Cohesin biology and the cohesinopathies: Abstracts from the Second Biennial Conference, Pontignano, Italy, 2009

Mutations in STAG2 cause an X-linked cohesinopathy associated with undergrowth, developmental delay, and dysmorphia: Expanding the phenotype in males

BACKGROUND

The cohesin complex is a multi-subunit protein complex which regulates sister chromatid cohesion and separation during cellular division. In addition, this evolutionarily conserved protein complex plays an integral role in DNA replication, DNA repair, and the regulation of transcription. The core complex is composed of four subunits: RAD21, SMC1A, SMC3, and STAG1/2. Mutations in these proteins have been implicated in human developmental disorders collectively termed "cohesinopathies."

METHODS

Using clinical exome sequencing, we have previously identified three female cases with heterozygous STAG2 mutations and overlapping syndromic phenotypes. Subsequently, a familial missense variant was identified in five male family members.

RESULTS

We now present the case of a 4-year-old male with developmental delay, failure to thrive, short stature, and polydactyly with a likely pathogenic STAG2 de novo missense hemizygous variant, c.3027A>T, p.Lys1009Asn. Furthermore, we compare the phenotypes of the four previously reported STAG2 variants with our case.

CONCLUSION

We conclude that mutations in STAG2 cause a novel constellation of sex-specific cohesinopathy-related phenotypes and are furthermore, essential for neurodevelopment, human growth, and behavioral development.

DNA Topoisomerase II modulates acetyl-regulation of cohesin-mediated chromosome dynamics

Cohesin is one of three multi-protein structural maintenance of chromosome (SMC) complexes that regulate eukaryotic chromosome dynamics. It forms a ring-shaped structure that embraces sister chromatids through interphase to promote their pairing. In preparation for mitosis, most cohesin is stripped from the chromosome arms in prophase by a poorly defined process that is associated with cohesin phosphorylation. In the fission yeast Schizosaccharomyces pombe this prophase pathway is dependent on the cohesin-related Smc5/6 complex, and this requirement is heightened in Smc5/6 hypomorphs by DNA damage, replication stress and Topoisomerase II (Top2) dysfunction. Cohesin interacts with chromosomes immediately upon mitotic exit and becomes cohesive coincident with DNA replication. Cohesiveness is promoted by acetylation of the Smc3 subunit by an acetyltransferase, known as Eso1 in the S. pombe, which counteracts the anti-cohesive function(s) of the cohesin regulators Pds5 and Wpl1. We recently showed that Eso1 and Smc5/6 antagonize each other, and concurrent inactivation restores sister chromatid separation following genotoxic stress. Here, we have investigated the relationship between Top2 and Eso1 in successful completion of mitosis. We observe that partial inactivation of both results in a synthetic lethal mitotic block, but this is not overcome by deleting pds5 or wpl1. However, analysis of both acetyl-blocking and mimetic mutations in Smc3 indicates that the cycling of cohesin acetyl-regulation is more important than acetyl-status per se, highlighting the non-linear nature of the cohesin cycle.

De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies

The cohesin complex is an evolutionarily conserved multi-subunit protein complex which regulates sister chromatid cohesion during mitosis and meiosis. Additionally, the cohesin complex regulates DNA replication, DNA repair, and transcription. The core of the complex consists of four subunits: SMC1A, SMC3, RAD21, and STAG1/2. Loss-of-function mutations in many of these proteins have been implicated in human developmental disorders collectively termed "cohesinopathies." Through clinical exome sequencing (CES) of an 8-year-old girl with a clinical history of global developmental delay, microcephaly, microtia with hearing loss, language delay, ADHD, and dysmorphic features, we describe a heterozygous de novo variant (c.205C>T; p.(Arg69*)) in the integral cohesin structural protein, STAG2. This variant is associated with decreased STAG2 protein expression. The analyses of metaphase spreads did not exhibit premature sister chromatid separation; however, delayed sister chromatid cohesion was observed. To further support the pathogenicity of STAG2 variants, we identified two additional female cases from the DECIPHER research database with mutations in STAG2 and phenotypes similar to our patient. Interestingly, the clinical features of these three cases are remarkably similar to those observed in other well-established cohesinopathies. Herein, we suggest that STAG2 is a dosage-sensitive gene and that heterozygous loss-of-function variants lead to a cohesinopathy.