Patient reported outcomes for phosphomannomutase 2 congenital disorder of glycosylation (PMM2-CDG): listening to what matters for the patients and health professionals

Background

Congenital disorders of glycosylation (CDG) are a growing group of rare genetic disorders. The most common CDG is phosphomannomutase 2 (PMM2)-CDG which often has a severe clinical presentation and life-limiting consequences. There are no approved therapies for this condition. Also, there are no validated disease-specific quality of life (QoL) scales to assess the heterogeneous clinical burden of PMM2-CDG which presents a challenge for the assessment of the disease severity and the impact of a certain treatment on the course of the disease.

Aim and methods

This study aimed to identify the most impactful clinical signs and symptoms of PMM2-CDG, and specific patient and observer reported outcome measures (PROMs and ObsROMs, respectively) that can adequately measure such impact on patients’ QoL. The most burdensome signs and symptoms were identified through input from the CDG community using a survey targeting PMM2-CDG families and experts, followed by family interviews to understand the real burden of these symptoms in daily life. The list of signs and symptoms was then verified and refined by patient representatives and medical experts in the field. Finally, a literature search for PROMs and ObsROMs used in other rare or common diseases with similar signs and symptoms to those of PMM2-CDG was performed.

Results

Twenty-four signs/symptoms were identified as the most impactful throughout PMM2-CDG patients’ lifetime. We found 239 articles that included tools to measure those community-selected PMM2-CDG symptoms. Among them, we identified 80 QoL scales that address those signs and symptoms and, subsequently, their psychometric quality was analysed. These scales could be applied directly to the PMM2-CDG population or adapted to create the first PMM2-CDG-specific QoL questionnaire.

Conclusion

Identifying the impactful clinical manifestations of PMM2-CDG, along with the collection of PROMs/ObsROMs assessing QoL using a creative and community-centric methodology are the first step towards the development of a new, tailored, and specific PMM2-CDG QoL questionnaire. These findings can be used to fill a gap in PMM2-CDG clinical development. Importantly, this methodology is transferable to other CDG and rare diseases with multiple signs and symptoms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02551-y.

The challenge of CDG diagnosis

Congenital disorders of glycosylation (CDG) are a rapidly growing family of genetic diseases that currently includes some 130 different types. CDG diagnosis is a challenge, not only because of this large number but also because of the huge clinical heterogeneity even within a number of CDG. In addition, the classical screening test, serum transferrin isoelectrofocusing, is only positive in about 60% of CDG, and can even become negative in some CDG particularly in PMM2-CDG, the most frequent N-glycosylation defect. In order to facilitate CDG diagnosis, we hereby provide some practical tools: (1) a list of clinical features strongly suggestive of a distinctive CDG; (2) a table of clinical, biochemical and laboratory findings reported in CDG, arranged per organ/system; (3) an overview of the affected organs/systems in each CDG; and (4) a diagnostic decision tree in face of a patient with a suspicion of CDG. Most important is to keep in mind a CDG in any unexplained syndrome, in particular when there is neurological involvement. This mini-review enumerates clinical and biochemical hallmarks of these diseases and the biochemical and genetic testing available, and provides an updated list and information on identified CDG. The main aim is to act as a CDG diagnosis simplified guide for healthcare professionals and, additionally, as an awareness and lobbying tool to help in the effectiveness and promptness of CDG diagnosis.

Generation and characterization of two human iPSC lines from patients with methylmalonic acidemia cblB type

Two human induced pluripotent stem cell (iPSC) lines were generated from fibroblasts of two siblings with methylmalonic acidemia cblB type carrying mutations in the MMAB gene: c.287T➔C (p.Ile96Thr) and a splicing loss-of-function variant c.584G➔A affecting the last nucleotide of exon 7 in MMAB (p.Ser174Cysfs*23). Reprogramming factors OCT3/4, SOX2, KLF4 and c-MYC were delivered using a non-integrative method based on the Sendai virus. Once established, iPSCs have shown full pluripotency, differentiation capacity and genetic stability.

Molecular and phenotypic characteristics of seven novel mutations causing branched-chain organic acidurias

Specific mitochondrial enzymatic deficiencies in the catabolism of branched-chain amino acids cause methylmalonic aciduria (MMA), propionic acidemia (PA) and maple syrup urine disease (MSUD). Disease-causing mutations were identified in nine unrelated branched-chain organic acidurias (BCOA) patients. We detected eight previously described mutations: p.Asn219Tyr, p.Arg369His p.Val553Glyfs*17 in MUT, p.Thr198Serfs*6 in MMAA, p.Ile144_Leu181del in PCCB, p.Gly288Valfs*11, p.Tyr438Asn in BCKDHA and p.Ala137Val in BCKDHB gene. Interestingly, we identified seven novel genetic variants: p.Leu549Pro, p.Glu564*, p.Leu641Pro in MUT, p.Tyr206Cys in PCCB, p.His194Arg, p.Val298Met in BCKDHA and p.Glu286_Met290del in BCKDHB gene. In silico and/or eukaryotic expression studies confirmed pathogenic effect of all novel genetic variants. Aberrant enzymes p.Leu549Pro MUT, p.Leu641Pro MUT and p.Tyr206Cys PCCB did not show residual activity in activity assays. In addition, activity of MUT enzymes was not rescued in the presence of vitamin B12 precursor in vitro which was in accordance with non-responsiveness or partial responsiveness of patients to vitamin B12 therapy. Our study brings the first molecular genetic data and detailed phenotypic characteristics for MMA, PA and MSUD patients for Serbia and the whole South-Eastern European region. Therefore, our study contributes to the better understanding of molecular landscape of BCOA in Europe and to general knowledge on genotype-phenotype correlation for these rare diseases.

Methylmalonic aciduria cblB type: characterization of two novel mutations and mitochondrial dysfunction studies

Methylmalonic aciduria (MMA) cblB type is caused by mutations in the MMAB gene, which codes for the enzyme adenosine triphosphate (ATP): cobalamin adenosyltransferase (ATR). This study reports differences in the metabolic and disease outcomes of two pairs of siblings with MMA cblB type, respectively harbouring the novel changes p.His183Leu/p.Arg190dup (P1 and P2) and the previously described mutations p.Ile96Thr/p.Ser174fs (P3 and P4). Expression analysis showed p.His183Leu and p.Arg190dup to be destabilizing mutations. Both were associated with reduced ATR stability and a shorter half-life than wild-type ATR. Analysis of several parameters related to oxidative stress and mitochondrial function showed an increase in reactive oxygen species (ROS) content, a decrease in mitochondrial respiration and changes in mitochondria morphology and structure in patient-derived fibroblasts compared to control cells. The impairment in energy production and the presence of oxidative stress and fission of the mitochondrial reticulum suggested mitochondrial dysfunction in cblB patients' fibroblasts. The recovery of mitochondrial function should be a goal in efforts to improve the clinical outcome of MMA cblB type.