Detection of abnormal sulphur-containing amino acid excretion in a mass urine-screening programme

Chaperone therapy for homocystinuria: the rescue of CBS mutations by heme arginate

Classical homocystinuria is caused by mutations in the cystathionine β-synthase (CBS) gene. Previous experiments in bacterial and yeast cells showed that many mutant CBS enzymes misfold and that chemical chaperones enable proper folding of a number of mutations. In the present study, we tested the extent of misfolding of 27 CBS mutations previously tested in E. coli under the more folding-permissive conditions of mammalian CHO-K1 cells and the ability of chaperones to rescue the conformation of these mutations. Expression of mutations in mammalian cells increased the median activity 16-fold and the amount of tetramers 3.2-fold compared with expression in bacteria. Subsequently, we tested the responses of seven selected mutations to three compounds with chaperone-like activity. Aminooxyacetic acid and 4-phenylbutyric acid exhibited only a weak effect. In contrast, heme arginate substantially increased the formation of mutant CBS protein tetramers (up to sixfold) and rescued catalytic activity (up to ninefold) of five out of seven mutations (p.A114V, p.K102N, p.R125Q, p.R266K, and p.R369C). The greatest effect of heme arginate was observed for the mutation p.R125Q, which is non-responsive to in vivo treatment with vitamin B(6). Moreover, the heme responsiveness of the p.R125Q mutation was confirmed in fibroblasts derived from a patient homozygous for this genetic variant. Based on these data, we propose that a distinct group of heme-responsive CBS mutations may exist and that the heme pocket of CBS may become an important target for designing novel therapies for homocystinuria.

Homocystinuria in New South Wales

Homocystinuria was studied in 27 patients from 15 families in New South Wales. All 2 had biochemical findings consistent with cystathionine synthetase deficiency. One patient was ascertained by newborn screening, but the remaining index cases were detected because of symptoms: poor eyesight 6, mental retardation 3, thromboses 2, skeletal abnormalities 2, and urinary infection1. 9 patients, one-third of all cases, were mildly affected: either they had no features of the disease, or these did not occur until the late teens. Pyridoxine responsiveness was found in 8 sibships, and clinically there were two distinct kinds of response. For patients born in the decade 1960-69 the ascertainment rate for the total population was 1:58 000. The true incidence must be much higher. Our series indicates that homocystinuria occurs more frequently than has heretofore been thought, and that mild cases are common. It is likely that cases are often missed in current newborn screening programmes.

A new form of prolonged transient tyrosinemia presenting with severe metabolic acidosis

Yet another form of tyrosinemia is described, in a young baby who developed metabolic acidosis and ceased to grow when weaned from breast milk onto a higher protein formula. Severe tyrosyluria and mild tyrosinemia cleared on a low-protein diet which also corrected the acidosis. However, restoration of growth required a normal protein intake with very greatly reduced amounts of phenylalanine and tyrosine. The metabolic fault later resolved spontaneously at about 12 months of age. Mental development appears normal and liver disease was never apparent. The patient and her mother both excrete quite large quantities of an unidentified peptide.