Genetic analysis of the cause of exencephaly in the SELH/Bc mouse stock

A new mouse stock, SELH/Bc, having a high liability to exencephaly has been developed. About 17% of SELH fetuses are exencephalic. The genetic cause of this exencephaly was investigated in a cross to a normal related ICR/Bc strain and in subsequent classical genetic crosses (F2, first and second backcrosses). The data were compared with a number of genetic models, including that of a single recessive mutation with 17% penetrance. The data did not fit single-locus inheritance. The expectations from the multifactorial threshold model based on an underlying quantitative liability trait with additive inheritance were found to fit the data very well. The number of loci involved was estimated to be about two or three. About 70% of exencephalic SELH fetuses are female, and there is no overall deficiency of males. The relatively higher risk in females was constant across the genetic backgrounds in the experiment. In summary, the liability to exencephaly in SELH mice appears to be a multifactorial threshold trait, and it therefore resembles human neural tube defects in type of genetic etiology. SELH therefore may be a valuable animal model in the study of neural tube defects.     

Further genetic studies of the cause of exencephaly in SELH mice.

We have developed an inbred stock of mice called SELH that has a high frequency of the neural tube defect exencephaly at birth. A previous genetic study indicated that the exencephaly is due to two to three additive loci differing between SELH and a closely related normal strain, ICR/Bc, but this analysis was not designed to detect genetic maternal effects. Recently, we demonstrated that there is genetic polymorphism among normal mouse strains leading to differences in site of initiation of closure of the cranial neural tube. In the present study, an inbred substrain of SELH mice, with 24% exencephaly among embryos, was crossed with an unrelated normal strain, SWV/Bc, and the frequency of exencephaly in subsequent generations used to extend our understanding of the genetic cause of exencephaly in SELH mice. The purposes of the genetic studies reported here were twofold. First, based on the influence of genetic maternal effects on other genetically complex birth defects in mice, we hypothesized that the exencephaly of SELH mice would exhibit strong genetic maternal effects. This hypothesis was tested by comparisons among the four possible reciprocal backcrosses to SELH. The result was an overall frequency of 2.3% exencephaly in first backcross embryos with no difference among the four crosses and no evidence of genetic maternal effects. Second, the frequency of exencephaly recovered in the backcross and F1 embryos was compared with the previous genetic study and with various genetic models. The frequencies were similar to those obtained from the cross to ICR/Bc mice and were compatible with a hypothesis of additive gene action at a few loci.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies of the effect of retinoic acid on anterior neural tube closure in mice genetically liable to exencephaly

Previously we have shown that all SELH/Bc mouse embryos close their anterior neural tubes by an abnormal mechanism and that 10-20% of SELH/Bc embryos are exencephalic. The purposes of these studies were (1) to observe the effects of retinoic acid on the frequency of exencephaly in SELH/Bc embryos; (2) to compare the SELH/Bc response with those of normal strains and of other neural tube mutants; and (3) to compare, between SELH/Bc and a normal strain (SWV/Bc), the effects of retinoic acid on morphology of the closing anterior neural tube. SELH/Bc was more liable to retinoic acid-induced exencephaly than were normal strains. After maternal treatment with 5 mg/kg retinoic acid on day 8.5 of gestation, 53% of SELH/Bc embryos had exencephaly, compared with 22% in ICR/Bc and 14% in SWV/Bc. When these results were transformed according to the assumptions of the developmental threshold model, the effects of genotype and retinoic acid appeared to be additive. Similar treatment on day 9 or 10 of gestation had little or no effect on the frequency of exencephaly in SELH/Bc mice. These results are similar to the reported responses of the curly-tail and Splotch mutants, where frequencies of spina bifida but not exencephaly were decreased. This pattern suggests that studies of effects of periconceptional vitamin treatment on risk of human neural tube defects should consider anencephaly and spina bifida separately. The study comparing the morphology of anterior neural tube closure in SELH/Bc and normal SWV/Bc embryos showed that retinoic acid delays the elevation of the mesencephalic neural folds. This results in a "stalling" of many embryos in the first steps of neural tube closure, with their neural folds remaining convex and splayed wide apart. The delay in fold elevation was superimposed on the different closure patterns of the two strains. The overall conclusion is that there is no nonadditive interaction in the parameters studied between retinoic acid treatment and the SELH/Bc genotype.     

Developmental study of neural tube closure in a mouse stock with a high incidence of exencephaly

About 17% of embryos and fetuses in the SELH/Bc mouse stock have the anterior neural tube defect, exencephaly. No other malformations are seen. The genetic liability to exencephaly was shown to be probably genetically fixed in the SELH/Bc stock. This means that SELH/Bc embryos with successful neural tube closure are genetically the same as exencephalics. Females were significantly more likely to be affected than males (66% females). The pattern of morphological developmental events during anterior neural tube closure on days 8 and 9 of gestation was compared among 322 ICR/Bc (normal), 304 SWV/Bc (normal), and 265 SELH/Bc embryos. Anterior neural tube closure was found to follow a strikingly different pattern in almost all SELH/Bc embryos than in either of the normal strains or in previous published studies. SELH/Bc embryos lack the initial contact between the anterior folds in the posterior prosencephalon/anterior mesencephalon region (Closure 2). In spite of this, all but 17% manage to close the anterior neural tube by extending caudally the later occurring normal anterior zone of contact and fusion at the most rostral aspect of the prosencephalon (Closure 3) through the region of Closure 2 to meet the zone of closure of the rhombencephalon, Closure 4. Anterior neural tube closure was completed late, and in some SELH/Bc embryos, elevation and fusion in the mesencephalon did not occur at all. In histological sections of six- and eight-somite embryos, elevated numbers of pyknotic cells in the neuroepithelium and mesenchyme, and elevated numbers of unstained inclusions in the neuroepithelium were found; but their relationship, if any, to the abnormal pattern of neural tube closure is not clear.     

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.     

QTL on mouse chromosomes 1 and 4 causing sperm-head morphological abnormality and male subfertility

The B10.M mouse strain represents a model for male subfertility as it produces a significantly low number of offspring. The only known male reproductive phenotype of this strain is its high frequency of sperm-head morphological abnormalities (44.7 ± 2.4 %). We previously reported that this phenotype was the product of two recessive loci. In this study we mapped the loci causing the high frequency of sperm-head morphological abnormalities in this strain using F2 animals produced by crossing B10.M and C3H mice. Quantitative trait loci (QTL) analysis (n = 178) identified two recessive genes, one on Chromosome (Chr) 1 (LOD score = 30.585) and one on Chr 4 (LOD score = 4.532). Further analysis (n = 854) mapped the locus on Chr 1 between Ercc5 (23.55 cM) and D1Mit528 (25.95 cM) and the locus on Chr 4 between D4Mit148 (69.48 cM) and D4Mit170 (70.47 cM). It was also found that the effects of these two loci were not independent. The major locus on Chr 1 determines the expression of sperm-head abnormalities, while the locus on Chr 4 enhances the frequency of abnormalities only when the genotype of the Chr 1 locus is homozygous for the B10.M allele. The major locus on Chr 1 was named sperm-head morphology 1 (Shm1), while the modifier locus on Chr 4 was named sperm-head morphology 2 (Shm2).     

Polygenetic susceptibility and resistance to 4-nitroquinoline 1-oxide-induced tongue carcinomas in the rat

Oral administration of 4-nitroquinoline 1-oxide (4NQO) to rats induced a high incidence of tongue carcinomas (TCs). The inbred Dark-Agouti (DA) strain of rats showed much higher susceptibility to 4NQO-induced TCs than the Wistar-Furth (WF) strain. Our previous study on crosses between the two strains postulated a semidominant susceptibility gene in DA and a semidominant resistance gene in WF rats. This hypothesis was confirmed by the genetic analysis of the back-crosses to either parent with PCR-based microsatellite assay. Using the number of TCS with >5 mm diameter as a quantitative parameter, we mapped a quantitative trait locus Stc1 (Susceptibility to TC) favouring TC development near the locus D19Mit9 on Chr. 19 with a peak LOD score of 6.08. Two other regions in Chr. 3 and Chr. 14 showed weak linkage for susceptibility, but were not statistically significant. On the other hand, another quantitative trait locus Rtc1 (Resistance to TC) providing resistance to TCs was mapped on Chr. 1 between the loci of D1Mit1 and D1Mit3 with a peak LOD score of 3.30. Quantitative parameters such as the number of tumours in the tongue or upper alimentary tract, the frequency of larger tumours and their maximum size were closely correlated and principally determined by Stc1 and Rtc1. Therefore the susceptibility to 4NQO-induced TCs in crosses between DA and WF is explained by the combinations of genotypes at these two loci. Possible candidate genes for Stc1 and Rtc1 are discussed.     

Gaping lids, gp, a mutation on centromeric Chromosome 11 that causes defective eyelid development in mice

In mammals, during fetal development, the eyelids grow and flatten over the eyes and temporarily fuse closed. Failure of this normal developmental process in mice leads to the defect, open-eyelids-at-birth. Nearly all newborns of the GP/Bc strain, homozygous for the spontaneous recessive mutation, gaping lids (gp), have bilateral open eyelids at birth, with essentially no fusion between the upper and lower eyelids. Histological sections and scanning electron microscopy of GP/Bc eyes during the normal period of eyelid growth and fusion indicate that gp/gp mutant fetuses have deficient upper and lower eyelids; surface periderm cells that appear to have some role in eyelid growth and fusion are present, but lack a normal ``streaming' pattern toward the fusion zone. No other defects due to the gaping lids mutation were detected. A genetic analysis based on outcrosses of GP/Bc to various linkage marker stocks and to CBA/J and ICR/Bc normal strains was done. Penetrance in F₂ segregants, but not in BC1 segregants, was usually significantly less than 100%, was strongly affected by the identity of the normal strain used, ranging from 44% to 92%, and indicated a potential complexity of modifiers. Forty-one affected F₂ and 120 BC₁ segregants from the outcross of GP/Bc to CBA/J, and 23 affected F₂ segregants from the outcross to ICR/Bc, were used to map gp to proximal Chr 11 between the centromere and D11Dal1 (Camk2b), an interval previously defined as less than 1 cM. Sets of whole F₂ litters from the crosses to CBA/J (n = 106) and ICR/Bc (n = 65) strains were typed for informative SSLPs near gp (D11Mit62 and D11Mit74, respectively) and demonstrated that the segregation ratios in the region are Mendelian. The known genes in the interval, Nf2 and Lif, do not seem to be obvious candidate genes for gp. An Egfr-null allele was used to confirm the previously reported map position of the potential candidate locus, Egfr, to a more distal interval, between D11Mit62/226 and D11Mit151, from which gp had been excluded. Tests for allelism showed that the Egfr mutation and the gp mutation complement each other, and therefore also indicate that they are at different gene loci. Open-eyelids-at-birth is associated with several mutations at other loci with variable penetrance owing to modifiers and in other more complex genetic liabilities in inbred strains, and the genetics of this trait is a model for other genetically complex developmental threshold traits. The gaping lids mutation identifies a previously unknown locus on proximal Chromosome (Chr) 11 that has a strong role in fetal eyelid growth. Received: 13 January 2000 / Accepted: 23 February 2000     

Mapping of genetic factors influencing the weight of cystic fibrosis knockout mice

One of the poorly understood clinical manifestations of cystic fibrosis (CF) is low body weight. Mice in which the CF causative gene, cystic fibrosis transmembrane conductance regulator (Cftr), has been knocked out reflect this as they are smaller than age-matched littermates. The variable weight of F2 Cftr -/- (CF) mice derived from a cross between congenic C57BL/6J and BALB/cJ Cftr heterozygotic mice permits the mapping of modifiers of this cystic fibrosis phenotype. In this report, quantitative trait loci (QTL) mapping was used to identify the chromosomal locations of genes that contribute to the body weight of 12-week-old F2 CF mice. Five loci of CF body weight were detected with four of the five acting in a sex-specific manner. Significant linkage of the phenotype to a region of Chromosome (Chr) 13 from D13Mit179 to D13Mit254 (LOD = 4.2) was established in female mice; and suggestive loci on Chrs 7 and 10 were identified. The weights of F2 male CF mice were suggestively linked to regions of Chrs 1 and 6, and to the same locus on Chr 7 as in female mice. The suggestive loci did not influence the weight of the limited set of control mice and thus are presumed to be CF specific in their effects. Further study of these putative CF body weight modifiers may provide insight on the pathogenesis of cystic fibrosis.     

Mapping genetic loci that regulate lipid levels in a NZB/B1NJxRF/J intercross and a combined intercross involving NZB/B1NJ, RF/J, MRL/MpJ, and SJL/J mouse strains

The NZB/B1NJ (NZB) mouse strain exhibits high cholesterol and HDL levels in blood compared with several other strains of mice. To study the genetic regulation of blood lipid levels, we performed a genome-wide linkage analysis in 542 chow-fed F2 female mice from an NZBxRF/J (RF) intercross and in a combined data set that included NZBxRF and MRL/MpJxSJL/J intercrosses. In the NZBxRF F2 mice, the cholesterol and HDL concentrations were influenced by quantitative trait loci (QTL) on chromosome (Chr) 5 [logarithm of odds (LOD) 17-19; D5Mit10] that was in the region identified earlier in crosses involving NZB mice, but two QTLs on Chr 12 (LOD 4.7; D12Mit182) and Chr 19 (LOD 5.7; D19Mit1) were specific to the NZBxRF intercross. Triglyceride levels were affected by two novel QTLs at D12Mit182 (LOD 8.7) and D15Mit13 (LOD 3.5). The combined-cross linkage analysis (1,054 mice, 231 markers) 1) identified four shared QTLs (Chrs 5, 7, 14, and 17) that were not detected in one of the parental crosses and 2) improved the resolution of two shared QTLs. In summary, we report additional loci regulating lipid levels in NZB mice that had not been identified earlier in crosses involving the NZB strain of mice. The identification of shared loci from multiple crosses increases confidence toward finding the QTL gene.     

The open-eyelid mutation, lidgap-Gates, is an eight-exon deletion in the mouse Map3k1 gene

The BALB/cGa mouse strain and its descendants, now called the SELH/Bc strain, have produced two waves of high frequency of spontaneous heritable mutations. One of these, the recessive lidgap-Gates (lg(Ga)) mutation, causes the same open-eyelids-at-birth phenotype as the gene knockout mutations of Map3k1 and co-maps to distal Chr 13. The lg(Ga) mutation is demonstrated to be a 27.5-kb deletion of exons 2-9 in the Map3k1 gene, the first spontaneous mutant allele described at this locus. The lg(Ga) mutation is consistent with a pattern suggesting that the waves of mutation in BALB/cGa and its descendants tend to be large deletions or ETn insertions, whose elevated rate of occurrence is due to an unknown mechanism.     

Identification of genetic determinants of IGF‐1 levels and longevity among mouse inbred strains

The IGF‐1 signaling pathway plays an important role in regulating longevity. To identify the genetic loci and genes that regulate plasma IGF‐1 levels, we intercrossed MRL/MpJ and SM/J, inbred mouse strains that differ in IGF‐1 levels. Quantitative trait loci (QTL) analysis of IGF‐1 levels of these F2 mice detected four QTL on chromosomes (Chrs) 9 (48 Mb), 10 (86 Mb), 15 (18 Mb), and 17 (85 Mb). Haplotype association mapping of IGF‐1 levels in 28 domesticated inbred strains identified three suggestive loci in females on Chrs 2 (13 Mb), 10 (88 Mb), and 17 (28 Mb) and in four males on Chrs 1 (159 Mb), 3 (52 and 58 Mb), and 16 (74 Mb). Except for the QTL on Chr 9 and 16, all loci co‐localized with IGF‐1 QTL previously identified in other mouse crosses. The most significant locus was the QTL on Chr 10, which contains the Igf1 gene and which had a LOD score of 31.8. Haplotype analysis among 28 domesticated inbred strains revealed a major QTL on Chr 10 overlapping with the QTL identified in the F2 mice. This locus showed three major haplotypes; strains with haplotype 1 had significantly lower plasma IGF‐1 and extended longevity (P < 0.05) than strains with haplotype 2 or 3. Bioinformatic analysis, combined with sequencing and expression studies, showed that Igf1 is the most likely QTL gene, but that other genes may also play a role in this strong QTL.     

Candidate genes for plasma triglyceride, FFA, and glucose revealed from an intercross between inbred mouse strains NZB/B1NJ and NZW/LacJ

To identify the genes controlling plasma concentrations of triglycerides (TGs), FFAs, and glucose, we carried out a quantitative trait loci (QTL) analysis of the closely related mouse strains New Zealand Black (NZB/B1NJ) and New Zealand White (NZW/LacJ), which share 63% of their genomes. The NZB x NZW F(2) progeny were genotyped and phenotyped to detect QTL, and then comparative genomics, bioinformatics, and sequencing were used to narrow the QTL and reduce the number of candidate genes. Triglyceride concentrations were linked to loci on chromosomes (Chr) 4, 7, 8, 10, and 18. FFA concentrations were affected by a significant locus on Chr 4, a suggestive locus on Chr 16, and two interacting loci on Chr 2 and 15. Plasma glucose concentrations were affected by QTL on Chr 2, 4, 7, 8, 10, 15, 17, and 18. Comparative genomics narrowed the QTL by 31% to 86%; haplotype analysis was usually able to further narrow it by 80%. We suggest several candidate genes: Gba2 on Chr 4, Irs2 on Chr 8, and Ppargc1b on Chr 18 for TG; A2bp1 on Chr 16 for FFA; and G6pc2 on Chr 2 and Timp3 on Chr 10 for glucose.     

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.     

Unravelling the complex genetics of cleft lip in the mouse model

Nonsyndromic cleft lip in ``A' strain mice and humans is genetically complex and is distinct from isolated cleft palate. Cleft lip embryos recovered in 2.4% of 1485 first backcross (BC₁) segregants from a cross of A/WySnJ (24% cleft lip) and C57BL/6J (no cleft lip) in A/WySnJ mothers, and in testcrosses of 10 recombinant inbred (RI) strains (AXB/Pgn or BXA/Pgn), were used for gene mapping and for inference of genetic architecture. The A/WySnJ maternal genotype increased cleft lip risk in reciprocal crosses; the relevant genetic difference between AXB-6/Pgn (8%) and A/WySnJ (24%) is entirely maternal. A combination of new mapping panels (325 meioses), new markers, and a recombinant cleft lip embryo redefined the location of a recessive factor essential to cleft lip risk, clf1, and candidate genes Itgb3 and Crhr, to between D11Mit146/360 and D11Mit166/147. A screen of 54 YACs for 46 genes and SSLP loci located Wnt15, Wnt3, Crhr, Mtapt, Itgb3, Dlx3, and Dlx7 within the clf1 candidate region. The clf2 locus was newly mapped to Chromosome (Chr) 13 by a genome screen of BC₁ segregants, and further defined to a 4-cM region between D13Mit13/54 and D13Mit231 by strain distribution patterns of cleft lip liability and markers in testcrossed RI strains. Specific combinations of marker genotypes associated with cleft lip risk indicated that high risk in A/WySnJ mice is caused by epistatic interaction between clf1 and clf2 in the context of a genetic maternal effect. Human homologs of clf1 and clf2 are expected to be on 17q and 5q/9q. Received: 17 May 2000 / Accepted: 30 November 2000     

Cleft palate: more genetic lessons from mice

Cleft palate occurred in high frequency (14%) in the F2 generation of the cross between two stocks of mice, LGG and SELH, neither of which produces more than 2% cleft palate. The cleft palate trait results from a new combination of alleles that is not present in either parental stock. The lack of cleft palate in the F2 generation after outcrosses of both parental stocks to other strains shows that this new combination of alleles has specific contributions from both parental strains, and also that there must be at least two loci involved. A deficiency of Mod-1 homozygotes in the SELH/LGG F2 adults suggests that one of the loci involved may be linked to Mod-1 and that the number of loci involved is few. Significantly more F2 males (19%) than females (9%) were affected with cleft palate. The data can be explained by a two-locus epistatic model with a dominant mutation (P) at one locus that causes cleft palate when not suppressed by or compensated for by a dominant allele (S) at a second locus. The parental stocks would be PPSS and ppss. In the F2 generation, the new combinations PPss and Ppss would express cleft palate, a total expected of 19%. Similar new combinations of alleles at two loci may explain some instances of high occurrence of cleft palate or other developmental threshold traits in previously unaffected human families.     

Gender-influenced obesity QTLs identified in a cross involving the KK type II diabetes-prone mouse strain

The inheritance of adiposity and related traits has been investigated in the obese, diabetes-prone KK/HlLt (KK) and the lean, normoglycemic C57BL/6J (B6) mouse strains, their F₁ hybrids, and a large intercross generation. Adiposity index (AI) was defined as the sum of four fat depot weights divided by body weight. Both male and female KK mice were obese, but AI values averaged twofold higher in females than in males. In contrast, B6 females were slightly more lean than males. A genome-wide search revealed several qualitative trait loci (QTLs) affecting AI. The proximal region of Chromosome (Chr) 9 has a large effect on AI, with a much stronger effect in females (lod = 6.3) than in males (lod = 2.7). The data for females fit a model in which a dominant allele from KK increases AI by 30%, with the lod score peak falling between markers D9Mit66 and D9Mit328. This QTL has large effects on inguinal and mesenteric fat pad weights, with smaller effects on gonadal and retroperitoneal fat pads. The region of Chr 9 containing this QTL has extensive homology to human Chr 11q. An X-linked QTL affecting AI was evident in males (lod = 3.77), but not females (lod = 0.7). Exclusion of mesenteric fat from male AI resulted in an increased lod score (lod = 5.0) at 8 cM distal to DXMit166. A suggestive AI QTL (lod = 4.2), differentially affecting males, was localized to Chr 18 near the glucocorticoid receptor locus. A region of Chr 7 had a strong effect on body weight (lod = 6.9), a significant effect on inguinal fat% (lod = 4.4), and a suggestive effect on AI in females (lod = 4.1). Plasma leptin levels were associated with genotypes on Chr 9 (lod = 5.9) and Chr 7 (lod = 4.2). A region of Chr 1 had a suggestive effect on fasted blood glucose (lod = 3.6). Received: 23 March 1999 / Accepted: 2 June 1999     

Mouse mutants with neural tube closure defects and their role in understanding human neural tube defects

BACKGROUND: The number of mouse mutants and strains with neural tube closure defects (NTDs) now exceeds 190, including 155 involving known genes, 33 with unidentified genes, and eight "multifactorial" strains. METHODS: The emerging patterns of mouse NTDs are considered in relation to the unknown genetics of the common human NTDs, anencephaly, and spina bifida aperta. RESULTS: Of the 150 mouse mutants that survive past midgestation, 20% have risk of either exencephaly and spina bifida aperta or both, parallel to the majority of human NTDs, whereas 70% have only exencephaly, 5% have only spina bifida, and 5% have craniorachischisis. The primary defect in most mouse NTDs is failure of neural fold elevation. Most null mutations (>90%) produce syndromes of multiple affected structures with high penetrance in homozygotes, whereas the "multifactorial" strains and several null-mutant heterozygotes and mutants with partial gene function (hypomorphs) have low-penetrance nonsyndromic NTDs, like the majority of human NTDs. The normal functions of the mutated genes are diverse, with clusters in pathways of actin function, apoptosis, and chromatin methylation and structure. The female excess observed in human anencephaly is found in all mouse exencephaly mutants for which gender has been studied. Maternal agents, including folate, methionine, inositol, or alternative commercial diets, have specific preventative effects in eight mutants and strains. CONCLUSIONS: If the human homologs of the mouse NTD mutants contribute to risk of common human NTDs, it seems likely to be in multifactorial combinations of hypomorphs and low-penetrance heterozygotes, as exemplified by mouse digenic mutants and the oligogenic SELH/Bc strain.     

A Radiation Hybrid Map of Mouse Chromosome 13

A mouse radiation hybrid (RH) panel was used to make a framework map for the entire length of mouse chromosome (Chr) 13. Forty-one loci were typed, and while most used primers flanking simple sequence repeats, some genes were included. The most proximal and distal loci are D13Mit132 and D13Mit35. The estimate of map length for Chr 13 is 1328 cR. The map is compared with the same set of loci from the consensus map for Chr 13, which is 70 cM in length, and also with a recombinational map derived from an intraspecies cross typed for many of the same loci. The mouse RH panel gave good resolution for Chr 13 and at the distal end allowed separation of previously nonrecombinant markers that are present on a single 620-kb YAC clone. Data analysis was performed using the RH option for Map Manager QT. This framework RH map of Chr 13 is the second of a series of RH maps for mouse chromosomes.