Tetra-amelia, lung hypo-/aplasia, cleft lip-palate, and heart defect: a new syndrome?

Pseudo-Roberts Syndrome: An Entity or Not?

Background: Roberts syndrome is a genetic disorder characterized by tetra-phocomelia with abnormalities of ESCO2. We report a male stillborn with tetra-phocomelia and no ESCO2 mutation. Case report: Pre- and post-natal imaging and autopsy findings included schizencephaly, phocomelia of four limbs, micrognathia, oligodactyly, and cardiopulmonary malformations. Microcephaly on pre-natal imaging was not confirmed by autopsy examination. Karyotype, prenatal chromosome microarray and ESCO2 gene testing were normal. Conclusion: Given the various skeletal anomalies found on autopsy and imaging evaluations, at least phenotypically, our case appeared to conform into Roberts syndrome spectrum. Since the infant did not have the mutation associated with this disorder, this infant could be labeled as the first report of a pseudo-Roberts syndrome because many of his phenotypic anomalies are characteristic of Roberts syndrome in absence of the ESCO2 gene mutation.

A 50-kb deletion disrupting the RSPO2 gene is associated with tetradysmelia in Holstein Friesian cattle

Background

Tetradysmelia is a rare genetic disorder that is characterized by an extremely severe reduction of all limb parts distal of the scapula and pelvic girdle. We studied a Holstein Friesian backcross family with 24 offspring, among which six calves displayed autosomal recessive tetradysmelia. In order to identify the genetic basis of the disorder, we genotyped three affected calves, five dams and nine unaffected siblings using a Bovine Illumina 50 k BeadChip and sequenced the whole genome of the sire.

Results

Pathological examination of four tetradysmelia cases revealed a uniform and severe dysmelia of all limbs. Applying a homozygosity mapping approach, we identified a homozygous region of 10.54 Mb on chromosome 14 (Bos taurus BTA14). Only calves that were diagnosed with tetradysmelia shared a distinct homozygous haplotype for this region. We sequenced the whole genome of the cases’ sire and searched for heterozygous single nucleotide polymorphisms (SNPs) and small variants on BTA14 that were uniquely present in the sire and absent from 3102 control whole-genome sequences of the 1000 Bull Genomes Project, but none were identified in the 10.54-Mb candidate region on BTA14. Therefore, we subsequently performed a more comprehensive analysis by also considering structural variants and detected a 50-kb deletion in the targeted chromosomal region that was in the heterozygous state in the cases’ sire. Using PCR, we confirmed that this detected deletion segregated perfectly within the family with tetradysmelia. The deletion spanned three exons of the bovine R-spondin 2 (RSPO2) gene, which encode three domains of the respective protein. R-spondin 2 is a secreted ligand of leucine-rich repeats containing G protein-coupled receptors that enhance Wnt signalling and is involved in a broad range of developmental processes during embryogenesis.

Conclusions

We identified a 50-kb deletion on BTA14 that disrupts the coding sequence of the RSPO2 gene and is associated with bovine tetradysmelia. To our knowledge, this is the first reported candidate causal mutation for tetradysmelia in a large animal model. Since signalling pathways involved in limb development are conserved across species, the observed inherited defect may serve as a model to further elucidate fundamental pathways of limb development.

R-spondin2 signaling is required for oocyte-driven intercellular communication and follicular growth

R-spondin2 (RSPO2) is a member of the R-spondin family, which are secreted activators of the WNT/β-catenin (CTNNB1) signaling pathway. In the mouse postnatal ovary, WNT/CTNNB1 signaling is active in the oocyte and in the neighboring supporting cells, the granulosa cells. Although the role of Rspo2 has been previously studied using in vitro experiments, the results are conflicting and the in vivo ovarian function of Rspo2 remains unclear. In the present study, we found that RSPO2/Rspo2 expression is restricted to the oocyte of developing follicles in both human and mouse ovaries from the beginning of the follicular growth. In mice, genetic deletion of Rspo2 does not impair oocyte growth, but instead prevents cell cycle progression of neighboring granulosa cells, thus resulting in an arrest of follicular growth. We further show this cell cycle arrest to be independent of growth promoting GDF9 signaling, but rather associated with a downregulation of WNT/CTNNB1 signaling in granulosa cells. To confirm the contribution of WNT/CTNNB1 signaling in granulosa cell proliferation, we induced cell type specific deletion of Ctnnb1 postnatally. Strikingly, follicles lacking Ctnnb1 failed to develop beyond the primary stage. These results show that RSPO2 acts in a paracrine manner to sustain granulosa cell proliferation in early developing follicles. Taken together, our data demonstrate that the activation of WNT/CTNNB1 signaling by RSPO2 is essential for oocyte-granulosa cell interactions that drive maturation of the ovarian follicles and eventually female fertility.

RSPO2 inhibition of RNF43 and ZNRF3 governs limb development independently of LGR4/5/6

The four R-spondin secreted ligands (RSPO1-RSPO4) act via their cognate LGR4, LGR5 and LGR6 receptors to amplify WNT signalling^1-3. Here we report an allelic series of recessive RSPO2 mutations in humans that cause tetra-amelia syndrome, which is characterized by lung aplasia and a total absence of the four limbs. Functional studies revealed impaired binding to the LGR4/5/6 receptors and the RNF43 and ZNRF3 transmembrane ligases, and reduced WNT potentiation, which correlated with allele severity. Unexpectedly, however, the triple and ubiquitous knockout of Lgr4, Lgr5 and Lgr6 in mice did not recapitulate the known Rspo2 or Rspo3 loss-of-function phenotypes. Moreover, endogenous depletion or addition of exogenous RSPO2 or RSPO3 in triple-knockout Lgr4/5/6 cells could still affect WNT responsiveness. Instead, we found that the concurrent deletion of rnf43 and znrf3 in Xenopus embryos was sufficient to trigger the outgrowth of supernumerary limbs. Our results establish that RSPO2, without the LGR4/5/6 receptors, serves as a direct antagonistic ligand to RNF43 and ZNRF3, which together constitute a master switch that governs limb specification. These findings have direct implications for regenerative medicine and WNT-associated cancers.

A review of supernumerary and absent limbs and digits of the upper limb

For years people have been enamored by anomalies of the human limbs, particularly supernumerary and absent limbs and digits. Historically, there are a number of examples of such anomalies, including royal families of ancient Chaldea, tribes from Arabia, and examples from across nineteenth century Europe. The development of the upper limbs in a growing embryo is still being elucidated with the recent advent of homeobox genes, but researchers agree that upper limbs develop between stages 12-23 through a complex embryological process. Maternal thalidomide intake during limb development is known to cause limb reduction and subsequent amelia or phocomelia. Additionally, a number of clinical reports have illustrated different limb anomaly cases, with each situation unique in phenotype and developmental abnormality. Supernumerary and absent limbs and digits are not unique to humans, and a number of animal cases have also been reported. This review of the literature illustrates the historical, anatomical, and clinical aspects of supernumerary and absent limbs and digits for the upper limb.

Tetra-amelia with lung hypoplasia and facial clefts, Roberts-SC syndrome: report of two cases

Although limb anomalies are a common clinical problem, they are rarely studied. The spectrum of limb anomalies ranges from very mild disorders such as syndactyly to very severe forms such as absent limb (amelia). Tetra-amelia is a rare anomaly with complete or partial deficiency of all four limbs. It may be isolated or associated with other anomalies. Roberts-SC phocomelia syndrome comprises four limb deficiencies, lung hypoplasia, facial clefts and other anomalies. We describe two cases that presented to us.

Homozygous WNT3 mutation causes tetra-amelia in a large consanguineous family

Tetra-amelia is a rare human genetic disorder characterized by complete absence of all four limbs and other anomalies. We studied a consanguineous family with four affected fetuses displaying autosomal recessive tetra-amelia and craniofacial and urogenital defects. By homozygosity mapping, the disease locus was assigned to chromosome 17q21, with a maximum multipoint LOD score of 2.9 at markers D17S931, D17S1785, D17SS1827, and D17S1868. Further fine mapping defined a critical interval of approximately 8.9 Mb between D17S1299 and D17S797. We identified a homozygous nonsense mutation (Q83X) in the WNT3 gene in affected fetuses of the family. WNT3, a human homologue of the Drosophila wingless gene, encodes a member of the WNT family known to play key roles in embryonic development. The Q83X mutation truncates WNT3 at its amino terminus, suggesting that loss of function is the most likely cause of the disorder. Our findings contrast with the observation of early lethality in mice homozygous for null alleles of Wnt3. To our knowledge, this is the first report of a mutation in a WNT gene associated with a Mendelian disorder. The identification of a WNT3 mutation in tetra-amelia indicates that WNT3 is required at the earliest stages of human limb formation and for craniofacial and urogenital development.

New case of bilateral upper limb amelia, facial clefts, and renal hypoplasia

We report on a male patient with bilateral upper limb amelia, facial clefts, and bilateral renal hypoplasia. We compare the clinical findings in our patient with those of the other three similar cases reported. This is the first long-surviving patient described with this association of malformations.

Tetra-amelia and splenogonadal fusion in Roberts syndrome

Roberts-SC phocomelia syndrome comprises limb deficiencies of variable severity, facial clefts, and other anomalies. Tetra-amelia may also be associated with facial clefts and similar anomalies. We report on a female infant with severe tetra-amelia, micrognathia, cleft palate, splenogonadal fusion, and premature centromere separation. We propose that this represents the severe expression of the Roberts-SC phocomelia syndrome.

Zimmer phocomelia: delineation by principal coordinate analysis

We present a 46,XX stillborn fetus with tetraphocomelia, absence of ears, severe hypoplasia of nose, cleft palate, pulmonary hypoplasia, imperforate anus and vagina, and phallus-like structure on an otherwise undefined perineum. The pattern of abnormalities resembles the tetraphocomelic condition described by Zimmer et al. in 1985. Tetraphocomelia, ear/nose hypoplasia with facial clefts, pulmonary hypoplasia, and defects of the caudal end including imperforate anus, and abnormal genitalia constitute a distinct pattern of malformation termed Zimmer phocomelia. Principal coordinate analysis with Gower's similarity index supported the clinical impression that cases reported by Zimmer and the present case are distinct from other phocomelic conditions. Although Zimmer phocomelia is currently referred to as "X-linked amelia," documentation of a female case with a penis-like structure in this report as well as consanguinity in the original family in Zimmer's report indicates that this condition is likely inherited in an autosomal recessive fashion. Zimmer phocomelia may be a more appropriate name than X-linked amelia.

Tri-amelia and phocomelia with multiple malformations resembling Roberts syndrome in a fetus: is it a variant or a new syndrome?

Roberts-SC phocomelia syndrome is a rare autosomal recessive disorder characterized by morphological anomalies such as limb defects and midfacial clefting, and by premature centromeric division in chromosomal study. Although it has been regarded as a single genetic entity and includes various morphologic defects, babies are being reported nowadays with severe facial defects, tetra-amelia, and pulmonary abnormality, yet with normal chromosomal findings. We have added a case resembling Roberts syndrome with various congenital anomalies. A gestation with a fetus was terminated at 24 weeks of gestational age because of multiple fetal anomalies. Postmortem examination revealed a severe mid-facial cleft, tri-amelia and phocomelia, multiple encephaloceles, protruding and hypoteloric eyes, low-set ears, atrial septal defect of ostium secundum type, patent ductus arteriosus, bilateral two-lobed lungs with incomplete lobation, multiple visceral anomalies, a penis without scrotum, abnormal dermoglyphics, and absence of nipples.

New autosomal recessive form of amelia

Amelia is a rare, usually sporadic malformation. We report on a family in which three fetuses had amelia of the upper limbs and variable deficiency of the lower limbs. The fetuses also had minor facial anomalies. Recurrence of the condition in sibs of both sexes suggests autosomal recessive inheritance. Recurrent amelia has been documented in only a few families most often associated with a different set of malformations. Possibly, mutations in more than one gene with different modes of transmission can lead to this severe limb deficiency. We speculate that the mutation found in our cases interferes with formation of the apical ectodermal ridge in the upper limbs and results in its premature degeneration in the lower limbs.