Tibial aplasia-hypoplasia and ectrodactyly in monozygotic twins with a discordant phenotype

Perspective: Is Random Monoallelic Expression a Contributor to Phenotypic Variability of Autosomal Dominant Disorders?

Several factors have been proposed as contributors to interfamilial and intrafamilial phenotypic variability in autosomal dominant disorders, including allelic variation, modifier genes, environmental factors and complex genetic and environmental interactions. However, regardless of the similarity of genetic background and environmental factors, asymmetric limb or trunk anomalies in a single individual and variable manifestation between monozygotic twins have been observed, indicating other mechanisms possibly involved in expressivity of autosomal dominant diseases. One such example is Holt-Oram syndrome (HOS), which is characterized by congenital cardiac defects and forelimb anomalies, mainly attributed to mutations in the TBX5 gene. We hypothesize that monoallelic expression of the TBX5 gene occurs during embryo development, and, in the context of a mutation, random monoallelic expression (RME) can create discrepant functions in a proportion of cells and thus contribute to variable phenotypes. A hybrid mouse model was used to investigate the occurrence of RME with the Tbx5 gene, and single-cell reverse transcription PCR and restriction digestion were performed for limb bud cells from developing embryos (E11.5) of the hybrid mice. RME of Tbx5 was observed in approximately two-thirds of limb bud cells. These results indicate that RME of the Tbx5 gene occurs frequently during embryo development, resulting in a mosaic expression signature (monoallelic, biallelic, or null) that may provide a potential explanation for the widespread phenotypic variability in HOS. This model will further provide novel insights into the variability of autosomal dominant traits and a better understanding of the complex expressivity of disease conditions.

Bilateral tibial agenesis and syndactyly in a cat

CASE DESCRIPTION

A three-year-old cat was referred to the Veterinary Teaching Hospital, University of Naples, Italy. The cat had severe pelvic limb deformity, and abnormal development of all four paws.

CLINICAL FINDINGS

Radiographs revealed bilateral tibial agenesis, syndactyly, and digital hypoplasia.

TREATMENT AND OUTCOME

No treatment was instituted because of the severity of the injury, the adaptation of the cat to the abnormal condition, and the owner's refusal to permit any treatment.

CLINICAL RELEVANCE

Congenital limb deformities are rarely reported in the cat and tibial agenesis is considered a very rare disease. This congenital anomaly is well documented and classified in man, and it has been associated with other abnormalities in more complex syndromes. This paper reports clinical and radiographic findings in a cat affected by bilateral complete tibial agenesis associated with other congenital anomalies.

[Congenital tibial hemimelia]

BACKGROUND

The congenital tibia hemimelia, also called tibial deficiency, is a rare disorder with unknown cause, showing many associated abnormalities or varying syndromes.

METHODS

The correct diagnosis can be easily established using radiographs and/or magnetic resonance imaging in the postpartum setting. However, treatment may be difficult and needs to take into consideration the given anatomic situation in the knee and ankle joint.

CONCLUSION

Prosthetic fitting may be possible in mild cases. Nevertheless, the majority of patients need to undergo surgical reconstruction in order to restore a functional, mobile, and stable knee and ankle joint.

A case of bilateral tibial hemimelia type VIIa

Congenital absence of tibia is a rare anomaly, and may be total or partial, unilateral or bilateral. Total absence is more frequent than partial, unilateral absence occurs more often than bilateral, with right limb more commonly affected than the left. In partial defect, almost always the distal end of the bone is affected, and of the bilateral cases, there may be total absence on both sides, or total on one side and partial on the other. Males are slightly more commonly affected than the females. Though, the family history is usually negative for congenital abnormalities and other diseases, there is a considerable chance of occurrence of congenital defect of the tibia or of other abnormalities, in near or remote relatives. We report a case of newborn having bilateral tibial hemimelia type VIIa.

Tibial hemimelia in one of the identical twins

Tibial hemimelia, a rare congenital anomaly, is characterized by deficiency of the tibia with relatively intact fibula. The incidence of congenital deficiency of the tibia is approximately 1 per 1 million live births. The percentage of monozygotic twins is 0.8% of all newborns. Therefore, the occurrence of tibial hemimelia in monozygotic twins is 1 case per 125 million. The purpose of this article is to present a case of identical twins, in which 1 had tibial hemimelia, a double metatarsal bone and preaxial polydactyly and the other had no alterations. The patient that was affected had an anomaly in the left leg and foot. She had 7 fingers in her left foot and a left duplicated great toe. Her legs had a 3-cm difference in length, rigid inversion, adduction of feet, and severe bowing. A roentgenogram showed right tibial hemimelia with intact fibula corresponding to Jones type 3 and the first metacarpo was duplicated with a proximal osseus synostosis. Karyotyping confirmed that the twins were monozygotic. Only 1 of the identical twins had tibial hemimelia, a double metatarsal bone, and preaxial polydactyly. Congenital defects may result from aberrations in the developmental stage, the growth stage, or both. Hemimelia may result from a primary failure to form an anlage, or a primary failure to chronologically synthesize collagen. This case shows that tibial hemimelia can have variable phenotypic manifestations.

Identical but not the same: the value of discordant monozygotic twins in genetic research

Monozygotic (MZ) twins show remarkable resemblance in many aspects of behavior, health, and disease. Until recently, MZ twins were usually called "genetically identical"; however, evidence for genetic and epigenetic differences within rare MZ twin pairs has accumulated. Here, we summarize the literature on MZ twins discordant for Mendelian inherited disorders and chromosomal abnormalities. A systematic literature search for English articles on discordant MZ twin pairs was performed in Web of Science and PubMed. A total number of 2,016 publications were retrieved and reviewed and 439 reports were retained. Discordant MZ twin pairs are informative in respect to variability of phenotypic expression, pathogenetic mechanisms, epigenetics, and post-zygotic mutagenesis and may serve as a model for research on genetic defects. The analysis of single discordant MZ twin pairs may represent an elegant approach to identify genes in inherited disorders.