Natural history, with clinical, biochemical, and molecular characterization of classical homocystinuria in the Qatari population

Current and Novel Therapeutical Approaches of Classical Homocystinuria in Childhood With Special Focus on Enzyme Replacement Therapy, Liver-Directed Therapy and Gene Therapy

Classical homocystinuria is a hereditary defect of the enzyme cystathionine beta synthase, which is produced in the liver. If this enzyme fails, the synthesis pathway of cysteine from methionine is interrupted, leading to the accumulation of homocysteine in the blood plasma and homocysteine in the urine. After birth, the children are unremarkable except for the characteristic laboratory findings. Symptoms rarely appear before the second year of life. The most common symptom is a prolapse of the crystalline lens. This finding is seen in 70% of untreated 10-year-old affected individuals. As the earliest symptom, psychomotor retardation occurs in the majority of patients already during the first two years of life. Limiting factors in terms of life expectancy are thromboembolism, peripheral arterial disease, myocardial infarction, and stroke. These symptoms are due to the damage to the vessels caused by the elevated amino acid levels. About 30% suffer a thromboembolic event by the age of 20, about half by the age of 30. This review focus on present and new therapeutical approaches like the role of enzyme replacement with presentation of different novel targets in research like pegtibatinase, pegtarviliase, CDX-6512, erymethionase, chaperones, proteasome inhibitors and probiotic treatment with SYNB 1353. Furthermore, we analyze the role of liver-directed therapy with three dimensional (3D) bioprinting, liver bioengineering of liver organoids in vitro and liver transplantation. The role of different gene therapy options to treat and cure this extremely rare disease in childhood will be discussed.

Tracheobronchomegaly (Mounier-Kuhn syndrome) and Bronchiectasis as rare manifestations of Homocystinuria

Homocystinuria (HCU) is a rare autosomal recessive inherited disorder usually diagnosed in childhood. It is characterized by a deficiency of the enzyme that converts homocysteine to cystathionine. The accumulation of homocysteine leads to abnormalities in the ocular, skeletal, cardiovascular, and central nervous systems. HCU shares several clinical features with Marfan syndrome; however, respiratory system involvement in HCU is uncommon and rarely reported. Bronchiectasis has been previously reported in a few cases of HCU, and it was attributed mainly to fibrillin deficiency. This case describes a young girl diagnosed with classical HCU since childhood who presented with a chronic productive cough and was initially misdiagnosed as bronchial Asthma. However, upon further evaluation, she was eventually diagnosed with tracheobronchomegaly (TBM), or Mounier-Kuhn Syndrome, and bronchiectasis based on the computed tomography (CT) scan of chest findings. To our knowledge, this is the first reported case of TBM and bronchiectasis in HCU. We believe that fibrillin degeneration may be the key to understanding this unusual association in HCU.

Association of selected genetic variants in CBS and MTHFR genes in a cohort of children with homocystinuria in Sri Lanka

BACKGROUND

Homocystinuria is an inherited, inborn error of homocysteine metabolism, which leads to the abnormal accumulation of homocysteine and its metabolites in blood and urine, resulting in various complications. Variants in the cystathionine β-synthase (CBS) and methylenetetrahydrofolate reductase (MTHFR) genes interrupt the formation of the corresponding enzymes and prevent homocysteine from being metabolised; hence, the homocysteine levels in plasma increase than the optimum levels.

MATERIALS AND METHODS

In the current study, eight clinically confirmed children with homocystinuria were detected to study the chosen variants in the CBS gene (c.833 T>C and c.19del) and in the MTHFR gene (c.665 C>T, c.1286 A>C) using SNaPshot mini-sequencing and direct sequencing.

RESULTS

After screening eight patients, none had the c.833T>C, but four patients were in the homozygous state for the c.19del variant in the CBS gene. Furthermore, seven were heterozygous for c.1286A>C, while one patient was heterozygous for c.665C>T in the MTHFR gene.

CONCLUSION

According to the results, c.19del is common in the studied cohort of Sri Lankan children, while c.833T>C is absent, whereas c.1286A>C was more frequent than c.665C>T. To our knowledge, the current study was the first report to discuss the genetic impact of homocystinuria in Sri Lanka; further comprehensive studies are necessary with a larger sample size to establish the association of these variants with the disease in Sri Lanka, which can be beneficial in enhanced patient care and for prospective studies.

Noninvasive Prenatal Testing of Methylmalonic Acidemia cblC Type Using the cSMART Assay for MMACHC Gene Mutations

Noninvasive prenatal testing (NIPT) for monogenic disorders has been developed in recent years; however, there are still significant technical and analytical challenges for clinical use. The clinical feasibility of NIPT for methylmalonic acidemia cblC type (cblC type MMA) was investigated using our circulating single-molecule amplification and re-sequencing technology (cSMART). Trios molecular diagnosis was performed in 29 cblC type MMA-affected children and their parents by traditional Sanger sequencing. In the second pregnancy, invasive prenatal diagnosis (IPD) of the pathogenic MMACHC gene was used to determine fetal genotypes, and NIPT was performed using a novel MMACHC gene–specific cSMART assay. Maternal–fetal genotypes were deduced based on the mutation ratio in maternal plasma DNA. Concordance of fetal genotypes between IPD and NIPT, and the sensitivity and specificity of NIPT were determined. After removing two cases with a low P value or reads, the concordance ratio for NIPT and IPD was 100.00% (27/27), and the sensitivity and specificity were 100.00% (54.07–100.00%) and 100.00% (83.89–100.00%), respectively. This study demonstrates that NIPT using the cSMART assay for cblC type MMA was accurate in detecting fetal genotypes. cSMART has a potential clinical application as a prenatal diagnosis and screening tool for carrier and low-risk genotypes of cblC type MMA and other monogenic diseases.

Early Diagnosis of Classic Homocystinuria in Kuwait through Newborn Screening: A 6-Year Experience

Kuwait is a small Arabian Gulf country with a high rate of consanguinity and where a national newborn screening program was expanded in October 2014 to include a wide range of endocrine and metabolic disorders. A retrospective study conducted between January 2015 and December 2020 revealed a total of 304,086 newborns have been screened in Kuwait. Six newborns were diagnosed with classic homocystinuria with an incidence of 1:50,000, which is not as high as in Qatar but higher than the global incidence. Molecular testing for five of them has revealed three previously reported pathogenic variants in the CBS gene, c.969G>A, p.(Trp323Ter); c.982G>A, p.(Asp328Asn); and the Qatari founder variant c.1006C>T, p.(Arg336Cys). This is the first study to review the screening of newborns in Kuwait for classic homocystinuria, starting with the detection of elevated blood methionine and providing a follow-up strategy for positive results, including plasma total homocysteine and amino acid analyses. Further, we have demonstrated an increase in the specificity of the current newborn screening test for classic homocystinuria by including the methionine to phenylalanine ratio along with the elevated methionine blood levels in first-tier testing. Here, we provide evidence that the newborn screening in Kuwait has led to the early detection of classic homocystinuria cases and enabled the affected individuals to lead active and productive lives.

Congenital cataract: An ocular manifestation of classical homocystinuria

Background

Homocystinuria is an autosomal recessive metabolic disorder occurring due to the defects in cystathionine‐β‐synthase enzyme. The study was carried out to investigate a Pakistani family presenting bilateral congenital cataract with symptoms of classical homocystinuria at LRBT Free Eye Hospital, Lahore, Pakistan.

Methods

Three affected individuals of the family presented skeletal deformations, intellectual disability, speech delay, and myopia with bilateral congenital cataract. Genetic analysis on DNA samples from affected individuals was done through whole exome sequencing to identify underlying genetic variant causing disease phenotypes in the family. In silico analysis was done to predict the effect of variation on the structure of mutant protein.

Results

A missense allelic variant (NM_000071.3: c.253G>A) of the CBS gene was revealed which may affect the catalytic activity of the substituted (NP_000062.1: p.G85R) protein by disrupting the folding of the enzymatic protein. High levels of homocysteine were observed in the plasma of affected individuals. This is the first report of this genetic variant from Pakistan causing homocystinuria and congenital cataract in association.

Conclusion

This variant was reported first time in association with congenital cataract instead of ectopia lentis. Congenital cataract was developed secondarily in these patients and provided a clue for the early diagnosis of metabolic disorders like homocystinuria to prevent further complications and morbidity.

Analysis of differential neonatal lethality in cystathionine β-synthase deficient mouse models using metabolic profiling

Cystathionine beta-synthase (CBS) is a key enzyme of the trans-sulfuration pathway that converts homocysteine to cystathionine. Loss of CBS activity due to mutation results in CBS deficiency, an inborn error of metabolism characterized by extreme elevation of plasma total homocysteine (tHcy). C57BL6 mice containing either a homozygous null mutation in the cystathionine β-synthase (Cbs^-/- ) gene or an inactive human CBS protein (Tg-G307S Cbs^-/- ) are born in mendelian numbers, but the vast majority die between 18 and 21 days of age due to liver failure. However, adult Cbs null mice that express a hypomorphic allele of human CBS as a transgene (Tg-I278T Cbs^-/- ) show almost no neonatal lethality despite having serum tHcy levels similar to mice with no CBS activity. Here, we characterize liver and serum metabolites in neonatal Cbs^+/- , Tg-G307S Cbs^-/- , and Tg-I278T Cbs^-/- mice at 6, 10, and 17 days of age to understand this difference. In serum, we observe similar elevations in tHcy in both Tg-G307S Cbs^-/- and Tg-I278T Cbs^-/- compared to control animals, but methionine is much more severely elevated in Tg-G307S Cbs^-/- mice. Large scale metabolomic analysis of liver tissue confirms that both methionine and methionine-sulfoxide are significantly more elevated in Tg-G307S Cbs^-/- animals, along with significant differences in several other metabolites including hexoses, amino acids, other amines, lipids, and carboxylic acids. Our data are consistent with a model that the neonatal lethality observed in CBS-null mice is driven by excess methionine resulting in increased stress on a variety of related pathways including the urea cycle, TCA cycle, gluconeogenesis, and phosphatidylcholine biosynthesis.

The Spectrum of Mutations of Homocystinuria in the MENA Region

Homocystinuria is an inborn error of metabolism due to the deficiency in cystathionine beta-synthase (CBS) enzyme activity. It leads to the elevation of both homocysteine and methionine levels in the blood and urine. Consequently, this build-up could lead to several complications such as nearsightedness, dislocated eye lenses, a variety of psychiatric and behavioral disorders, as well as vascular system complications. The prevalence of homocystinuria is around 1/200,000 births worldwide. However, its prevalence in the Gulf region, notably Qatar, is exceptionally high and reached 1:1800. To date, more than 191 pathogenic CBS mutations have been documented. The majority of these mutations were identified in Caucasians of European ancestry, whereas only a few mutations from African-Americans or Asians were reported. Approximately 87% of all CBS mutations are missense and do not target the CBS catalytic site, but rather result in unstable misfolded proteins lacking the normal biological function, designating them for degradation. The early detection of homocystinuria along with low protein and methionine-restricted diet is the best treatment approach for all types of homocystinuria patients. Yet, less than 50% of affected individuals show a significant reduction in plasma homocysteine levels after treatment. Patients who fail to lower the elevated homocysteine levels, through high protein-restricted diet or by B6 and folic acid supplements, are at higher risk for cardiovascular diseases, neurodegenerative diseases, neural tube defects, and other severe clinical complications. This review aims to examine the mutations spectrum of the CBS gene, the disease management, as well as the current and potential treatment approaches with a greater emphasis on studies reported in the Middle East and North Africa (MENA) region.

A proactive genotype-to-patient-phenotype map for cystathionine beta-synthase

BACKGROUND

For the majority of rare clinical missense variants, pathogenicity status cannot currently be classified. Classical homocystinuria, characterized by elevated homocysteine in plasma and urine, is caused by variants in the cystathionine beta-synthase (CBS) gene, most of which are rare. With early detection, existing therapies are highly effective.

METHODS

Damaging CBS variants can be detected based on their failure to restore growth in yeast cells lacking the yeast ortholog CYS4. This assay has only been applied reactively, after first observing a variant in patients. Using saturation codon-mutagenesis, en masse growth selection, and sequencing, we generated a comprehensive, proactive map of CBS missense variant function.

RESULTS

Our CBS variant effect map far exceeds the performance of computational predictors of disease variants. Map scores correlated strongly with both disease severity (Spearman's ϱ = 0.9) and human clinical response to vitamin B6 (ϱ = 0.93).

CONCLUSIONS

We demonstrate that highly multiplexed cell-based assays can yield proactive maps of variant function and patient response to therapy, even for rare variants not previously seen in the clinic.