Cobalamin C deficiency-associated thrombotic microangiopathy: uncommon or unrecognised?

Case report: Desquamating dermatitis, bilateral cerebellar lesions in a late-onset methylmalonic acidemia patient

Introduction

Cobalamin C (cblC) deficiency is a rare hereditary disorder affecting intracellular cobalamin metabolism, primarily caused by mutations in MMACHC. This condition is characterized by combined methylmalonic acidemia and hyperhomocysteinemia, displaying a wide range of clinical manifestations involving multiple organs. Owing to its uncommon occurrence and diverse clinical phenotypes, diagnosing cblC deficiency is challenging and often leads to delayed or missed diagnoses.

Case description

In this report, we present a case of late-onset cblC deficiency with brown desquamating dermatitis on the buttocks. Magnetic resonance imaging (MRI) of the brain revealed bilateral cerebellar abnormalities. The suspicion of an inherited metabolic disorder was raised by abnormal serum amino acid and acylcarnitine levels, along with increased urine methylmalonic acid and serum homocysteine levels. Whole-exome sequencing helped identify a homozygous variant (c.482G>A) in MMACHC, confirming the diagnosis of cblC deficiency. However, despite receiving treatment with hydroxocobalamin and betaine, the patient did not experience clinical improvement, which may be attributed to the delayed diagnosis as indicated by the declining homocysteine and methylmalonic acid levels.

Conclusion

Collectively, we emphasize the significance of recognizing the skin lesions and observing serial MRI changes in patients with cblC deficiency. Our case underscores the importance of early diagnosis and timely therapeutic intervention for this severe yet frequently manageable condition.

Concurrent Cobalamin C and Plasminogen Deficiencies in a Patient with Chronic Thrombotic Microangiopathy

BACKGROUND

Although most patients with atypical hemolytic uremic syndrome (aHUS) have variants in genes participating in alternative complement pathways, rare variants in non-complement pathway-related genes, including DGKE, INF2, MMACHC, PLG, and THBD, have also been described.

CASE PRESENTATION

We report an 18-year-old male patient with renal biopsy-proven chronic thrombotic microangiopathy that raised suspicion of aHUS. Whole-exome sequencing revealed a novel pathogenic homozygous MMACHC c.484G>T (p.Gly162Trp) variant. Subsequently, clinical and laboratory findings confirmed cobalamin C (Cbl C) deficiency. Also, homozygous missense c.1112C>T PLG (p.Thr371Ile) variant was detected (it had been reported as a variant of unknown significance). However, the low serum plasminogen (PLG) activity proved the pathogenicity of c.1112C>T. Hence, the patient was diagnosed with concurrent Cbl C and PLG deficiencies. Segregation analysis revealed that the mother and father had the same heterozygous PLG and MMACHC variants. PLG variants have generally been described in aHUS patients concomitant with complement gene variants in the literature; therefore, the association between aHUS and PLG variants is controversial. The possible contribution of PLG deficiency to thrombotic microangiopathy was also discussed in this case.

CONCLUSION

Non-complement-mediated aHUS is an exceptional disorder. A limited number of genes are involved in this entity. To our knowledge, this is the first aHUS patient diagnosed with both Cbl C and PLG deficiencies in the literature.

How I diagnose and treat atypical hemolytic uremic syndrome

Our understanding and management of atypical hemolytic uremic syndrome (aHUS) have dramatically improved in the last decade. aHUS has been established as a prototypic disease resulting from a dysregulation of the complement alternative C3 convertase. Subsequently, prospective nonrandomized studies and retrospective series have shown the efficacy of C5 blockade in the treatment of this devastating disease. C5 blockade has become the cornerstone of the treatment of aHUS. This therapeutic breakthrough has been dulled by persistent difficulties in the positive diagnosis of aHUS, and the latter remains, to date, a diagnosis by exclusion. Furthermore, the precise spectrum of complement-mediated renal thrombotic microangiopathy is still a matter of debate. Nevertheless, long-term management of aHUS is increasingly individualized and lifelong C5 blockade is no longer a paradigm that applies to all patients with this disease. The potential benefit of complement blockade in other forms of HUS, notably secondary HUS, remains uncertain.

How I treat thrombotic microangiopathy in the era of rapid genomics

Thrombotic microangiopathy (TMA) encompasses various genetically-driven diseases. The emergence of ultrafast genomic sequencing has recently opened up new avenues of research for genetic investigations in the setting of intensive care units. TMA is likely to be a suitable focus for fast-track genomic sequencing. By establishing an expeditious molecular diagnosis of patients with the complement-dependent hemolytic uremic syndrome, fast-track genomic sequencing allows for the timely implementation or withdrawal of anti-C5 treatment while averting unnecessary, costly, and potentially harmful therapy in patients testing negative for the syndrome. Furthermore, genomics has the potential to reshape the taxonomic classification of TMA owing to comprehensive genomic analysis. The most significant results from such analysis can be categorized as (1) new descriptions of genetic diseases previously not recognized as associated with TMA and (2) an enrichment of the phenotypic spectrum of diseases traditionally related to TMA. The latter draws on the concept of retrophenotyping, wherein genomic investigation precedes full clinical description. By taking precedence over a phenotypic approach, an unbiased genomic-focused analysis maximizes the chances of discovering new descriptions of a given variant. Presented here are 4 cases of TMA which highlight these issues and substantiate the promise of fast-track genomic sequencing.

Pseudo-thrombotic Microangiopathy Caused by Acquired Cobalamin Deficiency Due to Unintentional Neglect

Acquired vitamin B₁₂ (VB₁₂) deficiency is a rare cause of thrombotic microangiopathy (TMA). We experienced an 86-year-old Japanese woman who presented with coma, renal dysfunction, and microangiopathic hemolytic anemia. Although we initially considered thrombotic thrombocytopenic purpura, we eventually diagnosed her to have VB₁₂ deficiency due to inappropriate dietary care based on her low serum VB₁₂ level, social history, and negative parietal cell finding and the presence of intrinsic factor antibody. Because similar cases are expected to increase in today's aging society, our experience underscores the importance of including acquired VB₁₂ deficiency in the differential diagnosis of TMA, even in elderly patients without a history of gastrectomy.

Management of thrombotic microangiopathy in pregnancy and postpartum: report from an international working group

Pregnancy and postpartum are high-risk periods for different forms of thrombotic microangiopathy (TMA). However, the management of pregnancy-associated TMA remains ill defined. This report, by an international multidisciplinary working group of obstetricians, nephrologists, hematologists, intensivists, neonatologists, and complement biologists, summarizes the current knowledge of these potentially severe disorders and proposes a practical clinical approach to diagnose and manage an episode of pregnancy-associated TMA. This approach takes into account the timing of TMA in pregnancy or postpartum, coexisting symptoms, first-line laboratory workup, and probability-based assessment of possible causes of pregnancy-associated TMA. Its aims are: to rule thrombotic thrombocytopenic purpura (TTP) in or out, with urgency, using ADAMTS13 activity testing; to consider alternative disorders with features of TMA (preeclampsia/eclampsia; hemolysis elevated liver enzymes low platelets syndrome; antiphospholipid syndrome); or, ultimately, to diagnose complement-mediated atypical hemolytic uremic syndrome (aHUS; a diagnosis of exclusion). Although they are rare, diagnosing TTP and aHUS associated with pregnancy, and postpartum, is paramount as both require urgent specific treatment.

Vitamin B12 deficiency and metabolism-mediated thrombotic microangiopathy (MM-TMA)

Thrombotic microangiopathies (TMA) are characterized by microangiopathic hemolytic anemia, thrombocytopenia and organ damage resulting from mechanical factors, accumulation of the ultra-large von Willebrand factor multimers or complement-mediated abnormalities. Severe acquired vitamin B12 (Cobalamin - Cbl) deficiency or congenital defective Cbl metabolism could lead to a picture that mimics TMA. The later has been termed metabolism-mediated TMA (MM- TMA). This confusing picture is mediated partly by the large red cell fragmentation coupled with reduced platelet production in the absence of vitamin B12 and partly by the accumulated byproducts and metabolites that induce endothelial injury and hence organ damage. Expensive and complicated treatment for TMA is often initiated on an empiric basis, pending the results of confirmatory tests. In contrast, vitamin B12 Pseudo-TMA and MM-TMA could be treated with proper vitamin B12 supplementation. It is therefore important to identify these disorders promptly. The recent availability of a validated scoring system such as the PLASMIC score uses simple clinical and laboratory parameters. As it incorporates the mean corpuscular volume in its laboratory parameters, this helps in the identification of pseudo and MM-TMA. Perhaps some minor modification of this scoring system by changing the parameters of hemolysis to include reticulocytosis and rather than and/or other hemolytic parameters could even help refine this identification.

The PLASMIC Score May Be Useful in the Early Diagnosis of Complement-Mediated Thrombotic Microangiopathy via Early Exclusion of Thrombotic Thrombocytopenic Purpura

Complement-mediated thrombotic microangiopathy is a rare form of thrombotic microangiopathy but has high rates of mortality and morbidity. Effective treatment exists with eculizumab for this condition, but administration of treatment is often delayed because of overlapping symptoms with other causes of thrombotic microangiopathy. We present a case of a 78-year-old male who was eventually diagnosed with complement-mediated thrombotic microangiopathy. We also discuss the use of the PLASMIC scoring model to assist in more rapid diagnosis and discernment between various thrombotic microangiopathies.

Atypical adult-onset methylmalonic acidemia and homocystinuria presenting as hemolytic uremic syndrome

Thrombotic microangiopathy (TMA) syndromes can be secondary to a multitude of different diseases. Most can be identified with a systematic approach and, when excluded, TMA is generally attributed to a dysregulation in the activity of the complement alternative pathways-atypical hemolytic uremic syndrome (aHUS). We present a challenging case of a 19-year-old woman who presented with thrombotic microangiopathy, which was found to be caused by methylmalonic acidemia and homocystinuria, a rare vitamin B12 metabolism deficiency. To our knowledge, this is the first time that an adult-onset methylmalonic acidemia and homocystinuria presents as TMA preceding CNS involvement.

Monogenic diseases in respiratory medicine: Clinical perspectives

With the increasing awareness of genetics in respiratory medicine and improvements in molecular diagnostic techniques, many complicated and rare diseases in respiratory medicine can be diagnosed. Most respiratory diseases have no specific phenotype. However, the clinical spectrum of monogenic diseases in respiratory medicine varies, from pulmonary disease to other inherited disorders that involve the lung. The genes that mediate some of these diseases have been identified. Certain monogenic diseases remain poorly characterized clinically. Because of the specificity of the phenotype of respiratory disease, a future challenge will be to correlate the phenotype and genotype and understand its phenotypic variability. With the development of precision medicine, research on monogenic disorders has been intensive and vigorous. In this article, we provide a brief clinical introduction to monogenic diseases in pediatrics.

Haemolytic uraemic syndrome

Haemolytic uraemic syndrome is a form of thrombotic microangiopathy affecting predominantly the kidney and characterised by a triad of thrombocytopenia, mechanical haemolytic anaemia, and acute kidney injury. The term encompasses several disorders: shiga toxin-induced and pneumococcus-induced haemolytic uraemic syndrome, haemolytic uraemic syndrome associated with complement dysregulation or mutation of diacylglycerol kinase ɛ, haemolytic uraemic syndrome related to cobalamin C defect, and haemolytic uraemic syndrome secondary to a heterogeneous group of causes (infections, drugs, cancer, and systemic diseases). In the past two decades, experimental, genetic, and clinical studies have helped to decipher the pathophysiology of these various forms of haemolytic uraemic syndrome and undoubtedly improved diagnostic approaches. Moreover, a specific mechanism-based treatment has been made available for patients affected by atypical haemolytic uraemic syndrome due to complement dysregulation. Such treatment is, however, still absent for several other disease types, including shiga toxin-induced haemolytic uraemic syndrome.

Combined methylmalonic acidemia and homocysteinemia presenting predominantly with late-onset diffuse lung disease: a case series of four patients

Combined methylmalonic acidemia (MMA) and homocysteinemia are a group of autosomal recessive disorders caused by inborn errors of cobalamin metabolism, including CblC, D, F, and J, with cblC being the most common subtype. The clinical manifestations of combined MMA and homocysteinemia vary, but typically include neurologic, developmental and hematologic abnormalities.

We report 4 children with combined MMA and homocysteinemia who presented predominantly with late-onset diffuse lung diseases (DLD). Of these, 3 accompanied by pulmonary arterial hypertension (PAH), 1 accompanied by hypertension, and 2 accompanied by renal thrombotic microangiopathy (TMA), which was confirmed by renal biopsy. This confirms combined MMA and homocysteinemia should be considered in the differential diagnosis of DLD with or without PAH or renal TMA.