A nonsense loss-of-function mutation in PCSK1 contributes to dominantly inherited human obesity

BACKGROUND

A significant proportion of severe familial forms of obesity remain genetically elusive. Taking advantage of our unique cohort of multigenerational obese families, we aimed to assess the contribution of rare mutations in 29 common obesity-associated genes to familial obesity, and to evaluate in these families the putative presence of nine known monogenic forms of obesity.

METHODS

Through next-generation sequencing, we sequenced the coding regions of 34 genes involved in polygenic and/or monogenic forms of obesity in 201 participants (75 normal weight individuals, 54 overweight individuals and 72 individuals with obesity class I, II or III) from 13 French families. In vitro functional analyses were performed to investigate the mutation PCSK1-p.Arg80* which was identified in a family.

RESULTS

A novel heterozygous nonsense variant in PCSK1 (p.Arg80*), encoding a propeptide truncated to less than two exons (out of 14), was found to co-segregate with obesity in a three-generation family. We demonstrated that this mutation inhibits PCSK1 enzyme activity and that this inhibition most likely does not involve a strong physical interaction. Furthermore, both mutations PCSK1-p.Asn180Ser and POMC-p.Phe144Leu, which had previously been reported to be associated with severe obesity, were also identified in this study, but did not co-segregate with obesity. Finally, we did not identify any rare mutations co-segregating with obesity in common obesity susceptibility genes, except for CADM2 and QPCTL, where we found two novel variants (p.Arg81His and p.Leu98Pro, respectively) in three obese individuals.

CONCLUSIONS

We showed for the first time that a nonsense mutation in PCSK1 was likely to cause dominantly inherited human obesity, due to the inhibiting properties of the propeptide fragment encoded by the null allele. Furthermore, the present family sequencing design challenged the contribution of previously reported mutations to monogenic or at least severe obesity.

Une cause rare d’embolie cérébrale : la cardiopathie amyloïde

INTRODUCTION

We report a case of amyloid cardiopathy revealed by a cerebral embolism.

CASE REPORT

A 55-year-old patient was admitted with a right hemiplegia and aphasia due to an infarction in the middle cerebral artery territory. Echocardiography was suggestive of an amyloid cardiopathy, and an IgG lambda multiple myeloma with renal insufficiency was discovered. The patient died suddenly 4 months later after chemotherapy was initiated.

CONCLUSION

Embolic complications are rare and late in cardiac amyloidosis. The diagnosis may be suspected by echocardiography.

A murine ATFa-associated factor with transcriptional repressing activity

The ATFa proteins, which are members of the CREB/ATF family of transcription factors, have previously been shown to interact with the adenovirus E1a oncoprotein and to mediate its transcriptional activity; they heterodimerize with Jun, Fos or related transcription factors, possibly altering their DNA-binding specificity; they also stably bind JNK2, a stress-induced protein kinase. Here we report the identification and characterization of a novel protein isolated in a yeast two-hybrid screen using the N-terminal half of ATFa as a bait. This 1306-residue protein (mAM, for mouse ATFa-associated Modulator) is rather acidic (pHi 4.5) and contains high proportions of Ser/Thr (21%) and Pro (11%) residues. It colocalizes and interacts with ATFa in mammalian cells, contains a bipartite nuclear localization signal and possesses an ATPase activity. Transfection experiments show that mAM is able to downregulate transcriptional activity, in an ATPase-independent manner. Our results indicate that mAM interacts with several components of the basal transcription machinery (TFIIE and TFIIH), including RNAPII itself. Together, these findings suggest that mAM may be involved in the fine-tuning of ATFa-regulated gene expression, by interfering with the assembly or stability of specific preinitiation transcription complexes.

Role of the ATFa/JNK2 complex in Jun activation

The ATFa proteins, which are members of the CREB/ATF family of transcription factors, display quite versatile properties. We have previously shown that they interact with the adenovirus E1a oncoprotein, mediating part of its transcriptional activity and heterodimerize with the Jun, Fos or related transcription factors, thereby modulating their DNA-binding specificity. In the present study, we report the sequence requirement of the N-terminal activation domain of ATFa and demonstrate the importance of specific threonine residues (Thr51 and Thr53) in addition to that of the metal-binding domain, in transcriptional activation processes. We also show that the N-terminal domain of ATFa which stably binds the Jun N-terminal kinase-2 (JNK2) (Bocco et al., 1996), is not a substrate for this kinase in vivo but, instead, serves as a JNK2-docking site for ATFa-associated partners like JunD, allowing them to be phosphorylated by the bound kinase.

Point mutations causing Bloom's syndrome abolish ATPase and DNA helicase activities of the BLM protein

Bloom's syndrome (BS) is a rare human genetic disorder characterized by mutations within the BLM gene whose primary effects are excessive chromosome breakage and increased rates of sister chromatid interchange in somatic cells. We report the characterization of a murine protein (mBLM), highly related to the product of the human BLM gene. This protein exhibits an ATP-dependent DNA-helicase activity that unwinds DNA in a 3'-5' direction. Single amino acid substitutions found in BS cells, abolish both ATPase and helicase activities of this protein, indicating that defects in these BLM functions may be primarily responsible for BS establishment. These results provide the first evidence suggesting that the enzymatic activities of the BLM product are implicated in the upholding of genomic integrity.

Leukemia inhibitory factor-dependent transcriptional activation in embryonic stem cells

STAT transcription factors are induced by a number of growth factors and cytokines. Within minutes of induction, the STAT proteins are phosphorylated on tyrosine and serine residues and translocated to the nucleus, where they bind to their DNA targets. The leukemia inhibitory factor (LIF) mediates pleiotropic and sometimes opposite effects both in vivo and in cultured cells. It is known, for example, to prevent differentiation of embryonic stem (ES) cells in vitro. To get insights into LIF-regulated signaling in ES cells, we have analyzed protein-binding and transcriptional properties of STAT recognition sites in ES cells cultivated in the presence and in the absence of LIF. We have detected a specific LIF-regulated DNA-binding activity implicating the STAT3 protein. We show that STAT3 phosphorylation is essential for this LIF-dependent DNA-binding activity. The possibility that ERK2 or a closely related protein kinase, whose activity is modulated in a LIF-dependent manner, contributes to this phosphorylation is discussed. Finally, we show that the multimerized STAT3-binding DNA element confers LIF responsiveness to a minimal thymidine kinase promoter. This, together with our observation that overexpression of STAT3 dominant-negative mutants abrogates this LIF responsiveness, clearly indicates that STAT3 is involved in LIF-regulated transcriptional events in ES cells. Finally, stable expression of such a dominant negative mutant of STAT3 induces morphological differentiation of ES cells despite continuous LIF supply. Our results suggest that STAT3 is a critical target of the LIF signaling pathway, which maintains pluripotent cell proliferation.