Mitochondrial dysfunction causes moderate to profound hearing loss both in isolation and as a feature of multi-systemic mitochondrial disease. The m.1555A>G mitochondrial DNA (mtDNA) variant is associated with a predisposition to aminoglycoside ototoxicity and maternally inherited non-syndromic deafness. However, the reasons for the highly variable penetrance of the associated hearing loss have not yet been fully resolved. Aminoglycosides are a recognised modifier factor of the hearing loss, but cannot account for all hearing impaired carriers in multi-generational pedigrees, implicating additional co-segregating genetic factors. By identifying and characterising the c.3G>A SSBP1 variant as a nuclear modifier of m.1555A>G the work detailed in this thesis extends our understanding of mitochondrial-nuclear interactions in human disease. To ascertain the frequency of the m.1555A>G variant in patients with suspected mitochondrial hearing loss we surveyed the laboratories within the United Kingdom that undertake genetic testing for this variant. We determined that the variant was not found more frequently in patients with known hearing impairment providing further evidence that m.1555A>G does not cause hearing loss in isolation. These results strengthened the case for nuclear genetic modifiers as important contributors to m.1555A>G pathogenesis. We next identified a multi-generational family that transmitted the m.1555A>G variant with variable clinical penetrance of hearing loss. In addition, a cohort of sporadic individuals carrying m.1555A>G was used to test the hypothesis that a conserved genetic mechanism accounted for the phenotype in all carriers. To this effect, we undertook whole exome sequencing in selected familial and sporadic carriers of m.1555A>G, identifying a heterozygous start loss mutation in the core mtDNA replisome protein gene, SSBP1, that co-segregated with the m.1555A>G variant and the phenotype in the family. The SSBP1 variant lead to a perturbation of mtDNA metabolism, and was associated with multiple mtDNA deletions and mtDNA depletion in skeletal muscle. Fibroblasts from these patients also showed mitochondrial network fragmentation and reduced intra-mitochondrial protein synthesis in keeping with the co-existing m.1555A>G variant, leading to reduced proliferation rates under conditions of forced mitochondrial respiration. Our findings provide an explanation for the variable clinical penetrance of the disorder within these m.1555A>G carriers and highlight the importance of trans-acting modifiers in mitochondrial disease.