An electron microscopic analysis of notochordal and mesenchymal cell abnormalities in embryos of Danforth's short-tail (Sd) mice

Next-generation sequencing identifies the Danforth's short tail mouse mutation as a retrotransposon insertion affecting Ptf1a expression

The semidominant Danforth's short tail (Sd) mutation arose spontaneously in the 1920s. The homozygous Sd phenotype includes severe malformations of the axial skeleton with an absent tail, kidney agenesis, anal atresia, and persistent cloaca. The Sd mutant phenotype mirrors features seen in human caudal malformation syndromes including urorectal septum malformation, caudal regression, VACTERL association, and persistent cloaca. The Sd mutation was previously mapped to a 0.9 cM region on mouse chromosome 2qA3. We performed Sanger sequencing of exons and intron/exon boundaries mapping to the Sd critical region and did not identify any mutations. We then performed DNA enrichment/capture followed by next-generation sequencing (NGS) of the critical genomic region. Standard bioinformatic analysis of paired-end sequence data did not reveal any causative mutations. Interrogation of reads that had been discarded because only a single end mapped correctly to the Sd locus identified an early transposon (ETn) retroviral insertion at the Sd locus, located 12.5 kb upstream of the Ptf1a gene. We show that Ptf1a expression is significantly upregulated in Sd mutant embryos at E9.5. The identification of the Sd mutation will lead to improved understanding of the developmental pathways that are misregulated in human caudal malformation syndromes.

Birth defects are the leading cause of infant mortality in the United States, accounting for 1 in 5 infant deaths annually. Birth defects that affect development of the caudal portion of the embryo can include malformations of the spine, such as spina bifida, and malformations of the kidneys and lower gastrointestinal tract. Little is known regarding the genetic causes of human caudal birth defects. The Danforth's short tail (Sd) mouse shares many similarities with these caudal birth defects that occur in humans. In this manuscript, we used next-generation sequencing to identify the genetic cause of the Sd mouse phenotype. We found that the Sd mutation is a retrotransposon insertion that inappropriately turns on a nearby gene that is normally important for pancreas development. Future studies of Sd mice will help us understand the pathogenesis of caudal birth defects in humans.

Anorectal malformations associated with enteric dysganglionosis in Danforth's short tail (Sd) mice

BACKGROUND

The spontaneous mutant Danforth's short tail (Sd) mouse has been studied over the last 60 years from the morphological, embryological, and genetic point of view. The Sd mutation affects a gene essential to notochordal development, and the Sd mouse phenotype represents an analogue of human caudal regression syndrome. The Sd/Sd mouse presents different types of anorectal malformations (ARM) and was suggested as a simple and cheap model of investigation of ARM morphology and embryology. In the current study, the Sd mouse enteric nervous system (ENS) was thoroughly investigated with specific immunohistochemical markers.

METHODS

Macroscopic analysis, normal histology, and immunohistochemical techniques for detecting neurofilaments (NF) and NOS1 were used to study ENS of 138 Sd mice and 25 controls.

RESULTS

The surprising results of this study showed that Sd mutation is associated with different degrees of hypoganglionosis and aganglionosis. In 41% of Sd/SD-affected mice, the rectal pouch was aganglionic and in the remaining 58% was severely hypoganglionic. In addition, 4.1% of heterozygous mice presented a distal aganglionosis and 8.3% hypoganglionosis.

CONCLUSIONS

These results suggest that Sd mutation independently affects distinct cell lines during early organogenesis, as notochord cells, ventral hingut endoderm, and neuroblasts migrating from neural crest cells. Comparing the Sd murine model with human pathology, this study confirms that the association between ARM and intestinal dysganglionosis is not rare and underlines the importance of detecting in every ARM patient the innervation abnormalities of rectal pouch and fistulas.

Cytochemical analysis of the notochord in early rhesus monkey embryos

Functional differentiation of the notochord in rhesus monkey embryos at stages 11-12 (25-28 days of gestation) was analyzed by means of ultrastructural cytochemistry. The notochordal cells exhibited well developed Golgi complexes, rough endoplasmic reticulum, mitochondria, and numerous coated vesicles. Large irregular intercellular spaces were common, and some contained fibrils and particulate matter similar to that observed in the perinotochordal space immediately surrounding the notochord. With the glycogen-specific thiocarbohydrazide-silver proteinate technique, solitary particles as well as large aggregates of glycogen were present within the notochordal cells. The center of some aggregates was electron lucent and contained collapsed membranous structures. The results indicate that as early as stage 11 the nonhuman primate notochord exhibits ultrastructural features suggestive of secretory activity and cytological complexity.