This work demonstrates the application of whole-genome sequencing to the discovery of the cause of autosomal recessive DRD in an affected fraternal twin pair. Whole-genome sequencing of both twins implicated three genes with variants potentially causative of autosomal recessive DRD. Only one of these genes, SPR
was a high-priority candidate by functional criteria. This gene encodes the enzyme sepiapterin reductase, which catalyzes the reaction of 6-pyruvoyl-tetrahydropterin to BH4, and has been implicated previously in DRD. BH4 is an important coenzyme for the production of the neurotransmitters dopamine, noradrenalin, and serotonin. The involvement of SPR
was unexpected in the twin pair given their clinical symptoms including the lack of intellectual disability, their sustained response to monotherapy with l
), and a family history of neurological disorders that complicated the interpretation of the potential inheritance pattern (). Our observations highlight both the challenges of clinical phenotyping, especially for rare diseases by nonspecialists, and the ability of whole-genome sequencing to identify susceptibility variants, especially when initial candidate genes have been eliminated by more traditional locus-specific gene testing. Whole-genome sequencing facilitated both the diagnostic workup and the medical management of the fraternal twins with DRD.
The nonsense (p.Lys251X) and missense (p.Arg150Gly) mutations in the SPR
gene are predicted to be deleterious by disrupting the secondary structure and binding to NADP (nicotinamide adenine dinucleotide phosphate) of the SPR protein, respectively. Critically, both variants have been previously reported and deemed to be deleterious for SPR activity through functional biochemical assays (18
). Both SPR
mutations were originally reported as homozygous alleles but, in this study, were found to be compound heterozygous alleles.
The identification of the involvement of the SPR
gene in this family manifesting DRD has directly influenced clinical management of the individuals with DRD. Although DRD caused by mutations in SPR
can be treated with dopamine therapy alone, typically the response is limited when compared to treatment of DRD caused by autosomal dominant mutations in GCH1
. The administration of 5-HTP, a serotonin precursor, in addition to l
-dopa has been shown to greatly improve cognitive abilities and gait and to minimize pyramidal signs in DRD caused by mutations in SPR
). Moreover, supplementation with 5-HTP potentially allows tapering of the dosage of l
-dopa in these patients. Treatment with BH4 has also been shown to reduce clinical symptoms in DRD (22
). The identification of these medically actionable alleles in the fraternal twin pair in this study prompted administration of 5-HTP as a prelude to other possible adjuvant therapies that directly stimulate the serotonin pathways. 5-HTP therapy led to improvements in the male twin’s fine motor skills, including handwriting and athletic ability. Furthermore, in the more severely affected female twin, 5-HTP therapy appears to have led to abatement of a severe recurrent laryngospasm, which would often end in vomiting, preventing physical activity and greatly reducing quality of life. Although identification of mutations that cause recessive disorders has previously been demonstrated (12
) and in some cases has led to the alteration of treatment (24
), the application of whole-genome sequencing to a direct alteration in clinical management has not yet been fully realized. This study demonstrates the utility of whole-genome sequencing and databases of deleterious variants to facilitate both diagnostic and therapeutic medical management decisions.
Because genetic diagnosis using whole-genome sequencing in individual families is becoming more accessible, the debate concerning the underlying value of the extensive information that is generated has intensified (26
). Here, we demonstrate the application of the technique to a recessive disease of unknown cause. Further, the new knowledge afforded by the molecular testing resulted in the implementation of additional treatment options and further optimized patient care.
The identification of mutations in SPR accounts for the previous history and diagnosis of DRD in this family. With hindsight, SPR deficiency is a logical potential differential diagnosis. However, before whole-genome sequencing, implication of SPR mutations in this disease was hampered by an atypical DRD presentation and the presence of other neurological disorders in the family that were initially thought to be related. Further, it is tempting to suggest the use of candidate gene sequencing rather than whole-genome sequencing for this family now that the identified causative gene is a known good candidate. Candidate gene sequencing, however, suffers from several disadvantages that whole-genome sequencing does not. Candidate gene sequencing may fail to analyze the correct genes, typically does not interrogate all of the biologically important regions of a gene (for example, untranslated regions and promoters), does not eliminate other genes from consideration, and rapidly becomes more expensive and time-consuming compared with whole-genome sequencing as the number of loci deemed clinically relevant increases. Although candidate gene sequencing would have been successful in this case, clinical testing for SPR mutations was unavailable at the time of the initial evaluation and initiation of therapy, and attention to the potential gene involved was not directed by clinically observed phenotypic information.
In addition to providing comprehensive analysis of the potential underlying genetic contributions to specific diseases, whole-genome sequencing may provide insights into other phenotype/genotype relationships and potentially illuminate the genetic susceptibility of other family members. Initial pedigree analysis of the family of the DRD twins identified a family history of depression in three generations on the paternal side of the family and a diagnosis of fibromyalgia in two generations on the maternal side. The individual pathogenic alleles in the SPR gene cosegregate with the fibromyalgia phenotype (), although not with the depression phenotype. Because fibromyalgia can respond to serotonin reuptake inhibitor drugs, suggesting that pathogenesis of this disease may be related to reduced serotonin, we hypothesize that heterozygous loss-of-function SPR mutations may contribute to a genetic susceptibility to fibromyalgia.
Challenges in clinical application of whole-genome sequencing include informed consent of subjects, unknown medical significance of most variants, uncovering both susceptibility variants and recessive risk alleles, development of databases of medically actionable variants, and counseling based on variant interpretation (26
). The current case study of a fraternal twin pair with DRD, however, argues for the clinical utility of information gained from whole-genome sequencing in place of molecular genetic testing of individual genes, where there is even minimal suspicion of genetic heterogeneity for the clinical phenotype.