A study on genetic mutations involving mitochondrial disorders : a diagnostic approach and application of human iPSCs to understand disease pathogenesis

Padmini P. Polinati

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Mitochondrial diseases are generally caused by genetic variants that may affect cell function during the process of energy generation: right from the start of protein translocation to the fatty acid degradation by beta-oxidation (β-oxidation). The main objective of this PhD thesis is to study genetic variants that cause mitochondrial diseases and also to understand the disease pathogenesis of a known disease using the induced pluripotent stem cell (iPSC) method, a revolutionary approach in regenerative medicine. In the first study, we carried out a long-term follow up of six metabolic diseased patients and subsequently we performed a carrier frequency study of the identified carnitine palmitoyl transferase 1A (CPT1A) gene variant in the Finnish population. We identified a novel homozygous variant c.1364A>C (p.Lys455Thr) in exon 12 of the CPT1A gene. No carriers of the variant c.1364A>C were detected upon minisequencing of 150 control samples but the allele frequency of CPT1A variant in global population is 0.0002142 (ExAC Browser) whereas in the Finnish population (6614 allele number) the frequency is 0.001966. The identified variant was predicted to cause improper folding of the CPT1A protein, which leads to its degradation. All patients were treated with a high-carbohydrate and a low fat diet. In the second study, we focused on the human DnaJ (Hsp40 homolog) subfamily C, member 19 (DNAJC19) deficiency. Our studies showed that it causes early onset dilated cardiomyopathy syndrome (DCMA). This is the first report of a genetic defect in the mitochondrial protein, DNAJC19, outside of the Canadian Dariusleut Hutterite population. This defect is characterized by an unusual aetiology for an early onset recessively inherited dilated cardiomyopathy that is associated with ataxia and male genital anomalies. Sequencing of the human DNAJC19 gene revealed a homozygous single nucleotide (A) deletion in exon 6 that cause a frameshift and lead to the premature termination of the protein. In the third study, the pathogenesis of retinopathy in long-chain acyl-CoA dehydrogenase deficiency (LCHADD) was studied using iPSC technology. Retinopathy is an unusual manifestation of LCHADD, as mitochondrial fatty acid β-oxidation (FAβO) has not been considered to play a major role in the metabolism of the retina. Among all defects of mitochondrial FAβO, only long-chain acyl-CoA dehydrogenase (LCHAD) and mitochondrial trifunctional protein (TFP) deficiencies have developed pigmentary retinopathy and peripheral neuropathy. We elucidated how a genetic variant in the FAβO cycle can disrupt the retinal pigment epithelium (RPE) that can eventually lead to blindness. In addition, we developed a new in vitro cell model; iPSC clones were generated from LCHADD patient fibroblasts and further differentiated into RPE cells. Several changes were observed in patient RPE cells such as decreased cell size, lower pigmentation and irregular pattern of morphology. Electron microscopy analysis showed an accumulation of a few melanosomes, more melanolysosomes, and large sized lipid droplets in patient RPE cells. Furthermore, increased levels of triglycerides in patient RPE cells were observed upon mass spectrometric analysis. We concluded that all these changes had contributed to the disruption of the RPE layer that leads to blindness in LCHAD deficiency patients. Finally, the research done for this thesis succeeded in identifying novel variants in CPT1A and DNAJC19 genes in Finnish patients. Our long-term follow up studies on CPT1A deficiency can help patients in better diagnosis, which further helps clinicians to identify the genetic cause. We also found a novel phenotype with DNAJC19 deficiency. Further we established the groundwork to understand the pathogenesis of retinopathy in LCHADD patients using an advanced method that helps to study in depth pathogenesis mechanism.
Original languageEnglish
Place of PublicationHelsinki
Publisher
Print ISBNs978-951-51-1544-7
Publication statusPublished - 2015
MoE publication typeG5 Doctoral dissertation (article)

Bibliographical note

M1 - 77 s. + liitteet
Helsingin yliopisto

Fields of Science

  • Abnormalities, Multiple
  • +genetics
  • Anemia
  • Ataxia
  • Cardiomyopathy, Dilated
  • Carnitine O-Palmitoyltransferase
  • DNA Mutational Analysis
  • Genetic Association Studies
  • Genetic Predisposition to Disease
  • Induced Pluripotent Stem Cells
  • Mitochondrial Diseases
  • Mitochondrial Membrane Transport Proteins
  • Molecular Sequence Data
  • Mutation, Missense
  • Retinopathy
  • Syndrome
  • Urogenital Abnormalities
  • 3111 Biomedicine

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