Mapping in the region of Danforth's short tail and the localization of tail length modifiers

Development, Pathogenesis, and Regeneration of the Intervertebral Disc: Current and Future Insights Spanning Traditional to Omics Methods

The intervertebral disc (IVD) is the fibrocartilaginous joint located between each vertebral body that confers flexibility and weight bearing capabilities to the spine. The IVD plays an important role in absorbing shock and stress applied to the spine, which helps to protect not only the vertebral bones, but also the brain and the rest of the central nervous system. Degeneration of the IVD is correlated with back pain, which can be debilitating and severely affects quality of life. Indeed, back pain results in substantial socioeconomic losses and healthcare costs globally each year, with about 85% of the world population experiencing back pain at some point in their lifetimes. Currently, therapeutic strategies for treating IVD degeneration are limited, and as such, there is great interest in advancing treatments for back pain. Ideally, treatments for back pain would restore native structure and thereby function to the degenerated IVD. However, the complex developmental origin and tissue composition of the IVD along with the avascular nature of the mature disc makes regeneration of the IVD a uniquely challenging task. Investigators across the field of IVD research have been working to elucidate the mechanisms behind the formation of this multifaceted structure, which may identify new therapeutic targets and inform development of novel regenerative strategies. This review summarizes current knowledge base on IVD development, degeneration, and regenerative strategies taken from traditional genetic approaches and omics studies and discusses the future landscape of investigations in IVD research and advancement of clinical therapies.

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.

Ectopic expression of Ptf1a induces spinal defects, urogenital defects, and anorectal malformations in Danforth's short tail mice

Danforth's short tail (Sd) is a semidominant mutation on mouse chromosome 2, characterized by spinal defects, urogenital defects, and anorectal malformations. However, the gene responsible for the Sd phenotype was unknown. In this study, we identified the molecular basis of the Sd mutation. By positional cloning, we identified the insertion of an early transposon in the Sd candidate locus approximately 12-kb upstream of Ptf1a. We found that insertion of the transposon caused overexpression of three neighboring genes, Gm13344, Gm13336, and Ptf1a, in Sd mutant embryos and that the Sd phenotype was not caused by disruption of an as-yet-unknown gene in the candidate locus. Using multiple knockout and knock-in mouse models, we demonstrated that misexpression of Ptf1a, but not of Gm13344 or Gm13336, in the notochord, hindgut, cloaca, and mesonephros was sufficient to replicate the Sd phenotype. The ectopic expression of Ptf1a in the caudal embryo resulted in attenuated expression of Cdx2 and its downstream target genes T, Wnt3a, and Cyp26a1; we conclude that this is the molecular basis of the Sd phenotype. Analysis of Sd mutant mice will provide insight into the development of the spinal column, anus, and kidney.

Caudal regression syndrome (CRS) is a congenital heterogeneous constellation of caudal anomalies that includes varying degrees of agenesis of the spinal column, anorectal malformations, and genitourinary anomalies. Its pathogenesis is unclear. However, it could be the result of excessive physiologic regression of the embryonic caudal region based on analyses of the various mouse mutants carrying caudal agenesis. Among the mouse mutants, the Danforth's short tail (Sd) mouse is considered a best model for human CRS. Sd is a semidominant mutation, characterized by spinal defects, urogenital defects, and anorectal malformations, thus showing phenotypic similarity to human CRS. Although Sd is known to map to mouse chromosome 2, little is known about the molecular nature of the mutation. Here, we demonstrate an insertion of one type of retrotransposon near the Ptf1a gene. This resulted in ectopic expression of Ptf1a gene in the caudal region of the embryo and downregulation of Cdx2 and its downstream targets, leading to characteristic phenotypes in Sd mouse. Thus, Sd mutant mice will provide insight into the development of the spinal column, anus, and kidney.

A retrotransposon insertion in the 5' regulatory domain of Ptf1a results in ectopic gene expression and multiple congenital defects in Danforth's short tail mouse

Danforth's short tail mutant (Sd) mouse, first described in 1930, is a classic spontaneous mutant exhibiting defects of the axial skeleton, hindgut, and urogenital system. We used meiotic mapping in 1,497 segregants to localize the mutation to a 42.8-kb intergenic segment on chromosome 2. Resequencing of this region identified an 8.5-kb early retrotransposon (ETn) insertion within the highly conserved regulatory sequences upstream of Pancreas Specific Transcription Factor, 1a (Ptf1a). This mutation resulted in up to tenfold increased expression of Ptf1a as compared to wild-type embryos at E9.5 but no detectable changes in the expression levels of other neighboring genes. At E9.5, Sd mutants exhibit ectopic Ptf1a expression in embryonic progenitors of every organ that will manifest a developmental defect: the notochord, the hindgut, and the mesonephric ducts. Moreover, at E 8.5, Sd mutant mice exhibit ectopic Ptf1a expression in the lateral plate mesoderm, tail bud mesenchyme, and in the notochord, preceding the onset of visible defects such as notochord degeneration. The Sd heterozygote phenotype was not ameliorated by Ptf1a haploinsufficiency, further suggesting that the developmental defects result from ectopic expression of Ptf1a. These data identify disruption of the spatio-temporal pattern of Ptf1a expression as the unifying mechanism underlying the multiple congenital defects in Danforth's short tail mouse. This striking example of an enhancer mutation resulting in profound developmental defects suggests that disruption of conserved regulatory elements may also contribute to human malformation syndromes.

Birth defects are a major cause of childhood morbidity and mortality. We studied the Danforth's short tail mouse, a classic mouse model of birth defects involving the skeleton, gut, and urinary system. We precisely localized the mutation responsible for these birth defects to a 42.8-kb segment on chromosome 2 and identified the mutation as an 8.5-kb transposon that disrupts highly conserved regulatory sequences upstream of the Pancreas Specific Transcription Factor, 1a (Ptf1a). The insertion disrupts a Ptf1a regulatory domain that is highly conserved across evolution and results in spatiotemporal defects in Ptf1a expression: we detected increased expression, temporally premature expression, and (most important for elucidating the mutant phenotype) the ectopic expression of Ptf1a in the notochord, hindgut, and mesonephros—the three sites that will give rise to organ defects in Danforth's short tail mouse. Our data also provide a striking example of how a noncoding, regulatory mutation can produce transient spatio-temporal dsyregulation of gene expression and result in profound developmental defects, highlighting the critical role of noncoding elements for coordinated gene expression in the vertebrate genome. Finally, these data provide novel insight into the role of Ptf1a in embryogenesis and lay the groundwork for elucidation of novel mechanisms underlying birth defects in humans.

The Skt gene, required for anorectal development, is a candidate for a molecular marker of the cloacal plate

BACKGROUND AND AIMS

It has been reported that a dorsal cloacal plate defect is associated with anorectal malformations (ARMs); however, there has been very little information reported about the developmental mechanisms involved with cloacal plate formation. Danforth's short tail (Sd) mutant mice show ARMs. In our previous study, the co-presence of Skt ( Gt ) mutation, in which Skt gene is disrupted by the gene-trap vector (p-U8), increased the incidence of ARMs in Sd mutant to 100%. Our aims in this study are determining the Skt expression around the cloaca during the anorectal development and demonstrating the role of Skt gene in ARMs.

METHODS

Embryos, normal controls [+Skt ( Gt )/+Skt ( Gt )] and ARMs models [Sd Skt ( Gt )/+Skt ( Gt )], from embryonic day (E) 9.5 to E12.5, were evaluated with X-gal staining.

RESULTS

In control embryos, Skt expression was detected both in the endoderm and ectoderm of the cloacal plate from E9.5 onward. At E12.5, Skt expression was also detected in the mesenchyme neighboring the dorsal cloacal plates. In [Sd Skt ( Gt )/+Skt ( Gt )] mutant embryos, the cloacal plates failed to extend proximodistally and, consequently, the dorsal part of cloacal plate was defective at E11.5. Skt expressing cells were detected in the shortened cloacal plate and in the thickened mesenchyme dorsal to it.

CONCLUSIONS

We showed the spatial and temporal expression of Skt gene in the cloacal plate formation. This gene could be a marker for the cloacal plate during the anorectal development. Furthermore, Skt was considered to be associated with the embryogenesis of ARMs.

The floor plate is sufficient for development of the sclerotome and spine without the notochord

Danforth'sshort-tail (Sd) mouse is a semi-dominant mutation affecting the development of the vertebral column. Although the notochord degenerates completely by embryonic day 9.5, the vertebral column exists up to the lumber region, suggesting that the floor plate can substitute for notochord function. We previously established the mutant mouse line, Skt(Gt), through gene trap mutagenesis and identified the novel gene, Skt, which was mapped 0.95cM distal to the Sd locus. Taking advantage of the fact that monitoring notochordal development and genotyping of the Sd locus can be performed using the Skt(Gt) allele, we assessed the development of the vertebra, notochord, somite, floor plate and sclerotome in +-+/+-Skt(Gt), Sd-+/+-+, Sd-Skt(Gt)/+-+, Sd-Skt(Gt)/+-Skt(Gt), Sd-+/Sd-+ and Sd-Skt(Gt)/Sd-Skt(Gt) embryos. In Sd homozygous mutants with a C57BL/6 genetic background, the vertebral column was truncated in the 6th thoracic vertebra, which was more severe than previously reported. The floor plate and sclerotome developed to the level of somite before notochord degeneration and the number of remaining vertebrae corresponded well with the level of development of the floor plate and sclerotome. Defects to the sclerotome and subsequent vertebral development were not due to failure of somitogenesis. Taken together, these results suggest that the notochord induced floor plate development before degeneration, and that the remaining floor plate is sufficient for maintenance of differentiation of the somite into the sclerotome and vertebra in the absence of the notochord.

A novel murine gene, Sickle tail, linked to the Danforth's short tail locus, is required for normal development of the intervertebral disc

We established the mutant mouse line, B6;CB-SktGtAyu8021IMEG (SktGt), through gene-trap mutagenesis in embryonic stem cells. The novel gene identified, called Sickle tail (Skt), is composed of 19 exons and encodes a protein of 1352 amino acids. Expression of a reporter gene was detected in the notochord during embryogenesis and in the nucleus pulposus of mice. Compression of some of the nuclei pulposi in the intervertebral discs (IVDs) appeared at embryonic day (E) 17.5, resulting in a kinky-tail phenotype showing defects in the nucleus pulposus and annulus fibrosus of IVDs in SktGt/Gt mice. These phenotypes were different from those in Danforth's short tail (Sd) mice in which the nucleus pulposus was totally absent and replaced by peripheral fibers similar to those seen in the annulus fibrosus in all IVDs. The Skt gene maps to the proximal part of mouse chromosome 2, near the Sd locus. The genetic distance between them was 0.95 cM. The number of vertebrae in both [Sd +/+ SktGt] and [Sd SktGt/+ +] compound heterozygotes was less than that of Sd heterozygotes. Furthermore, the enhancer trap locus Etl4lacZ, which was previously reported to be an allele of Sd, was located in the third intron of the Skt gene.

Mapping Modifiers Affecting Muscularity of the Myostatin Mutant (MstnCmpt-dl1Abc) Compact Mouse

The hypermuscular Compact phenotype was first noted in a line of mice selected for high body weight and protein content. A new line, based on mice showing the Compact phenotype, was formed and selected for maximum expression of the Compact phenotype. Previously we mapped and identified a 12-bp deletion in the myostatin gene, denoted MstnCmpt-dl1Abc, which can be considered as a major gene responsible for the hypermuscular phenotype. Genetic analysis revealed that full expression of the hypermuscular phenotype requires the action of modifier loci in addition to MstnCmpt-dl1Abc. To map these modifier loci, an interspecific F2 population was generated between Comp9, an inbred line homozygous for MstnCmpt-dl1Abc, and CAST/Ei, an inbred line generated from Mus musculus castaneus. Selective DNA pooling and genotyping, separately by gender, was carried out within a subpopulation of the F2 consisting of individuals homozygous for MstnCmpt-dl1Abc. Significant association with hypermuscularity at a false discovery rate (FDR) of 0.05 was found for markers on chromosomes 3, 5, 7, 11, 16, and X. In all cases, the marker allele derived from the Comp9 parent showed a higher frequency in the hypermuscular group and the CAST/Ei allele in the normal group. The modifier loci apparently exerted their effects on muscularity only in the presence of MstnCmpt-dl1Abc.

Vestibular dysfunction in the epistatic circler mouse is caused by phenotypic interaction of one recessive gene and three modifier genes

Vestibular dysfunction is a frequent clinical problem, leading to dizziness and imbalance. Genes play an important role in its etiology, but the genetics are complex and poorly understood. In this study we have analyzed the complex inheritance pattern in the Epistatic circler mouse, which shows circling behavior indicative of vestibular dysfunction in the mouse. This phenotype exists in a proportion of the F2-generation from an intercross between C57L/J and SWR/J mouse strains. Genetic investigation indicates that the circling behavior is caused by a major recessively inherited gene derived from the SWR/J strain (the Ecs-gene) in combination with at least three different modifier genes derived from C57L/J (the Ecl-genes). Genetic mapping made it possible to localize the Ecs-gene to chromosome 14 and the Ecl-genes to chromosome 3, 4, and 13. This study illustrates the feasibility of identifying genes for multifactorial traits in mice.

Genetic interactions suggest that Danforth's short tail (Sd) is a gain-of-function mutation

Danforth's short tail (Sd) is a semidominant mutation on mouse chromosome 2 that acts cell autonomously in the notochord and leads to its distintegration, and thus causes severe defects in somite patterning and vertebral column development. The molecular nature of the Sd gene and mutation is unknown, and it is unclear whether Sd is a loss-of-function mutation and the semidominant inheritance of the Sd phenotype is due to haploinsufficiency, or whether Sd represents a gain-of-function mutation in a gene essential for notochord development and maintenance. Here, we report on the genetic interaction between Sd and an insertional mutation called Etl4lacZ, which provides genetic evidence that Sd is a gain-of-function mutation. Etl4lacZ is an enhancer trap insertion, which gives rise to lacZ expression in distinct cell types, including the notochord. In homozygosity, the lacZ insertion leads to abnormal vertebrae in the caudal part of the vertebral column. Etl4lacZ maps approximately 0.75 cM distal to Sd, and in double heterozy gotes modifies the Sd phenotype contrarily, depending on the chromosomal configuration of the Sd and Etl4lacZ mutations: when Etl4lacZ is present on the chromosome wild type for Sd (Sd+/+ Etl4lacZ; trans configuration), the Sd phenotype is enhanced, i.e., vertebral malformations extend to more anterior positions and the vertebral body of the axis is further reduced. Conversely, when Etl4lacZ is present on the same chromosome as Sd (Sd Etl4lacZ/+ +; cis configuration), the Sd phenotype is attenuated, i.e., vertebral malformations are confined to more posterior levels, and the dens axis, which is severely reduced or absent in Sd heterozygotes, is restored. The different effect of the Etl4lacZ insertion on Sd, depending on its presence in trans or cis, suggests a direct interaction of the transgene insertion with the Sd gene. Additionally, the attenuation of the Sd phenotype by Etl4lacZ in cis suggests that Sd is a gain-of-function mutation and lends support to the idea that Etl4lacZ is a new allele of Sd. Dev. Genet. 23:86-96, 1998.

Strategy for uncovering complex determinants of hypertension using animal models

Hypertension is a complex, multifactorial disorder resulting from the interaction of multiple genetic and environmental factors. While rodent models of hypertension have proved useful for identifying chromosomal regions containing blood pressure quantitative trait loci (QTLs), the gene(s) responsible for strain-differences in blood pressure remain to be identified. A strategy for identifying the genetic factors underlying blood pressure in animal models is presented, grouped according to the following themes: 1) choice of hypertension model, 2) identification of chromosomal regions containing QTLs, 3) confirming the presence of QTLs and delimiting the chromosomal region containing them, 4) developing a physical map of the QTL-containing region of the chromosome, 5) identification of strong candidate gene(s), and 6) requirements for proving that a gene is responsible, in part, for blood pressure differences.

The development of the kidney

This chapter describes the earlier stages of development of the vertebrate metanephric kidney. It focuses on the mouse and descriptive morphology is used for considering both molecular mechanisms, underpinning kidney morphogenesis and differentiation, and the ways in which these processes can go awry and lead to congenital kidney disorders—particularly in humans. The mature kidney is a fairly complex organ attached to an arterial input vessel and two output vessels, the vein and the ureter. Inside, the artery and vein are connected by a complex network of capillaries that invade a large number of glomeruli, the proximal entrance to nephrons, which are filtration units that link to an arborized collecting-duct system that drains into the ureter. The ability of the kidney and isolated metanephrogenic mesenchyme, to develop in culture means that the developing tissues can be subjected to a wide variety of experimental procedures designed to investigate their molecular and cellular properties and to test hypotheses about developmental mechanisms.