The homeoprotein SIX1 controls cellular senescence through the regulation of p16INK4A and differentiation-related genes

Cellular senescence is an antiproliferative response with essential functions in tumor suppression and tissue homeostasis. Here we show that SIX1, a member of the SIX family of homeobox transcriptional factors, is a novel repressor of senescence. Our data show that SIX1 is specifically downregulated in fibroblasts upon oncogenic stress and other pro-senescence stimuli, as well as in senescent skin premalignant lesions. Silencing of SIX1 in human fibroblasts suffices to trigger senescence, which is mediated by p16INK4A and lacks a canonical senescence-associated secretory phenotype. Interestingly, SIX1-associated senescence is further characterized by the expression of a set of development and differentiation-related genes that significantly overlap with genes associated with SIX1 in organogenesis or human tumors, and show coincident regulation in oncogene-induced senescence. Mechanistically, we show that gene regulation by SIX1 during senescence is mediated, at least in part, by cooperation with Polycomb repressive complexes. In summary, our results identify SIX1, a key development regulator altered in human tumors, as a critical repressor of cellular senescence, providing a novel connection between senescence, differentiation and tumorigenesis.

[Eosinophilic myositis in a 9 year old boy]

INTRODUCTION

Eosinophil infiltration of skeletal muscle is rare, but often no etiological factor can be identified and these are isolated eosinophilic myositis. They may be associated with parasite infections or drugs, or be features of rare systemic disorders of hypereosinophilia, such as the myalgia eosinophilia syndrome and the idiopathic hypereosinophilic syndrome. The eosinophilic myopathies should be distinguished from the commoner inflammatory myopathies such as polymyositis and dermatomyositis.

CLINICAL CASE

A nine year old boy with slight motor clumsiness but normal psychomotor development and neurological findings. Laboratory findings showed slightly raised serum transaminases (SGOT 271, SGPT 157 UI/L), CPK 7517 UI/L and eosinophilia (707/mL). Investigations for myoglobin cysticercosis, trichinosis, hydatidosis and toxicariasis were negative. No parasites were found in the faeces. The gammaglobulins were normal. Anti smooth muscle, antinuclear and anti KLM antibodies were negative. Cardiological studies were normal. His father, mother and two siblings had normal results of laboratory tests. Muscle biopsy showed inflammatory myopathy with abundant eosinophils, no evidence of parasites, no alteration of membrane proteins: dystrophin, sarcoglycan and merosine. Two years later he remains asymptomatic, maintains raised muscle enzyme levels in all tests with figures for CPK between 3,065 and 9,616UI/L, and eosinophilia ranging between 634 and 1,026/mL. Corticosteroid treatment was tried but no response obtained.

CONCLUSION

We consider this to be a case of eosinophilic polymyositis which gives rise to many questions regarding etiopathogenesis, management and prognosis.