An autosomal dominant mutation of facial development in a transgenic mouse

Naso-maxillary deformity due to frontonasal expression of human transthyretin gene in transgenic mice

BACKGROUND

Retinoic acid, a metabolic product of retinol, is essential for craniofacial morphogenesis. Transthyretin (TTR) is a plasma protein delivering retinol to tissues. We produced several transgenic mouse lines using the human mutant TTR (hTTRMet30) gene to establish a mouse model of familial amyloidotic polyneuropathy. One of the lines showed an autosomal dominant inheritance of naso-maxillary deformity termed Nax.

RESULTS

The Nax malformation was characterized by a hypoplastic developmental defect of the frontonasal region. Homozygous mice with higher transgene expressions showed more severe phenotypes, but a subline, in which the copy number and expression of the transgene was reduced, showed a normal phenotype, indicating that the hTTRMet30 expression caused the malformation. Nax mice began to express the hTTRMet30 gene in the nasal placode from embryonic day 10.5 (E10.5), which was 2 days earlier than in the other transgenic lines with a normal phenotype. Excessive cell death was observed in the nasal placode of the E10.5 Nax embryos. In addition, the forced expression of hTTRMet30 in the nasal placode of transgenic mice resulted in similar phenotypes.

CONCLUSION

The expression of the hTTRMet30 gene in the nasal placode at E10.5 induced apoptotic cell death, leading to hypoplastic deformity in the frontonasal region.

Validity of the Hfm transgenic mouse as a model for hemifacial microsomia

OBJECTIVE

Our comprehension of hemifacial microsomia (HFM) has been hindered by its diverse phenotype and unclear etiopathogenesis. The conventional view has been that HFM's facial defects result from embryonic hemorrhages in the region of the first and second branchial arches. A more recent model based on a transgenic mutation of a locus termed Hfm (B1 to B3 on chromosome 10) appears to provide an insight into HFM causation. This study investigated the validity of this model by examining the Hfm craniofacial phenotype and histological development of the embryonic head (E13.5 to 17.5).

RESULTS

The results confirmed that although the loss-of-function mutation was transmitted in an autosomal dominant manner, the penetrance rate was significantly reduced and only Hfm heterozygotes were viable. The observations here extend the Hfm phenotype beyond microtia and jaw asymmetry to include structural and positional anomalies affecting the external auditory meatus, middle ear, cranial base, maxilla, and pharyngeal structures. Temporomandibular joint (TMJ) development and palatal shelf fusion were also affected in a small number of cases. In addition, some Hfm embryos displayed a novel finding: transposition of the developing inner ear between the tubotympanic recess and cranial base.

CONCLUSIONS

These craniofacial features, especially the ear anomalies and facial asymmetry indicate that the Hfm transgenic mouse represents a useful model for the HFM-microtia spectrum. In particular, it supports the hypothesis that at least a proportion of HFM anomalies has a genetic causation mediated via mesenchymal disruptions and possibly embryonic hemorrhages.

Isolated maxillary bending in CL/FR strain mice: observation of craniofacial deformity and inheritance pattern

OBJECTIVE

The CL/Fr mouse, known as a strain with spontaneous cleft lip and/or palate (CL/P), has been used as an animal model to investigate etiology in CL/P.

METHOD

We examined a facial asymmetry mutant discovered in a CL/Fr mouse colony that was not associated with CL/P and was shown to be inheritable in subsequent generations. Facial asymmetry became apparent with postnatal growth, whereas it was not detectable at birth, and was termed "maxillary bending" (MB) based on the characteristic bending of the maxilla.

RESULTS

As a result of selective breeding, an 'MB line', In which MB was observed in 21.68% (67/309) in addition to CL/P in 17.80% (55/309) of the offspring, was developed in the CL/Fr colony. In mating experiments between the MB line and C57BL/6J, all F1 progeny showed the normal phenotype. MB was observed in 0.72% (1/139) of the F2 generation, and the backcross generation showed segregation of MB in 6.25% (22/352) and CL/P in 1.42% (5/352). These instances suggested the occurrence of an additional mutation in the CL/Fr mouse genome controlled by an autosomal recessive gene with low penetrance. However, since the CL/Fr mouse primarily has a developmental deficiency in the maxilla, the possibility that CL/P and MB share common etiologic factors cannot be completely ruled out.

CONCLUSION

The maxillary bending retains significance, as this mutant can serve as an animal model of abnormal facial growth. Elucidation of the etiologic relationship between MB and CL/P may provide clues to clarifying the deficiency in first branchial arch in the mouse.

Morphological abnormalities, neonatal mortality, and reproductive abnormalities in mice transgenic for diphtheria toxin genes that are driven by the promoter for adipocyte lipid binding protein

Transgenic mice were used in an experiment that was designed to serve as a model of a possible approach to reducing the amount of carcass fat in meat animals. The objective was to reduce the number of adipocytes in transgenic mice thereby restricting the capacity to accumulate lipid. Our approach employed the technique of genetic ablation. The promoter for the adipocyte lipid binding protein gene was used in an attempt to direct expression of diphtheria toxin genes specifically to adipocytes. Three diphtheria toxin genes were used; they encode, respectively, an extremely cytotoxic wild type toxin, a less toxic attenuated toxin, and a nonfunctional toxin. While it was not possible to accurately assess effects of the transgenes on lipid accumulation, several informative observations were noted. A large percentage of transgenic founder mice that harbor either wild type or attenuated toxin genes are morphologically abnormal, die as neonates, or exhibit reproductive abnormalities including sterility or failure to transmit the transgene to offspring. In contrast, mice that harbor the nonfunctional toxin gene or are nontransgenic rarely have these same abnormalities. These results suggest that the transgenic mice are expressing the transgenes in cells other than adipocytes and that the aberrant production of functional toxin is responsible for the congenital abnormalities. The production of morphological and reproductive abnormalities in transgenic animals should be useful for investigating normal developmental processes.

Ectopic expression of chloramphenicol acetyltransferase (CAT) in the cerebellum in mice transgenic for a carbonic anhydrase II promoter-CAT construct that is without apparent phenotypic effect

We have developed six transgenic lines of mice with constructs containing presumptive 5' regulatory regions of carbonic anhydrase II (CA II). Four of the lines contained 1,100 bases of the 5' flanking region of the human CA II gene, and two transgenic lines resulted from a construct containing 500 bases of the 5' flanking region of the mouse CA II gene. Tissue-specific expression of the chloramphenicol acetyltransferase (CAT) gene was not obtained in any of the transgenic lines. One of the transgenic lines was found to have high levels of expression of CAT in cerebellum. This expression persisted through multiple generations and was independent of the parental origin of the transgene. On the assumption that the expression was due to the insertion of the transgene in or near a gene expressed normally in cerebellum, homozygous mice were bred for the transgenic insert to see if a mutation might have been induced. Homozygous mice were found and seemed to be normal in all aspects of their phenotype studied. Thus, in this case, neither the insertion of the gene nor the ectopic expression of CAT seemed to be harmful to the animals.