Increased nuchal translucency in arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome and discovery of a Portuguese specific mutation in the VPS33B gene

CORVET, CHEVI and HOPS – multisubunit tethers of the endo-lysosomal system in health and disease

Multisubunit tethering complexes (MTCs) are multitasking hubs that form a link between membrane fusion, organelle motility and signaling. CORVET, CHEVI and HOPS are MTCs of the endo-lysosomal system. They regulate the major membrane flows required for endocytosis, lysosome biogenesis, autophagy and phagocytosis. In addition, individual subunits control complex-independent transport of specific cargoes and exert functions beyond tethering, such as attachment to microtubules and SNARE activation. Mutations in CHEVI subunits lead to arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome, while defects in CORVET and, particularly, HOPS are associated with neurodegeneration, pigmentation disorders, liver malfunction and various forms of cancer. Diseases and phenotypes, however, vary per affected subunit and a concise overview of MTC protein function and associated human pathologies is currently lacking. Here, we provide an integrated overview on the cellular functions and pathological defects associated with CORVET, CHEVI or HOPS proteins, both with regard to their complexes and as individual subunits. The combination of these data provides novel insights into how mutations in endo-lysosomal proteins lead to human pathologies.

Novel VIPAS39 mutation in a syndromic patient with arthrogryposis, renal tubular dysfunction and intrahepatic cholestasis

ARC syndrome is a rare autosomal recessive disease, characterized by arthrogryposis, renal tubular dysfunction and cholestasis. Herein a 2.5 month old infant with dysmorphic features, including small anterior fontanel, low set ears, beaked nose and high arched palate is presented who was referred because of icterus. He also suffered from some additional anomalies, including unilateral choanal atresia, club foot, and bilateral developmental dislocation of hip, while further studies showed renal tubular acidosis and hearing impairment in addition to cholestasis. Genetic studies showed a homozygous mutation in the VIPAS39 gene. Making the definite diagnosis of the syndrome is important, while increased risk of mutation in other siblings highlights the importance of prenatal diagnosis.

Arthrogryposis-renal dysfunction-cholestasis (ARC) syndrome: from molecular genetics to clinical features

Arthrogryposis-renal dysfunction-cholestasis (ARC) syndrome is a rare but fatal autosomal recessive multisystem disorder caused by mutations in the VPS33B or VIPAR gene. The classical presentation of ARC includes congenital joint contractures, renal tubular dysfunction, and cholestasis. Additional features include ichthyosis, central nervous system malformation, platelet anomalies, and severe failure to thrive. Diagnosis of ARC syndrome relies on clinical features, organ biopsy, and mutational analysis. However, no specific treatment currently exists for this syndrome.

CONCLUSION

This is an overview of the latest knowledge regarding the genetic features and clinical manifestations of ARC syndrome. Greater awareness and understanding of this syndrome should allow more timely intervention with potential for improving long-term outcome.

Agranular platelets as a cardinal feature of ARC syndrome

We aimed to describe abnormal platelet morphology and its clinical significance in infants who were diagnosed with arthrogryposis renal dysfunction and cholestasis (ARC) syndrome. We collected all of the cases of ARC syndrome referred to a single pediatric referral center. In all patients, platelet counts and analysis of platelet morphology were performed with peripheral blood smear specimens. Electron microscopy images were obtained to examine the ultrastructure of the platelets. Over the 12-year period, 12 cases of ARC syndrome were identified. The sex ratio (male:female) was 1:1. The median birth weight was 3.15 kg (range, 2.3 to 3.8 kg). Failure to thrive was observed in all the patients. The major cause of death was recurrent febrile illness and pneumonia. The median age at death was 8.9 months (range, 2.6 to 28.8 kg). Their median body weight at death was 3.1 kg (range, 2.6 to 6.0 kg). Close examination of their peripheral blood smear (n=11) specimens showed large, pale, agranular platelets similar to those seen in gray platelet syndrome. Electron microscopic images of the platelets (n=7) revealed a lack of alpha; granules. Agranular platelets are a common finding in ARC syndrome. Agranular platelets should be considered as a cardinal feature of ARC syndrome and can be useful as a noninvasive diagnostic marker for ARC syndrome.

Increased nuchal translucency in euploid fetuses--what should we be telling the parents?

Nuchal translucency (NT) measurement between 11 and 14 weeks' gestation is an undisputed marker for aneuploidies. When conventional karyotyping is normal, enlarged NT is a strong marker for adverse pregnancy outcome, associated with miscarriage, intrauterine death, congenital heart defects, and numerous other structural defects and genetic syndromes. The risk of adverse outcome is proportional to the degree of NT enlargement. Although the majority of structural anomalies are amenable to ultrasound detection, unspecified genetic syndromes involving developmental delay may only emerge after birth. Concern over these prenatally undetectable conditions is a heavy burden for parents. However, following detection of enlarged NT the majority of babies with normal detailed ultrasound examination and echocardiography will have an uneventful outcome with no increased risk for developmental delay when compared to the general population. Counseling should emphasize this to help parents restore hope in normal pregnancy outcome and infant development.

Screening for structural fetal anomalies during the nuchal translucency ultrasound examination

OBJECTIVE

The purpose of this study was to evaluate the ability to screen for structural fetal anomalies during the nuchal translucency (NT) ultrasound examination, without performing a complete anatomic fetal scan, by using the sagittal views of the fetus.

STUDY DESIGN

In a prospective study, we evaluated all the suspected structural findings observed during the NT examinations performed in our Division of Maternal-Fetal Medicine in 2004-2005. The purpose of the examination was to screen for fetal chromosome abnormalities by using the fetal NT measurements. However, the sonographers were instructed to pay attention to any abnormality observed while obtaining the sagittal views of the fetus. Other views were not to be obtained and fetal anatomy scan was performed only if a structural fetal anomaly was suspected when viewing the fetus in sagittal planes. When a structural fetal anomaly was suspected, a fetal anatomy scan was performed, and then a diagnosis was established at 14-16 weeks' gestation or later.

RESULTS

We performed 1723 NT examinations during the study period. The sonographers suspected structural fetal anomalies in 22 cases (1.3%), most of them performed between 11.2 and 13 weeks' gestation. Further evaluation of these cases diagnosed 9 fetuses (0.52%) with structural anomalies including: acrania, holoprosencephaly, Dandy-Walker syndrome, cerebellar agenesis, prune belly syndrome, 2 cases of omphalocele, and 2 cases of cleft lip. The NT was abnormal (greater than 3 mm) in only 1 case (omphalocele). None of the additional 8 cases diagnosed with structural anomalies had a positive maternal serum screening result for trisomy 21. Eight of these 9 fetal structural anomalies were sonographically confirmed at 14-16 weeks' gestation and the remaining 1 was confirmed at 20 weeks' gestation. An additional 13 noncardiac structural anomalies were detected in the study group during routine fetal anatomy scan performed at 14-16 or at 18-24 weeks' gestation. Four of these 9 fetal cardiac defects (44%) were diagnosed by an early fetal echocardiography performed for an increased fetal NT.

CONCLUSION

In addition to chromosomal anomalies and congenital cardiac defects, the NT examination can provide an opportunity to screen for structural fetal anomalies when viewing within the sagittal planes of the fetus. The NT examination can be used as a screening test for those who require an early fetal anatomy scan without performing an additional early anatomy scan to all patients.

Clinical and molecular genetic features of ARC syndrome

Arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome (MIM 208085) is an autosomal recessive multisystem disorder that may be associated with germline VPS33B mutations. VPS33B is involved in regulation of vesicular membrane fusion by interacting with SNARE proteins, and evidence of abnormal polarised membrane protein trafficking has been reported in ARC patients. We characterised clinical and molecular features of ARC syndrome in order to identify potential genotype-phenotype correlations. The clinical phenotype of 62 ARC syndrome patients was analysed. In addition to classical features described previously, all patients had severe failure to thrive, which was not adequately explained by the degree of liver disease and 10% had structural cardiac defects. Almost half of the patients who underwent diagnostic organ biopsy (7/16) developed life-threatening haemorrhage. We found that most patients (9/11) who suffered severe haemorrhage (7 post biopsy and 4 spontaneous) had normal platelet count and morphology. Germline VPS33B mutations were detected in 28/35 families (48/62 individuals) with ARC syndrome. Several mutations were restricted to specific ethnic groups. Thus p.Arg438X mutation was common in the UK Pakistani families and haplotyping was consistent with a founder mutation with the most recent common ancestor 900-1,000 years ago. Heterozygosity was found in the VPS33B locus in some cases of ARC providing the first evidence of a possible second ARC syndrome gene. In conclusion we state that molecular diagnosis is possible for most children in whom ARC syndrome is suspected and VPS33B mutation analysis should replace organ biopsy as a first line diagnostic test for ARC syndrome.