genes in mice cause a variety of defects including the NTD, craniorachischisis, and sometimes SB or exencephaly (EX); they also demonstrate the role of digenic combinations of PCP mutants in NTDs. Recent studies have sought rare predicted-to-be-deleterious alterations (putative mutations) in coding sequence of PCP genes in human cases with various anomalies of the neural tube. This review summarizes the cumulative results of these studies according to a framework based on the embryopathogenesis of NTDs, and considers some of the insights from the approaches used and the limitations. Rare putative mutations in the PCP genes VANGL2, SCRIB, DACT1, and CELSR1 cumulatively contributed to over 20% of cases with craniorachischisis, a rare defect; no contributing variants were found for PRICKLE1 or PTK7. PCP rare putative mutations had a weaker role in myelomeningocele (SB), being found in approximately 6% of cases and cumulated across CELSR1, FUZ, FZD6, PRICKLE1, VANGL1, and VANGL2. These results demonstrate that PCP gene alterations contribute to the etiology of human NTDs. We recommend that future research should explore other types of PCP gene variant such as regulatory mutations and low frequency (1 to 5%) deleterious polymorphisms.

Development and maturation of the spinal cord: implications of molecular and genetic defects

The human central nervous system (CNS) may be the most complex structure in the universe. Its development and appropriate specification into phenotypically and spatially distinct neural subpopulations involves a precisely orchestrated response, with thousands of transcriptional regulators combining with epigenetic controls and specific temporal cues in perfect synchrony. Understandably, our insight into the sophisticated molecular mechanisms which underlie spinal cord development are as yet limited. Even less is known about abnormalities of this process - putative genetic and molecular causes of well-described defects have only begun to emerge in recent years. Nonetheless, modern scientific techniques are beginning to demonstrate common patterns and principles amid the tremendous complexity of spinal cord development and maldevelopment. These advances are important, given that developmental anomalies of the spinal cord are an important cause of mortality and morbidity (Sadler, 2000); it is hoped that research advances will lead to better methods to detect, treat, and prevent these lesions.

Maternal diet alters exencephaly frequency in SELH/Bc strain mouse embryos

BACKGROUND

The SELH/Bc mouse inbred strain, with a high frequency of nonsyndromic, genetically-multifactorial exencephaly, is a model for human cranial neural tube defects (NTDs). Maternal diet affects risk of human NTDs.

METHODS

Exencephaly frequencies in SELH/Bc embryos were compared in 8 studies in which dams were fed alternative commercial Purina diets (5015 and 5001) or semisynthetic diets, and in several studies in which maternal diet was supplemented with a specific nutrient, either in drinking water or food before and during pregnancy, or by intraperitoneal injection on E7 and/or E8.

RESULTS

The exencephaly frequency in SELH/Bc embryos was 2- to 8-fold higher when the dams were fed Purina 5015 (averaging 23% exencephaly) or a semisynthetic diet modeled on Purina 5015 (averaging 28%) or NIH-31 standard diet (23%), compared with Purina 5001 (averaging 7%). The exencephaly frequency remained high (41%) on a semisynthetic diet modeled on Purina 5001. The exencephaly frequency was not reduced significantly by maternal supplementation with folic acid, nor with each of zinc, methionine, niacin, brewers' yeast, riboflavin, vitamin B12, or inositol. Nor was it reduced by maternal diets with supplemental methyl donors and cofactors or with reduced fat.

CONCLUSIONS

The frequency of exencephaly in SELH/Bc embryos is strongly influenced by a specific unidentified aspect of the commercial ration Purina 5001 that prevents 55-85% of exencephaly in SELH/Bc embryos, when directly compared with an alternative commercial ration Purina 5015 or its semisynthetic mimic. This strong maternal diet effect on NTD frequency may point to novel nutritional approaches to prevention of human NTDs.

Multifactorial genetics of exencephaly in SELH/Bc mice

BACKGROUND

The SELH/Bc mouse strain has 10-30% exencephaly and is an animal model for human neural tube closure defects. This study examined the number of causative genes, their dominance relationships, and linkage map positions.

METHODS

The SELH/Bc strain (S) was crossed to the normal LM/Bc strain (L) and frequencies of exencephaly were observed in the F(1), BC(1), and F(2) generations. 102 F(2) males were individually testcrossed by SELH/Bc. The extremes, the 10 highest and 10 zero exencephaly-producing F(2) sires, were typed for 109 SSLP marker loci in a genome screen. Next, the resultant five provisional chromosomal regions were tested for linkage in 31 F(2) exencephalic embryos. Finally, 12 males, SS or LL for the Chr 13 region on an LM/Bc background, were testcrossed by SELH/Bc.

RESULTS

The exencephaly frequencies in the F(1) (0.3%), BC(1) (4.4%), and F(2) (3.7%), and the distribution of F(2) males' testcross values (0-15.5%), indicated that the high risk of exencephaly in SELH/Bc is due to the cumulative effect of two or three loci. Linkage studies indicated the location of semidominant exencephaly-risk genes on Chr 13 near D13Mit13 (P < 0.001), Chr 5 near D5Mit168 (P < 0.025), and possibly Chr 11 near D11Mit10 (P < 0.07). The gene on Chr 13, Exen1, and the strong role of other loci were confirmed by the congenic males.

CONCLUSIONS

The high risk of exencephaly in SELH/Bc mice is caused by the cumulative effect of two to three semidominant genes. Candidate genes include Msx2, Madh5, Ptch, and Irx1 (Chr 13) and Actb and Rac1 (Chr 5).

Mini-review: toward understanding mechanisms of genetic neural tube defects in mice

We review the data from studies of mouse mutants that lend insight to the mechanisms that lead to neural tube defects (NTDs). Most of the 50 single-gene mutations that cause neural tube defects (NTDs) in mice also cause severe embryonic-lethal syndromes, in which exencephaly is a nonspecific feature. In a few mutants (e.g., Trp53, Macs, Mlp or Sp), other defects may be present, but affected fetuses can survive to birth. Multifactorial genetic causes, as are present in the curly tail stock (15-20% spina bifida), or the SELH/Bc strain (15-20% exencephaly), lead to nonsyndromic NTDs. The mutations indicate that "spina bifida occulta," a dorsal gap in the vertebral arches over an intact neural tube, is usually genetically and developmentally unrelated to exencephaly or "spina bifida" (aperta). Almost all exencephaly or spina bifida aperta of genetic origin is caused by failure of neural fold elevation. The developmental mechanisms in genetic NTDs are considered in terms of distinct rostro-caudal zones along the neural folds that likely differ in mechanism of elevation. Failure of elevation leads to: split face (zone A), exencephaly (zone B), rachischisis (all of zone D), or spina bifida (caudal zone D). The developmental mechanisms leading to these genetic NTDs are heterogeneous, even within one zone. At the tissue level, the mutants show that the mechanism of failure of elevation can involve, e.g., (1) slow growth of adjacent tethered tissue (curly tail), (2) defective forebrain mesenchyme (Cart1 or twist), (3) defective basal lamina in surface ectoderm (Lama5), (4) excessive breadth of floorplate and notochord (Lp), (5) abnormal neuroepithelium (Apob, Sp, Tcfap2a), (6) morphological deformation of neural folds (jmj), (7) abnormal neuroepithelial and neural crest cell gap-junction communication (Gja1), or (8) incomplete compensation for a defective step in the elevation sequence (SELH/Bc). At the biochemical level, mutants suggest involvement of: (1) faulty regulation of apoptosis (Trp53 or p300), (2) premature differentiation (Hes1), (3) disruption of actin function (Macs or Mlp), (4) abnormal telomerase complex (Terc), or (5) faulty pyrimidine synthesis (Sp). The NTD preventative effect of maternal dietary supplementation is also heterogeneous, as demonstrated by: (1) methionine (Axd), (2) folic acid or thymidine (Sp), or (3) inositol (curly tail). The heterogeneity of mechanism of mouse NTDs suggests that human NTDs, including the common nonsyndromic anencephaly or spina bifida, may also reflect a variety of genetically caused defects in developmental mechanisms normally responsible for elevation of the neural folds.

Neurulation abnormalities secondary to altered gene expression in neural tube defect susceptible Splotch embryos

The murine mutant Splotch (Sp) is a well-established model for studying neural tube closure defects. In the current investigation, the progression through neural tube closure (NTC) as well as the expression patterns of 12 developmentally regulated genes were examined in the neural tissue of wildtype (+/+), Splotch heterozygous (Sp/+), and Splotch homozygous (Sp/Sp) embryos during neurulation. The overall growth of the embryos, as measured by the number of somite pairs, did not differ significantly between the three genotypes at any of the collection time-points. There was, however, a significant delay in the progression through NTC for both the Sp/+ and Sp/Sp embryos. A univariate analysis on the expression of the 12 candidate genes (bcl-2, FBP-2, Hmx-2, Msx-3, N-cam, N-cad, noggin, p53, Pax-3, Shh, Wee-1, wnt-1) revealed that although 11 were statistically altered, across time or by genotype, there were no significant interactions between gestation age and genotype for any of these genes during NTC. However, a multivariate statistical analysis on the simultaneous expression of these genes revealed interactions at both gestation day (GD) 8:12 (day:hour) and 9:00 among Pax-3, N-cam, N-cad, bcl-2, p53, and Wee-1 that could potentially explain the aberrant NTC. The data from these studies suggest that a disruption in the genes that govern the cell cycle or extracellular matrices of the developing neural tube might play a critical role in the occurrence of the NTDs observed in Splotch embryos.

Genetic landmarks for defects in mouse neural tube closure

Many mutations cause neural tube closure defects (NTDs, exencephaly or spina bifida) in mice and the gene loci are widely distributed in the mouse genome. This compilation summarizes the map position of 40 mouse NTD mutations and the corresponding human linkage homology of each. It includes the nature of the gene product where known, and whether the NTD is part of a syndrome involving other developmental systems. Also listed are the several mouse strains known to have genetic susceptibility to exencephaly, with multifactorial genetic cause in at least one case. The purposes of this mouse NTD compilation are to enable recognition of patterns in genetic causes of NTDs, of molecular pathways essential for closure of specific regions of the mammalian neural tube, and of candidate regions for mapping loci contributing to human multifactorial NTDs.

Exencephaly and cleft cerebellum in SELH/Bc mouse embryos are alternative developmental consequences of the same underlying genetic defect

SELH/Bc inbred mice have ataxia in 5-10% of young adults and exencephaly in 10-20% of newborns. SELH/Bc mice also have a high rate of spontaneous mutation and therefore it could not be assumed that these two abnormalities share the same genetic cause. Previously, we have shown that the liability to exencephaly in SELH/Bc mice is multifactorial, involving two to three loci, and that all the ataxics have a midline cleft cerebellum. The purpose of the present study was to resolve the genetic relationship between liability to exencephaly and liability to cleft cerebellum. We tested whether these traits were transmitted together by segregating F2 males; cotransmission would indicate that both traits are probably caused by the same genes. Approximately 100 embryos from each of 25 F2 sires from a cross between SELH/Bc and the normal LM/Bc strain were scored for exencephaly and the non-exencephalic embryos were scored for cleft cerebellum. The range of exencephaly production by these 25 F2 sires was 0% to 16%; the sires had been selected to represent the extremes of the range of exencephaly production. We found that the 10 sires that produced no exencephaly also produced no cleft cerebellum and 12 of the 15 sires that produced some exencephaly also produced some cleft cerebellum. This indicated strongly that the two traits are transmitted together (Fisher's exact test, P < 0.0002). Furthermore, within exencephaly-producing sires, the specific frequencies of the two traits were significantly positively correlated (Spearman rs = 0.58; P < 0.05), indicating that the same multifactorial risk factors influence both traits. All SELH/Bc embryos omit one normal initiation site of cranial neural tube closure, Closure 2. In a previous study, absence of the Closure 2 initiation site of cranial neural tube closure has been shown to be genetically correlated with liability to exencephaly. In the second part of the present study, the same Closure 2 data from eight of the F2 sires were observed to be significantly positively correlated with liability to cleft cerebellum (Spearman rs = 0.83; P < 0.05). The results of this genetic approach have supported the hypothesis, based on observation of embryos, that one basic multifactorial genetic defect in SELH mice leads to an abnormal cranial neural tube closure mechanism, to exencephaly to cleft cerebellum, and to ataxia.