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.

Defective tubulin detyrosination causes structural brain abnormalities with cognitive deficiency in humans and mice

Reversible detyrosination of tubulin, the building block of microtubules, is crucial for neuronal physiology. Enzymes responsible for detyrosination were recently identified as complexes of vasohibins (VASHs) one or two with small VASH-binding protein (SVBP). Here we report three consanguineous families, each containing multiple individuals with biallelic inactivation of SVBP caused by truncating variants (p.Q28* and p.K13Nfs*18). Affected individuals show brain abnormalities with microcephaly, intellectual disability and delayed gross motor and speech development. Immunoblot testing in cells with pathogenic SVBP variants demonstrated that the encoded proteins were unstable and non-functional, resulting in a complete loss of VASH detyrosination activity. Svbp knockout mice exhibit drastic accumulation of tyrosinated tubulin and a reduction of detyrosinated tubulin in brain tissue. Similar alterations in tubulin tyrosination levels were observed in cultured neurons and associated with defects in axonal differentiation and architecture. Morphological analysis of the Svbp knockout mouse brains by anatomical magnetic resonance imaging showed a broad impact of SVBP loss, with a 7% brain volume decrease, numerous structural defects and a 30% reduction of some white matter tracts. Svbp knockout mice display behavioural defects, including mild hyperactivity, lower anxiety and impaired social behaviour. They do not, however, show prominent memory defects. Thus, SVBP-deficient mice recapitulate several features observed in human patients. Altogether, our data demonstrate that deleterious variants in SVBP cause this neurodevelopmental pathology, by leading to a major change in brain tubulin tyrosination and alteration of microtubule dynamics and neuron physiology.

Expanding the clinical and genetic spectra of NKX6-2-related disorder

Hypomyelinating leukodystrophies (HLDs) affect the white matter of the central nervous system and manifest as neurological disorders. They are genetically heterogeneous. Very recently, biallelic variants in NKX6-2 have been suggested to cause a novel form of autosomal recessive HLD. Using whole-exome or whole-genome sequencing, we identified the previously reported c.196delC and c.487C>G variants in NKX6-2 in 3 and 2 unrelated index cases, respectively; the novel c.608G>A variant was identified in a sixth patient. All variants were homozygous in affected family members only. Our patients share a primary diagnosis of psychomotor delay, and they show spastic quadriparesis, nystagmus and hypotonia. Seizures and dysmorphic features (observed in 2 families each) represent an addition to the phenotype, while developmental regression (observed in 3 families) appears to be a notable and previously underestimated clinical feature. Our findings extend the clinical and mutational spectra associated with this novel form of HLD. Comparative analysis of our 10 patients and the 15 reported previously did, however, not reveal clear evidence for a genotype-phenotype correlation.