# Recessive PPTC7 deficiency triggers excessive mitophagy to cause a severe inborn error of metabolism with hypomyelinating leukodystrophy

## TL;DR

A genetic deficiency in PPTC7 causes severe metabolic and neurological issues in humans by triggering excessive mitophagy.

## Contribution

This study identifies the first known human inborn error of mitophagy caused by PPTC7 deficiency.

## Key findings

- PPTC7 deficiency in patients leads to excessive BNIP3- and NIX-mediated mitophagy.
- The p.D158N variant disrupts PPTC7's phosphatase function and regulation of mitophagy.
- Exogenous PPTC7 rescues increased mitophagy in patient-derived cells.

## Abstract

The mitochondrial phosphatase PPTC7 has emerged as a potent regulator of metabolism and mitophagy as its global knockout leads to perinatal lethality in mice. However, no known Mendelian diseases have been linked to PPTC7 deficiency, rendering its role in human pathophysiology unclear. Here, we identify two independent homozygous variants in PPTC7: a missense variant, p.D158N, and a duplication variant (c.*57dup) within the 3’ untranslated region (UTR). These variants were detected in three patients from two unrelated families presenting with a primary mitochondrial disease characterized by hypomyelinating leukodystrophy, recurrent metabolic and lactic acidosis, and anemia with immune dysregulation. Patient samples, including plasma and primary fibroblasts, showed robust metabolic and mitochondrial dysfunction, with substantial phenotypic overlap with Pptc7 knockout murine fibroblast models. PPTC7 patient fibroblasts carrying the p.D158N variant and CRISPR-knocked in cells to model the 3’UTR variant showed hallmarks of excessive BNIP3- and NIX-mediated mitophagy, including aberrant mitochondrial morphology, diminished mitochondrial protein expression, and increased mt-Keima flux. Critically, increased mitophagy in these cellular models was rescued by exogenous PPTC7 expression, confirming dysfunction derives from loss of this mitochondrial phosphatase. Mechanistically, we found that the p.D158N variant, affecting a highly conserved residue, disrupts metal binding to compromise both the enzymatic phosphatase function of PPTC7 as well as its negative regulation of BNIP3 and NIX. Collectively, these data provide the first known cases with a recessive inborn error of mitophagy due to PPTC7 deficiency and underscore the importance of this mitochondrial phosphatase in maintaining metabolic health and balanced mitophagy.