Cystathionine beta-synthase (CBS) deficiency is a rare inherited disorder in the methionine catabolic pathway, in which the impaired synthesis of cystathionine leads to accumulation of homocysteine. Patients can present to many different specialists and diagnosis is often delayed. Severely affected patients usually present in childhood with ectopia lentis, learning difficulties and skeletal abnormalities. These patients generally require treatment with a low-methionine diet and/or betaine. In contrast, mildly affected patients are likely to present as adults with thromboembolism and to respond to treatment with pyridoxine. In this article, we present recommendations for the diagnosis and management of CBS deficiency, based on a systematic review of the literature. Unfortunately, the quality of the evidence is poor, as it often is for rare diseases. We strongly recommend measuring the plasma total homocysteine concentrations in any patient whose clinical features suggest the diagnosis. Our recommendations may help to standardise testing for pyridoxine responsiveness. Current evidence suggests that patients are unlikely to develop complications if the plasma total homocysteine concentration is maintained below 120 μmol/L. Nevertheless, we recommend keeping the concentration below 100 μmol/L because levels fluctuate and the complications associated with high levels are so serious.

Newborn screening (NBS) is justified if early intervention is effective in a disorder generally not detected early in life on a clinical basis, and if sensitive and specific biochemical markers exist. Experience with NBS for homocystinurias and methylation disorders is limited. However, there is robust evidence for the success of early treatment with diet, betaine and/or pyridoxine for CBS deficiency and good evidence for the success of early betaine treatment in severe MTHFR deficiency. These conditions can be screened in dried blood spots by determining methionine (Met), methionine-to-phenylanine (Met/Phe) ratio, and total homocysteine (tHcy) as a second tier marker. Therefore, we recommend NBS for cystathionine beta-synthase and severe MTHFR deficiency. Weaker evidence is available for the disorders of intracellular cobalamin metabolism. Early treatment is clearly of advantage for patients with the late-onset cblC defect. In the early-onset type, survival and non-neurological symptoms improve but the effect on neurocognitive development is uncertain. The cblC defect can be screened by measuring propionylcarnitine, propionylcarnitine-to-acetylcarnitine ratio combined with the second tier markers methylmalonic acid and tHcy. For the cblE and cblG defects, evidence for the benefit of early treatment is weaker; and data on performance of Met, Met/Phe and tHcy even more limited. Individuals homozygous or compound heterozygous for MAT1A mutations may benefit from detection by NBS using Met, which on the other hand also detects asymptomatic heterozygotes. Clinical and laboratory data is insufficient to develop any recommendation on NBS for the cblD, cblF, cblJ defects, glycineN-methyltransferase-, S-adenosylhomocysteinehydrolase- and adenosine kinase deficiency.

The main remethylation defects include disorders which all have defective methionine synthesis in common. Methylenetetrahydrofolate reductase deficiency impairs methyltetrahydrofolate synthesis, defects in cytosolic reduction of hydroxocobalamin (CblC/D) impair the synthesis of both methyl- and adenosyl cobalamin and deficiencies of methionine synthase (CblE/G) are associated with defective methyl cobalamin synthesis. The clinical presentation is characterized by acute neurological distress in early infancy. In childhood, patients present with progressive encephalopathy with an end-stage which has many signs in common with the adult onset form. In fact, both have more or less severe signs of subacute degeneration of the cord. Cobalamin defective patients must be treated with parenteral supplementation of hydroxocobalamin (1-2 mg per dose). Some methylenetetrahydrofolate patients could be folate responsive and must have a high-dosage folate trial. In addition, oral betaine supplementation (2-9 g per day depending on age) appears an effective means to prevent further neurological deterioration.

We have studied a novel class of DNA sequences that cause DNA polymerases to pause. These sequences have the central consensus Py-G-C and are not necessarily adjacent to hairpins in the DNA template. Since most consensus sequences do not cause pauses under standard conditions, additional template features must exist that make it difficult to incorporate nucleotides at these positions. We believe that these pauses result from constraints that make the conformation change involved in nucleotide selection more difficult. These pauses can obscure parts of DNA sequencing ladders and prevent DNA amplification by the polymerase chain reaction. The addition of betaine, and some related compounds, relieves these pauses.