X-Linked Sinus Node Dysfunction and Atrial Fibrillation. Introduction: Atrial fibrillation (AF) is a heritable disorder with male predilection, suggesting a sex chromosome defect in certain patients. Loss-of-function truncation mutations in EMD, encoding the nuclear membrane protein emerin, cause X-linked Emery-Dreifuss muscular dystrophy (EDMD) characterized by localized contractures and skeletal myopathy in adolescence, sinus node dysfunction (SND) in early adulthood, and atrial fibrillation as a variably associated trait. This study sought to identify the genetic basis for male-restricted, nonsyndromic sinus node dysfunction and AF in a multigenerational family. Methods and Results: Genealogical and medical records, and DNA samples, were obtained. Progressive SND and AF occurred in four males related through maternal lineages, consistent with X-linked inheritance. Skeletal myopathy was absent, even at advanced ages. Targeted X chromosome genotyping mapped the disease locus to Xq28, implicating EMD as a positional candidate gene. DNA sequencing revealed hemizygosity for an in-frame 3-bp deletion in EMD (Lys37del) in affected males, disrupting a residue within the LEM binding domain critical for nuclear assembly but leaving the remainder of the protein intact. Buccal epithelial cell staining with emerin antibody demonstrated near-total functional loss of emerin. Female relatives underwent prospective electrocardiographic and genetic testing. Those heterozygous for Lys37del had ∼50-70% emerin-positive nuclei and variable degrees of paroxysmal supraventricular arrhythmia. Conclusions: Mutation of EMD can underlie X-linked familial AF. Lys37del is associated with epithelial cell emerin deficiency, as in EDMD, yet it causes electrical atriomyopathy in the absence of skeletal muscle disease. Targeted genetic testing of EMD should be considered in patients with SND-associated AF and/or family history suggesting X-linked inheritance.

The contents of nuclei are highly organized. Nuclear organization is facilitated by a dense protein network, called the nuclear lamina, which underlies the nuclear envelope. The nuclear lamina is composed of filamentous lamins and more than eighty lamin-associated proteins (LAPs). Among the first LAPs identified are LEM Domain (LEM-D) proteins, named after LAP2, emerin and MAN1. LEM-D proteins have many shared and unique functions that include providing structural support to the nucleus, regulating signal transduction pathways and gene expression, facilitating proper progression through the cell cycle and maintaining chromatin attachments at the nuclear periphery. Despite requirements for these processes in all cell types, loss of globally expressed LEM-D proteins causes tissue-restricted defects. Identification of the primary function in tissues susceptible to LEM-D protein loss is a persistent challenge in the nuclear lamina field. Research described here uses Drosophila as a model to understand LEM-D protein function. Loss of the Drosophila emerin homologue Otefin (Ote) causes a complex phenotype in the ovary wherein both somatic and germline cells are compromised. Using tissue-restricted expression experiments, it was determined that Ote function is only required in germline stem cells (GSCs) to maintain all cells in the ovary. Developmental, molecular and genetic analyses revealed that the primary defect in ote mutant ovaries is an early block in germline differentiation, followed by GSC death. Genetic rescue experiments revealed that both of these GSC defects are due to the activation of the DNA Damage Response (DDR) proteins ATR and Chk2. Interestingly, activation of ATR and Chk2 occurs independent of detectable canonical DDR triggers, including DNA damage. Immunohistochemical analyses suggest that Ote might be regulating chromatin condensation and/or heterochromatin organization in GSCs. Through studies of Ote, a rescue method was discovered that involves culturing animals at elevated temperatures. This novel rescue strategy, termed hyperthermia, acts independent of ATR or Chk2 inhibition. Interestingly, elevated temperatures leads to chromatin decondensation in Drosophila, suggesting that hyperthermia may rescue oogenesis by alleviating chromatin defects observed in ote mutant germ cells. Together, results from experiments discussed herein dissect a complex ovary phenotype to reveal the critical requirement for a nuclear lamina LEM-D protein. Investigations into Ote function have revealed several aspects of GSC biology. The ATR/Chk2 checkpoint activated in the absence of Ote uncovered a previously unidentified cause of female GSC death. Further, findings that ATR and Chk2 are activated in the absence of canonical triggers suggest that GSCs possess a system to monitor defects or changes in the nucleus that do not involve DNA damage. Therefore, studies of Ote function and ote mutant phenotypes have uncovered valuable insights into LEM-D protein function and revealed the existence of novel conditions required for GSC maintenance.