Mechanotransduction is the evolutionarily conserved ability of an organism to detect mechanical stimuli from the environment and convert them into electrochemical signals. In the non-sensory cells of mammals, this mechanotransductive role is performed by the PIEZO1 channels which are dynamically involved in a range of cellular processes relevant to both health and disease. Here, we investigate a recent clinical case of hydrops fetalis linked to novel compound heterozygous PIEZO1 mutations - P2190L and R2082H. Preliminary functional analyses revealed distinct impacts of these mutations: while the R2082H mutant exhibited reduced force sensitivity, the P2190L mutant completely abolished ion channel activity. However, it remained unclear whether these mutations alter PIEZO1’s interaction with its recently identified auxiliary subunit, MDFIC. To address this, we employed cell-attached patch-clamp experiments to assess the functionality of these mutants in the presence and absence of MDFIC. Since MDFIC was shown to delay the inactivation kinetics of PIEZO1, we hypothesised it could rescue the P2190L phenotype by prolonging the open state1. Our initial findings indicated that P2190L remained non-conductive even with MDFIC co-expression, suggesting that the mutation fundamentally disrupted ion channel activity. This observation prompted further investigation into the structural basis of P2190L’s loss of function. The P2190L PIEZO1 structure was obtained using cryogenic electron microscopy which revealed a straightening of the outer helix and loss of inner helix stability at the ion conduction pore. Additional all-atom molecular dynamics (MD) simulations help to understand the molecular mechanisms by which this mutation prevents ion conduction. Ultimately, these experiments aid in rationalising the pathological effects of PIEZO1 mutations by linking structural alterations to functional deficits, thereby enhancing our understanding of PIEZO1’s structure-function relationship.