We report the structural, electronic, elastic, and piezoelectric properties of some alpha -quartz SiO2 isotypes, namely, AlPO4 , GaPO4 , GaAsO4 , and FePO4 . This family of crystals is well known for its elastic and piezoelectric properties related to their MO4 and XO4 tetrahedral units, especially the M-O-X bridging angle theta and the tilt angle delta , the most distorted structures being expected to exhibit the highest piezoelectric coupling constant. We have then computed the optimized structure of each MXO4 isomorph compound in order to study the variation in the elastic and piezoelectric tensors with respect to theta and delta . A comparison between our results at the density-functional theory level and the available data (theoretical and experimental ones) has been made. The differences observed for the whole class of systems has been discussed and a comparison with the SiO2 alpha -quartz behavior is made.