Leucine zipper-EF-hand containing transmembrane protein 1 (LETM1) encodes an inner mitochondrial membrane protein with an osmoregulatory function controlling mitochondrial volume and ion homeostasis. The putative association of LETM1
M1 with a human disease was initially suggested in Wolf-Hirschhorn syndrome, a disorder that results from de novo monoallelic deletion of chromosome 4p16.3, a region encompassing LETM1. Utilizing exome sequencing and international gene-matching efforts, we have identified 18 affected individuals from 11 unrelated families harboring ultra-rare bi-allelic missense and loss-of-function LETM1 variants and clinical presentations highly suggestive of mitochondrial disease. These manifested as a spectrum of predominantly infantile-onset (14/18, 78%) and variably progressive neurological, metabolic, and dysmorphic symptoms, plus multiple organ dysfunction associated with neurodegeneration. The common features included respiratory chain complex deficiencies (100%), global developmental delay (94%), optic atrophy (83%), sensorineural hearing loss (78%), and cerebellar ataxia (78%) followed by epilepsy (67%), spasticity (53%), and myopathy (50%). Other features included bilateral cataracts (42%), cardiomyopathy (36%), and diabetes (27%). To better understand the pathogenic mechanism of the identified LETM1 variants, we performed biochemical and morphological studies on mitochondrial K+/H+ exchange activity, proteins, and shape in proband-derived fibroblasts and muscles and in Saccharomyces cerevisiae, which is an important model organism for mitochondrial osmotic regulation. Our results demonstrate that bi-allelic LETM1 variants are associated with defective mitochondrial K+ efflux, swollen mitochondrial matrix structures, and loss of important mitochondrial oxidative phosphorylation protein components, thus highlighting the implication of perturbed mitochondrial osmoregulation caused by LETM1 variants in neurological and mitochondrial pathologies.
Li N, etal., Int J Clin Exp Pathol. 2015 Oct 1;8(10):12893-900. eCollection 2015.
BACKGROUND: Leucine zipper/EF hand-containing transmembrane-1 (LETM1) is a mitochondrial inner membrane protein that was first identified in Wolf-Hirschhorn syndrome. However, high-level expression of LETM1 has been correlat
ed with multiple human malignancies, suggesting roles in carcinogenesis and tumor progression. This study is aimed to explore the clinicopathological characteristics and prognostic value of LETM1 overexpression in breast cancer. METHODS: Immunohistochemical (IHC) staining, and immunofluorescence (IF) were performed to examine LETM1 expression in breast cancer cell line/tissues compared with adjacent normal tissues. Statistical analysis was applied to evaluate the correlation between LETM1 overexpression and the clinicopathological features of breast cancer. Survival rates were calculated using the Kaplan-Meier method, and the relationship between prognostic factors and patient survival was analyzed using the Cox proportional hazard models. RESULTS: LETM1 protein showed cytoplasmic staining pattern in breast cancer. The strongly positive rate of LETM1 protein was 61.6% (98/159) in breast cancer, which was significantly higher than in DCIS (29.7%, 11/37), hyperplasia (16.7%, 3/18) and adjacent normal breast tissues (15.9%, 7/44). High-level expression of LETM1 protein was correlated with lymph node metastasis, poor differentiation, late clinical stage, disease-free survival (DFS) and overall survival (OS) rates in breast cancer. Moreover, multivariate analysis suggested that LETM1 emerged as a significant independent prognostic factor along with clinical stage of patients with breast cancer. CONCLUSIONS: LETM1 plays an important role in the progression of breast cancer. High level expression of LETM1 is an independent poor prognostic factor of breast cancer.
Deletions within human chromosome 4p16.3 cause Wolf-Hirschhorn syndrome (WHS), which is characterized by severe mental and developmental defects. It is thought that haploinsufficiency of more than one gene contributes to the complex phenotype. We have cloned and characterized a novel gene (LETM1
le='font-weight:700;'>LETM1) that is deleted in nearly all WHS patients. LETM1 encodes a putative member of the EF-hand family of Ca(2+)-binding proteins. The protein contains two EF-hands, a transmembrane domain, a leucine zipper, and several coiled-coil domains. On the basis of its possible Ca(2+)-binding property and involvement in Ca(2+) signaling and/or homeostasis, we propose that haploinsufficiency of LETM1 may contribute to the neuromuscular features of WHS patients.
