Modifier locus for exencephaly in Cecr2 mutant mice is syntenic to the 10q25.3 region associated with neural tube defects in humans

Neural tube defects (NTDs), the second most common birth defect in humans, are multifactorial with complex genetic and environmental causes, although the genetic factors are almost completely unknown. In mice, >100 single gene mutations cause NTDs; however, the penetrance in many of these single gene mutant lines is highly dependent on the genetic background. We previously reported that a homozygous Cecr2 mutation on a BALB/c background causes exencephaly at a frequency of 74% compared with 0% on an FVB/N background. We now report that a major genetic modifier on chromosome 19, mapped using whole genome linkage analysis, increases the relative risk of exencephaly by 3.74 times in homozygous BALB embryos vs. BALB/FVB heterozygotes. Scanning electron microscopy revealed that the modifier does not affect the location of neural tube closure site 2, a known murine susceptibility factor for exencephaly. Crossing the Sp ( Splotch) mutation in the Pax3 gene onto the FVB/N background for two generations indicated that this resistant strain also decreases the penetrance of spina bifida. The chromosome 19 modifier region corresponds to a linkage region on human chromosome 10q25.3 mapped in a whole genome scan of human NTD families. Since the FVB/N genetic background affects susceptibility to both exencephaly and spina bifida, the human homolog of the chromosome 19 modifier locus may be a better candidate for human NTD susceptibility factors than genes that when mutated actually cause NTDs in mice.

Interaction between splotch (Sp) and curly tail (ct) mouse mutants in the embryonic development of neural tube defects

The mouse mutations splotch (Sp) and curly tail (ct) both produce spinal neural tube defects with closely similar morphology, but achieve this by different embryonic mechanisms. To determine whether the mutants may interact during development, we constructed mice carrying both mutations. Double heterozygotes exhibited tail defects in 10% of cases, although the single heterozygotes do not express this phenotype. Backcrosses of double heterozygotes to ct/ct produced offspring with an elevated incidence of neural tube defects, both spina bifida and tail defects, compared with a control backcross in which Sp was not involved. Use of the deletion allele Sp2H permitted embryos carrying a splotch mutation to be recognised by polymerase chain reaction assay. This experiment showed that only embryos carrying Sp2H develop spina bifida in the backcross with ct/ct, suggesting that the genotype Sp2H/+, ct/ct is usually lethal around the time of birth as a result of severe disturbance of neurulation. The interaction between Sp and ct was investigated further by examining embryos in the backcross for developmental markers of the Sp/Sp and ct/ct genotypes. Sp/Sp embryos characteristically lack neural crest derivatives, such as dorsal root ganglia, and die on day 13 of gestation. Double mutant embryos from the backcross did not exhibit either of these characteristics suggesting that homozygosity for ct does not cause Sp/+ embryos to develop as if they were of genotype Sp/Sp. The angle of ventral curvature of the posterior neuropore region is enhanced in affected ct/ct embryos whereas it was found to be reduced in Sp/Sp embryos compared with their normal littermates.(ABSTRACT TRUNCATED AT 250 WORDS)