Loss of MTX2 causes mandibuloacral dysplasia and links mitochondrial dysfunction to altered nuclear morphology.

Mandibuloacral dysplasia syndromes are mainly due to recessive LMNA or ZMPSTE24 mutations, with cardinal nuclear morphological abnormalities and dysfunction. We report five homozygous null mutations in MTX2, encoding Metaxin-2 (MTX2), an outer mitochondrial membrane protein, in patients presenting with a severe laminopathy-like mandibuloacral dysplasia characterized by growth retardation, bone resorption, arterial calcification, renal glomerulosclerosis and severe hypertension. Loss of MTX2 in patients' primary fibroblasts leads to loss of Metaxin-1 (MTX1) and mitochondrial dysfunction, including network fragmentation and oxidative phosphorylation impairment. Furthermore, patients' fibroblasts are resistant to induced apoptosis, leading to increased cell senescence and mitophagy and reduced proliferation. Interestingly, secondary nuclear morphological defects are observed in both MTX2-mutant fibroblasts and mtx-2-depleted C. elegans. We thus report the identification of a severe premature aging syndrome revealing an unsuspected link between mitochondrial composition and function and nuclear morphology, establishing a pathophysiological link with premature aging laminopathies and likely explaining common clinical features.

Cutaneous and metabolic defects associated with nuclear abnormalities in a transgenic mouse model expressing R527H lamin A mutation causing mandibuloacral dysplasia type A (MADA) syndrome.

LMNA gene encodes for lamin A/C, attractive proteins linked to nuclear structure and functions. When mutated, it causes different rare diseases called laminopathies. In particular, an Arginine change in Histidine in position 527 (p.Arg527His) falling in the C-terminal domain of lamin A precursor form (prelamin A) causes mandibuloacral dysplasia Type A (MADA), a segmental progeroid syndrome characterized by skin, bone and metabolic anomalies. The well-characterized cellular models made difficult to assess the tissue-specific functions of 527His prelamin A. Here, we describe the generation and characterization of a MADA transgenic mouse overexpressing 527His LMNA gene, encoding mutated prelamin A. Bodyweight is slightly affected, while no difference in lifespan was observed in transgenic animals. Mild metabolic anomalies and thinning and loss of hairs from the back were the other observed phenotypic MADA manifestations. Histological analysis of tissues relevant for MADA syndrome revealed slight increase in adipose tissue inflammatory cells and a reduction of hypodermis due to a loss of subcutaneous adipose tissue. At cellular levels, transgenic cutaneous fibroblasts displayed nuclear envelope aberrations, presence of prelamin A, proliferation, and senescence rate defects. Gene transcriptional pattern was found differentially modulated between transgenic and wildtype animals, too. In conclusion, the presence of 527His Prelamin A accumulation is further linked to the appearance of mild progeroid features and metabolic disorder without lifespan reduction.

Mandibuloacral dysplasia: A premature ageing disease with aspects of physiological ageing.

Mandibuloacral dysplasia (MAD) is a rare genetic condition characterized by bone abnormalities including localized osteolysis and generalized osteoporosis, skin pigmentation, lipodystrophic signs and mildly accelerated ageing. The molecular defects associated with MAD are mutations in LMNA or ZMPSTE24 (FACE1) gene, causing type A or type B MAD, respectively. Downstream of LMNA or ZMPSTE24 mutations, the lamin A precursor, prelamin A, is accumulated in cells and affects chromatin dynamics and stress response. A new form of mandibuloacral dysplasia has been recently associated with mutations in POLD1 gene, encoding DNA polymerase delta, a major player in DNA replication. Of note, involvement of prelamin A in chromatin dynamics and recruitment of DNA repair factors has been also determined under physiological conditions, at the border between stress response and cellular senescence. Here, we review current knowledge on MAD clinical and pathogenetic aspects and highlight aspects typical of physiological ageing.

Rapamycin treatment of Mandibuloacral dysplasia cells rescues localization of chromatin-associated proteins and cell cycle dynamics.

Lamin A is a key component of the nuclear lamina produced through post-translational processing of its precursor known as prelamin A.LMNA mutations leading to farnesylated prelamin A accumulation are known to cause lipodystrophy, progeroid and developmental diseases, including Mandibuloacral dysplasia, a mild progeroid syndrome with partial lipodystrophy and altered bone turnover. Thus, degradation of prelamin A is expected to improve the disease phenotype. Here, we show different susceptibilities of prelamin A forms to proteolysis and further demonstrate that treatment with rapamycin efficiently and selectively triggers lysosomal degradation of farnesylated prelamin A, the most toxic processing intermediate. Importantly, rapamycin treatment of Mandibuloacral dysplasia cells, which feature very low levels of the NAD-dependent sirtuin SIRT-1 in the nuclear matrix, restores SIRT-1 localization and distribution of chromatin markers, elicits release of the transcription factor Oct-1 and determines shortening of the prolonged S-phase. These findings indicate the drug as a possible treatment for Mandibuloacral dysplasia.

Familial partial lipodystrophy, mandibuloacral dysplasia and restrictive dermopathy feature barrier-to-autointegration factor (BAF) nuclear redistribution.

Prelamin A processing impairment is a common feature of a restricted group of rare genetic alterations/disorders associated with a wide range of clinical phenotypes. Changes in histone posttranslational modifications, alterations in non-histone chromatin proteins and chromatin disorganization have been specifically linked to impairment of specific, distinct prelamin A processing steps, but the molecular mechanism involved in these processes is not yet understood . In this study, we show that the accumulation of wild-type prelamin A detected in restrictive dermopathy (RD), as well as the accumulation of mutated forms of prelamin A identified in familial partial lipodystrophy (FPLD) and mandibuloacral dysplasia (MADA), affect the nuclear localization of barrier-to-autointegration factor (BAF), a protein able to link lamin A precursor to chromatin remodeling functions. Our findings, in accordance with previously described results, support the hypothesis of a prelamin A involvement in BAF nuclear recruitment and suggest BAF-prelamin A complex as a protein platform usually activated in prelamin A-accumulating diseases. Finally, we demonstrate the involvement of the inner nuclear membrane protein emerin in the proper localization of BAF-prelamin A complex.

Vitamin D, parathyroid hormone, and acroosteolysis in systemic sclerosis.

OBJECTIVE: .Sclerodactyly with acroosteolysis (AO) and calcinosis are prominent features of systemic sclerosis (SSc), but the pathogenesis of these findings is poorly understood. Vitamin D and parathyroid hormone (PTH) have a crucial role in bone metabolism and resorption and may affect AO and calcinosis. We assessed vitamin D and PTH in patients with SSc. METHODS: Medical records of 134 consecutive patients with SSc (American College of Rheumatology criteria) followed at the rheumatology department during the years 2003-2006 were reviewed for clinical assessment, laboratory evaluation [including 25(OH) vitamin D, calcium, phosphorus, alkaline phosphatase, PTH, creatinine, and albumin]; imaging data confirming AO and/or calcinosis. Patients followed routinely at least once a year were included (81 patients). Of these, 60 patients' medical records were found to have complete, relevant clinical, laboratory, and radiographic imaging. RESULTS: Thirteen patients had diffuse disease and 47 limited disease - 51 women and 9 men, 44 Jews and 16 Arabs; mean age 55 +/- 14 years; disease duration 8 +/- 6 years. AO with or without calcinosis was observed in 42 patients (70%). Vitamin D deficiency was found in 46% of patients (16 out of 44 Jewish patients, 10 out of 16 Arab patients). PTH was elevated in 21.7% of patients. Significant correlations were observed between acroosteolysis and PTH (p = 0.015), calcinosis (p = 0.009), and disease duration (p = 0.008), and between PTH and vitamin D levels (p = 0.01). All patients had normal serum concentrations of calcium, phosphorus, magnesium, and albumin, and liver and kidney functions. CONCLUSION: In this group of Mediterranean patients with SSc, the incidence of vitamin D deficiency and secondary hyperparathyroidism was surprisingly high. This finding correlated with the occurrence of AO and calcinosis. Low levels of vitamin D may reflect silent malabsorption and might be a risk factor for secondary hyperparathyroidism and bone resorption. Traditional dress habits and low exposure to sun may contribute to vitamin D deficiency in an Arab population but do not explain all the findings. The pathogenesis of these findings needs to be corroborated in other SSc populations.

Hadju-Cheney syndrome: response to therapy with bisphosphonates in two patients.

Hadju-Cheney syndrome is characterized by short stature, distinctive facies, and a slowly progressive skeletal dysplasia including acro-osteolysis. Autosomal dominant inheritance is typical, but the genetic defect and molecular pathogenesis of the syndrome are unknown. Osteoporosis with atraumatic fracture is a frequent finding, and previous studies have documented biochemical and morphometric evidence of high bone turnover. Here, we report the clinical details and response to therapy with bisphosphonates in two patients (mother and son) with Hadju-Cheney syndrome and postulate that osteoclast-mediated bone resorption is important in the generalized osteoporosis commonly associated with this condition.