INTRODUCTIONThe PIEZO1 gene is located on chromosome 16 (16q24.3) and encodes the largest known human transmembrane protein, PIEZO1. The PIEZO1 protein has low tissue specificity, showing expression in at least 27 different tissue types.1 This mechanoreceptor is activated by mechanical force and controls potassium and calcium flow in multiple cell types, most prominently lung, bladder, skin, and red blood cells.1,2 Studies have shown that in fetal tissue the highest expression of this protein is in the liver, spleen, and lymphatic vessels.1,2 PIEZO1 channels are crucial during development of lymphatic vasculature, acting as baroreceptors in the lymphatic and vascular systems, and sensing frictional force to determine vascular structure.3 Pathogenic variants inPIEZO1 have therefore been phenotypically associated with lymphatic malformation, perinatal edema, and non-immune fetal hydrops, as well as hemolytic anemia and pseudohyperkalemia.2,4PIEZO1 -related disorders include autosomal recessive lymphatic malformation 6 (LMPHM6, OMIM 616843) due to biallelic loss-of-function variants and autosomal dominant dehydrated hereditary stomatocytosis (OMIM 194380) due to heterozygous gain-of-function variants.5 Despite significant phenotypic heterogeneity, both disorders may present with perinatal edema and non-immune hydrops fetalis.5Congenital lymphatic dysplasia is the hallmark feature of LMPHM6, resulting in severe impairment of vascular development, and commonly causing in utero demise. The associated primary lymphedema affects all body segments, causing pleural effusions, pulmonary and intestinal lymphangiectasia, facial and neck swelling, chylothoraces, and pericardial effusion. In some cases, hydrops fetalis has resolved postnatally with concomitant recurrence of peripheral edema during childhood, typically in the lower limbs. Most reported cases of LMPHM6 have resultant pleural effusions that become chylous with the introduction of enteral feeds.5More recently, PIEZO1 has been implicated as a potential mechanotransducer in osteoblastic bone formation. Osteoblasts respond to mechanical loading and bone growth and maintenance is known to be impaired in microgravity environments and non-weight bearing patients. However, the mechanism by which this occurs has previously been uncertain.6 Recent research found that knockout mice with PIEZO1-deficient osteoblasts sustain spontaneous fractures in the setting of impaired bone density and strength. Patients with osteoporosis have been found to have reduced expression of the PIEZO1 protein.6 Likewise, simulated microgravity environments result in decreased osteoblastic function via suppression of PIEZO1 activity.7 Here, we present a patient with LMPH6 due to likely pathogenic homozygous loss-of-function PIEZO1variants who has unexplained bone disease, the first reported case of significant bone disease in this phenotype.